Synapter {synapter} | R Documentation

## Class "Synapter"

### Description

A reference class to store, manage and process Synapt G2 data to combine identification and quantitation results.

The data, intermediate and final results are stored together in such a ad-how container called a class. In the frame of the analysis of a set of 3 or 5 data files, namely as identification peptide, a quantitation peptide and a quantitation Pep3D, and identification fragments and quantitation spectra, such a container is created and populated, updated according to the user's instructions and used to display and export results.

The functionality of the `synapter` package implemented in the `Synapter` class in described in the Details section below. Documentation for the individual methods is provided in the Methods section. Finally, a complete example of an analysis is provided in the Examples section, at the end of this document.

See also papers by Shliaha et al. for details about ion mobility separation and the manuscript describing the `synapter` methodology.

### Usage

    Synapter(filenames, master) ## creates an instance of class 'Synapter'

### Arguments

`filenames` | A named `list` of file names to be load. The names must be 'identpeptide', 'quantpeptide', 'quantpep3d' and 'fasta' (could be an RDS file created by `link{createUniquePeptideDbRds}`) and (optional 'identfragments' and 'quantspectra'). `identpeptide` can be a `csv` final peptide file (from PLGS) or a saved `"MasterPeptides"` data object as created by `makeMaster` if working with master peptide data. To serialise the `"MasterPeptides"` instance, use the `saveRDS` function, and file extenstion `rds`. This master file could contain a fragment library as well. In this case the 'identfragments' argument would be ignored.
`master` | A `logical` that defines if the identification file is a master file. See `makeMaster` for details about this strategy.

### Details

A `Synapter` object logs every operation that is applied to it. When displayed with `show` or when the name of the instance is typed at the R console, the original input file names, all operations and resulting the size of the respective data are displayed. This allows the user to trace the effect of respective operations.

#### Loading the data

The construction of the data and analysis container, technically defined as an instance or object of class `Synapter`, is created with the `Synapter` constructor. This function requires 4 or 6 files as input, namely, the identification final peptide csv file, the quantitation final peptide csv file, the quantitation Pep3D csv file (as exported from the PLGS software), the fasta file use for peptide identification, and optional the identification fragments csv file and the quantitation spectra xml file. The fasta file ('fasta') could be an RDS file generated by `link{createUniquePeptideDbRds}`, too. The file names need to be specified as a named list with names 'identpeptide', 'quantpeptide', 'quantpep3d', 'fasta', 'identfragments' and 'quantspectra' respectively. These files are read and the data is stored in the newly created `Synapter` instance.

The final peptide files are filtered to retain peptides with `matchType` corresponding to `PepFrag1` and `PepFrag2`, corresponding to unmodified round 1 and 2 peptide identification. Other types, like `NeutralLoss_NH3`, `NeutralLoss_H20`, `InSource`, `MissedCleavage` or `VarMod` are not considered in the rest of the analysis. The quantitation Pep3D data is filtered to retain `Function` equal to `1` and unique quantitation spectrum ids, i.e. unique entries for multiple charge states or isotopes of an EMRT (exact mass-retention time features).

Then, p-values for `Regular` peptides are computed based on the `Regular` and `Random` database types score distributions, as described in Käll et al., 2008a. Only unique peptide sequences are taken into account: in case of duplicated peptides, only one entry is kept. Empirical p-values are adjusted using Bonferroni and Benjamini and Hochberg, 1995 (`multtest` package) and q-values are computed using the `qvalue` package (Storey JD and Tibshirani R., 2003 and Käll et al., 2008b). Only `Regular` entries are stored in the resulting data for subsequent analysis.

The data tables can be exported as `csv` spreadsheets with the `writeIdentPeptides` and `writeQuantPeptides` methods.

#### Filtering identification and quantitation peptide

The first step of the analysis aims to match reliable peptide. The final peptide datasets are filtered based on the FDR (BH is default) using the `filterQuantPepScore` and `filterIdentPepScore` methods. Several plots are provided to illustrate peptide score densities (from which p-values are estimated, `plotPepScores`; use `getPepNumbers` to see how many peptides were available) and q-values (`plotFdr`).

Peptides matching to multiple proteins in the fasta file (non-unique tryptic identification and quantitation peptides) can be discarded with the `filterUniqueDbPeptides` method. One can also filter on the peptide length using `filterPeptideLength`.

Another filtering criterion is mass accuracy. Error tolerance quantiles (in ppm, parts per million) can be visualised with the `plotPpmError` method. The values can be retrieved with `getPpmErrorQs`. Filtering is then done separately for identification and quantitation peptide data using `filterIdentPpmError` and `filterQuantPpmError` respectively. The previous plotting functions can be used again to visualise the resulting distribution.

Filtering can also be performed at the level of protein false positive rate, as computed by the PLGS application (`protein.falsePositiveRate` column), which counts the percentage of decoy proteins that have been identified prior to the regular protein of interest. This can be done with the `filterIdentProtFpr` and `filterQuantProtFpr` methods. Note that this field is erroneously called a false positive rate in the PLGS software and the associated manuscript; it is a false discovery rate.

#### Merging identification and quantitation peptides

Common and reliable identification and quantitation peptides are then matched based on their sequences and merged using the `mergePeptides` method.

#### Retention time modelling

Systematic differences between identification features and quantitation features retention times are modelled by fitting a local regression (see the `loess` function for details), using the `modelRt` method. The smoothing parameter, or number of neighbour data points used the for local fit, is controlled by the `span` parameter that can be set in the above method.

The effect of this parameter can be observed with the `plotRt` method, specifying `what = "data"` as parameters. The resulting model can then be visualised with the above method specifying `what = "model"`, specifying up to 3 number of standard deviations to plot. A histogram of retention time differences can be produced with the `plotRtDiffs` method.

To visualise the feature space `plotFeatures` could be used. It generates one or two (if ion mobility is available) plots of retention time vs mass and mass vs ion mobility for each data source, namely, Identification data, Quantitation data and Quantitation Pep3D data.

#### Intensity modelling

Systematic differences between intensities of identification features and quantitation features depending on retention times are modelled by fitting a local regression (see the `loess` function for details), using the `modelIntensity` method. The smoothing parameter, or number of neighbour data points used the for local fit, is controlled by the `span` parameter that can be set in the above method.

The effect of this parameter can be observed with the `plotIntensity` method, specifying `what = "data"` as parameters. The resulting model can then be visualised with the above method specifying `what = "model"`, specifying up to 3 number of standard deviations to plot.

#### Grid search to optimise matching tolerances

Matching of identification peptides and quantitation EMRTs is done within a mass tolerance in parts per million (ppm) and the modelled retention time +/- a certain number of standard deviations. To help in the choice of these two parameters, a grid search over a set of possible values is performed and performance metrics are recorded, to guide in the selection of a 'best' pair of parameters.

The following metrics are computed: (1) the percentage of identification peptides that matched a single quantitation EMRT (called `prcntTotal`), (2) the percentage of identification peptides used in the retention time model that matched the quantitation EMRT corresponding to the correct quantitation peptide in ident/quant pair of the model (called `prcntModel`) and (3) the detailed about the matching of the features used for modelling (accessible with `getGridDetails`) and the corresponding `details` grid that reports the percentage of correct unique assignments. The detailed grid results specify the number of non matched identification peptides (0), the number of correctly (1) or wrongly (-1) uniquely matched identification peptides, the number of identification peptides that matched 2 or more peptides including (2+) or excluding (2-) the correct quantitation equivalent are also available.

See the next section for additional details about how matching. The search is performed with the `searchGrid` method, possibly on a subset of the data (see Methods and Examples sections for further details).

The parameters used for matching can be set manually with `setPpmError`, `setRtNsd`, `setImDiff` respectively, or using `setBestGridParams` to apply best parameters as defined using the grid search. See example and method documentation for details.

#### Identification transfer: matching identification peptides and quantitation EMRTs

The identification peptide - quantitation EMRT matching, termed identification transfer, is performed using the best parameters, as defined above with a grid search, or using user-defined parameters.

Matching is considered successful when one and only one EMRT is found in the mass tolerance/retention time/ion mobility window defined by the error ppm, number of retention time standard deviations, and ion mobility difference parameters. The values of uniquely matched EMRTs are reported in the final matched dataframe that can be exported (see below). If however, none or more than one EMRTs are matched, 0 or the number of matches are reported.

As identification peptides are serially individually matched to 'close' EMRTs, it is possible for peptides to be matched the same EMRT independently. Such cases are reported as -1 in the results dataframes.

The results can be assess using the `plotEMRTtable` (or `getEMRTtable` to retrieve the values) and `performace` methods. The former shows the number of identification peptides assigned to none (0), exactly 1 (1) or more (> 2) EMRTs. The latter method reports matched identification peptides, the number of (q-value and protein FPR filtered) identification and quantitation peptides. Matched EMRT and quantitation peptide numbers are then compared calculating the synapter enrichment (100 * ( synapter - quant ) / quant) and Venn counts.

#### Remove Less Intense Peaks

As an additional step it is possible to remove less intense peaks from the spectra and fragment data. Use `plotCumulativeNumberOfFragments` to plot the number of fragments vs the intensity and to find a good threshold. The `filterFragments` method could remove peaks if the intensity is below a specified threshold via the `minIntensity` argument. Set the `maxNumber` argument to keep only the `maxNumber` highest peaks/fragments. The `what` argument controls the data on which the filter is applied. Use `what = "fragments.ident"` for the identification fragments and `what = "spectra.quant"` for the quantiation spectra data.

#### Fragment Matching

After importing fragment and spectra data it is possible to match peaks between the identification fragments and the quantitation spectra using the `fragmentMatching` method. Use `setFragmentMatchingPpmTolerance` to set the maximal allowed tolerance for considering a peak as identical. There are two different methods to visualise the results of the fragment matching procedure. `plotFragmentMatching` plots the fragments and spectra for each considered pair. `plotFragmentMatchingPerformance` draws two plots. On the left panel you could see the performance of different thresholds for the number of common peaks for unique matches. The right panel visualizes the performance of different differences (delta) of common peaks between the best match (highest number of common peaks) and the second best match in each non unique match group. `plotFragmentMatchingPerformance` returns the corresponding values invisible or use `fragmentMatchingPerformance` to access these data. Use `filterUniqueMatches` and `filterNonUniqueMatches` to remove unique or non unique matches below the threshold of common peaks respective the difference in common peaks from the MatchedEMRTs data.frame.

#### Exporting and saving data

The merged identification and quantitation peptides can be exported to csv using the `writeMergedPeptides` method. Similarly, the matched identification peptides and quantitation EMRTs are exported with `writeMatchedEMRTs`.

Complete `Synapter` instances can be serialised with `save`, as any R object, and reloaded with `load` for further analysis.

It is possible to get the fragment and spectra data from the identification and quantitation run using `getIdentificationFragments` respectively `getQuantitationSpectra`.

### Methods

#### Analysis methods

mergePeptides

  `signature(object = "Synapter")`: Merges quantitation and identification final peptide data, used to perform retention time modelling (see `modelRt` below).

modelRt

  `signature(object = "Synapter", span = "numeric")`: Performs local polynomial regression fitting (see `loess`) retention time alignment using `span` parameter value to control the degree of smoothing.

modelIntensity

  `signature(object = "Synapter", span = "numeric")`: Performs local polynomial regression fitting (see `loess`) intensity values using `span` parameter value to control the degree of smoothing.

findEMRTs

  `signature(object = "Synapter", ppm = "numeric", nsd = "numeric", imdiff = "numeric")`: Finds EMRTs matching identification peptides using `ppm` mass tolerance, `nsd` number of retention time standard deviations and `imdiff` difference in ion mobility. The last three parameters are optional if previously set with `setPpmError`, `setRtNsd`, `setImDiff`, or, better, `setBestGridParams` (see below).

rescueEMRTs

  `signature(object = "Synapter", method = c("rescue", "copy"))`: The `method` parameter defined the behaviour for those high confidence features that where identified in both identification and quantitation acquisitions and used for the retention time model (see `mergePeptides`). Prior to version 1.1.1, these features were transferred from the quantitation pep3d file if unique matches were found, as any feature (`"transfer"`). As a result, those matching 0 or > 1 EMRTs were quantified as `NA`. The default is now to `"rescue"` the quantitation values of these by directly retrieving the data from the quantification peptide data. Alternatively, the quantitation values for these features can be directly taken from the quantitation peptide data using `"copy"`, thus effectively bypassing identification transfer.

searchGrid

  `signature(object="Synapter", ppms="numeric", nsds="numeric", imdiffs = "numeric", subset="numeric", n = "numeric", verbose="logical")`: Performs a grid search. The grid is defined by the `ppm`, `nsd` and `imdiffs` numerical vectors, representing the sequence of values to be tested. Default are `seq(5, 20, 2)`, `seq(0.5, 5, 0.5)`, `seq(0.2, 2, 0.2)` respectively. To ignore ion mobility set `imdiffs = Inf`. `subset` and `n` allow to use a randomly chosen subset of the data for the grid search to reduce search time. `subset` is a numeric, between 0 and 1, describing the percentage of data to be used; `n` specifies the absolute number of feature to use. The default is to use all data. `verbose` controls whether textual output should be printed to the console. (Note, the `mergedEMRTs` value used in internal calls to `findEMRTs` is `"transfer"` - see `findEMRTs` for details).

fragmentMatching

  `signature(object="Synapter", ppm = "numeric", verbose = "logical"`: Performs a fragment matching between spectra and fragment data. The `ppm` argument controls the tolerance that is used to consider two peaks (MZ values) as identical. If `verbose` is `TRUE` (default) a progress bar is shown.

##### Methods to display, access and set data

show

  `signature(object = "Synapter")`: Display `object` by printing a summary to the console.

dim

  `signature(x="Synapter")`: Returns a `list` of dimensions for the identification peptide, quantitation peptide, merged peptides and matched features data sets.

inputFiles

  `signature(object="Synapter")`: Returns a `character` of length 6 with the names of the input files used as `identpeptide`, `quantpeptide`, `quantpep3d`, `fasta`, `identfragments` and `quantspectra`.

getLog

  `signature(object="Synapter")`: Returns a `character` of variable length with a summary of processing undergone by `object`.

getGrid

  `signature(object="Synapter", digits = "numeric")`: Returns a named `list` of length 3 with the precent of total (`prcntTotal`), percent of model (`prcntModel`) and detailed (`details`) grid search results. The `details` grid search reports the proportion of correctly assigned features (+1) to all unique assignments (+1 and -1). Values are rounded to 3 `digits` by default.

getGridDetails

  `signature(object="Synapter")`: Returns a `list` of number of ..., -2, -1, 0, +1, +2, ... results found for each of the `ppm`/`nsd` pairs tested during the grid search.

getBestGridValue

  `signature(object="Synapter")`: Returns a named `numeric` of length 3 with best grid values for the 3 searches. Names are `prcntTotal`, `prcntModel` and `details`.

getBestGridParams

  `signature(object="Synapter")`: Returns a named `list` of matrices (`prcntTotal`, `prcntModel` and `details`). Each matrix gives the `ppm` and `nsd` pairs that yielded the best grid values (see `getBestGridValue` above).

setBestGridParams

  `signature(object="Synapter", what="character")`: This methods set the best parameter pair, as determined by `what`. Possible values are `auto` (default), `model`, `total` and `details`. The 3 last ones use the (first) best parameter values as reported by `getBestGridParams`. `auto` uses the best `model` parameters and, if several best pairs exists, the one that maximises `details` is selected.

setPepScoreFdr

  `signature(object="Synapter", fdr = "numeric")`: Sets the peptide score false discovery rate (default is 0.01) threshold used by `filterQuantPepScore` and `filterIdentPepScore`.

getPepScoreFdr

  `signature(object="Synapter")`: Returns the peptide false discrovery rate threshold.

setIdentPpmError

  `signature(object="Synapter", ppm = "numeric")`: Set the identification mass tolerance to `ppm` (default 10).

getIdentPpmError

  `signature(object="Synapter")`: Returns the identification mass tolerance.

setQuantPpmError

  `signature(object="Synapter", ppm = "numeric")`: Set the quantitation mass tolerance to `ppm` (default 10).

getQuantPpmError

  `signature(object="Synapter")`: Returns the quantitation mass tolerance.

setPpmError

  `signature(object="Synapter", ppm = "numeric")`: Sets the identification and quantitation mass tolerance `ppm` (default is 10).

setLowessSpan

  `signature(object="Synapter", span = "numeric")`: Sets the `loess` `span` parameter; default is 0.05.

getLowessSpan

  `signature(object="Synapter")`: Returns the `span` parameter value.

setRtNsd

  `signature(object="Synapter", nsd = "numeric")`: Sets the retention time tolerance `nsd`, default is 2.

getRtNsd

  `signature(object="Synapter")`: Returns the value of the retention time tolerance `nsd`.

setImDiff

  `signature(object="Synapter", imdiff = "numeric")`: Sets the ion mobility tolerance `imdiff`, default is 0.5.

getImDiff

  `signature(object="Synapter")`: Returns the value of the ion mobility tolerance `imdiff`.

getPpmErrorQs

  `signature(object="Synapter", qs = "numeric", digits = "numeric")`: Returns the mass tolerance `qs` quantiles (default is `c(0.25, 0.5, 0.75, seq(0.9, 1, 0.01)`) for the identification and quantitation peptides. Default is 3 `digits`.

getRtQs

  `signature(object="Synapter", qs = "numeric", digits = "numeric")`: Returns the retention time tolerance `qs` quantiles (default is `c(0.25, 0.5, 0.75, seq(0.9, 1, 0.01)`) for the identification and quantitation peptides. Default is 3 `digits`.

getPepNumbers

  `signature(object="Synapter")`: Returns the number of regular and random quantitation and identification peptide considered for p-value calculation and used to plot the score densities (see `plotPepScores`). Especially the difference between random and regular entries are informative in respect with the confidence of the random scores distribution.

setFragmentMatchingPpmTolerance

  `signature(object="Synapter", ppm = "numeric")`: Sets the fragment matching mass tolerance `ppm` (default is 25).

getFragmentMatchingPpmTolerance

  `signature(object="Synapter")`: Returns the fragment matching mass tolerance in ppm.

showFdrStats

  `signature(object="Synapter", k = "numeric")`: Returns a named `list` of length 2 with the proportion of identification and quantitation peptides that are considered significant with a threshold of `k` (default is `c(0.001, 0.01, 0.5, 0.1)`) using raw and adjusted p-values/q-values.

getEMRTtable

  `signature(object="Synapter")`: Returns a `table` with the number of 0, 1, 2, ... assigned EMRTs.

performance

  `signatute(object="Synapter", verbose = TRUE)`: Returns (and displays, if `verbose`) the performance of the synapter analysis.

performance2

  `signatute(object="Synapter", verbose = TRUE)`: Returns (and displays, if `verbose`) information about number of missing values and identification source of transfered EMRTs.

fragmentMatchingPerformance

  `signature(object="Synapter", what = c("unique", "non-unique")`: Returns the performance of the fragment matching for `unqiue` or `non-unique` matches. The return valus is a `matrix` with seven columns. The first column `ncommon`/`deltacommon` contains the thresholds. Column 2 to 5 are the true positives `tp`, false positives `fp`, true negatives `tn`, false negatives `fn` for the merged peptide data. The sixth column `all` shows the corresponding number of peptides for all peptides (not just the merged ones) and the last column shows the FDR `fdr` for the current threshold (in that row) for the merged data.

  Please note that the FDR is overfitted/underestimated because the merged peptides are the peptides from the highest quality spectra were PLGS could easily identify the peptides. The peptides that are not present in the merged data are often of lower quality hence the FDR would be higher by trend.

  See `plotFragmentMatchingPerformance` for a graphical representation.

##### Filters

filterUniqueDbPeptides

  `signature(object="Synapter", missedCleavages = 0, IisL = TRUE, verbose = TRUE)`: This method first digests the fasta database file and keeps unique tryptic peptides. (NOTE: since version 1.5.3, the tryptic digestion uses the `cleaver` package, replacing the more simplistic inbuild function. The effect of this change is documented in https://github.com/lgatto/synapter/pull/47). The number of maximal missed cleavages can be set as `missedCleavages` (default is 0). If `IisL = TRUE` Isoleucin and Leucin are treated as the same aminoacid. In this case sequences like "ABCI", "ABCL" are removed because they are not unqiue anymore. If `IisL = FALSE` (default) "ABCI" and "ABCL" are reported as unique. The peptide sequences are then used as a filter against the identification and quantitation peptides, where only unique proteotyptic instances (no miscleavage allowed by default) are eventually kept in the `object` instance. This method also removes any additional duplicated peptides, that would not match any peptides identified in the fasta database.

filterUniqueQuantDbPeptides

  `signature(object="Synapter", missedCleavages = 0, IisL = TRUE, verbose = TRUE)`: As `filterUniqueDbPeptides` for quantitation peptides only.

filterUniqueIdentDbPeptides

  `signature(object="Synapter", missedCleavages = 0, IisL = TRUE, verbose = TRUE)`: As `filterUniqueDbPeptides` for identification peptides only.

filterQuantPepScore

  `signature(object="Synapter", fdr = "numeric", method = "character")`: Filters the quantitation peptides using `fdr` false discovery rate. `fdr` is missing by default and is retrieved with `getPepScoreFdr` automatically. If not set, default value of 0.01 is used. `method` defines how to performe p-value adjustment; one of `BH`, `Bonferrone` or `qval`. See details section for more information.

filterIdentPepScore

  `signature(object="Synapter", fdr = "numeric", method = "charactet")`: As `filterQuantPepScore`, but for identification peptides.

filterQuantProtFpr

  `signature(object="Synapter", fpr = "numeric")`: Filters quantitation peptides using the protein false positive rate (erroneously defined as a FPR, should be FDR), as reported by PLGS, using threshold set by `fpr` (missing by default) or retrieved by `getProtFpr`.

filterIdentProtFpr

  `signature(object="Synapter", fpr = "numeric")`: as `filterQuantProtFpr`, but for identification peptides.

filterQuantPpmError

  `signature(object="Synapter", ppm = "numeric")`: Filters the quantitation peptides based on the mass tolerance `ppm` (default missing) provided or retrieved automatically using `getPpmError`.

filterIdentPpmError

  `signature(object="Synapter")`: as `filterQuantPpmError`, but for identification peptides.

filterFragments

  `signature(object = "Synapter", what = c("fragments.ident", "spectra.quant"), minIntensity = "numeric", maxNumber = "numeric", verbose = "logical")`: Filters the spectra/fragment data using a minimal intensity threshold (`minIntensity`) or a maximal number of peaks/fragments threshold (`maxNumber`). Please note that the maximal number is transfered to an intensity threshold and the result could contain less peaks than specified by `maxNumber`. If both arguments are given, the more aggressive one is chosen. Use the `what` argument to specify the data that should be filtered. Set `what = "fragments.ident"` for the identification fragment data or `what = "spectra.quant"` for the quantiation spectra. If `verbose` is `TRUE` (default) a progress bar is shown.

filterUniqueMatches

  `signature(object="Synapter", minNumber = "numeric")`: Removes all unique matches that have less than `minNumber` of peaks/fragments in common. Use `fragmentMatchingPerformance(..., what="unique")`/ `plotFragmentMatchingPerformance` (left panel) to find an ideal threshold.

filterNonUniqueMatches

  `signature(object="Synapter", minDelta = "numeric")`: Removes all non unique matches that have a difference between the best match (highest number of common peaks/fragments, treated as true match) and the second best match (second highest number of common peaks/fragments) less than `minDelta`. For the matches above the threshold only the one with the highest number of common peaks/fragments in each match group is kept. Use `fragmentMatchingPerformance(..., what="non-unique")`/ `plotFragmentMatchingPerformance` (right panel) to find an ideal threshold.

filterNonUniqueIdentMatches

  `signature(object="Synapter")`: Removes all non unique identification matches. In rare circumstances (if the grid search parameters are to wide/relaxed or a fragment library is used) it could happen that the `searchGrid` methods matches a single quantification EMRT to multiple identification EMRTs. This methods removes all these non unique matches.

##### Plotting

plotPpmError

  `signature(object="Synapter", what = "character")`: Plots the proportion of data against the mass error tolerance in ppms. Depending on `what`, the data for identification (`what = "Ident"`), quantitation (`what = "Quant"`) or `"both"` is plotted.

plotRtDiffs

  `signature(object="Synapter", ...)`: Plots a histogram of retention time differences after alignments. `...` is passed to `hist`.

plotRt

  `signature(object="Synapter", what = "character", f = "numeric", nsd = "numeric")`: Plots the Identification - Quantitation retention time difference as a function of the Identification retention time. If `what` is `"data"`, two plots are generated: one ranging the full range of retention time differences and one focusing on the highest data point density and showing models with various `span` parameter values, as defined by `f` (default is 2/3, 1/2, 1/4, 1/10, 1/16, 1/25, 1/50, passed as a numed numeric). If `what` is `"model"`, a focused plot with the applied span parameter is plotted and areas of `nsd` (default is `x(1, 3, 5)` number of standard deviations are shaded around the model.

plotIntensity

  `signature(object="Synapter", what = "character", f = "numeric", nsd = "numeric")`: Plots the (log2) ratio of Identification and Quantitation intensities as a function of the Identification retention time. If `what` is `"data"`, two plots are generated: one ranging the full range of ratios and one focusing on the highest data point density and showing models with various `span` parameter values, as defined by `f` (default is 2/3, 1/2, 1/4, 1/10, 1/16, 1/25, 1/50, passed as a numed numeric). If `what` is `"model"`, a focused plot with the applied span parameter is plotted and areas of `nsd` (default is `x(1, 3, 5)` number of standard deviations are shaded around the model.

plotPepScores

  `signature(object="Synapter")`: Plots the distribution of random and regular peptide scores for identification and quantitation features. This reflects how peptide p-values are computed. See also `getPepNumbers`.

plotFdr

  `signature(object="Synapter", method = "character")`: Displays 2 plots per identification and quantitation peptides, showing the number of significant peptides as a function of the FDR cut-off and the expected false number of false positive as a number of significant tests. PepFrag 1 and 2 peptides are illustrated on the same figures. These figures are adapted from `plot.qvalue`. `method`, one of `"BH"`, `"Bonferroni"` or `"qval"`, defines what identification statistics to use.

plotEMRTtable

  `signature(object="Synapter")`: Plots the barchart of number or 0, 1, 2, ... assigned EMRTs (see `getEMRTtable`) .

plotGrid

  `signature(object="Synapter", what = "character"), maindim = "character"`: Plots a heatmap of the respective grid search results. This grid to be plotted is controlled by `what`: `"total"`, `"model"` or `"details"` are available. If ion mobility was used in the grid search you can use `maindim` to decided which dimensions should be shown. `maindim` could be one of `"im"` (default), `"rt"` and `"mz"`. If `maindim = "im"` a heatmap for each ion mobility threshold is drawn. For `maindim = "rt"` and `maindim` you get a heatmap for each retention time respective mass threshold.

plotFeatures

  `signature(object="Synapter", what = "character", xlim = "numeric", ylim = "numeric", ionmobiltiy = "logical")`: Plots the retention time against precursor mass space. If `what` is `"all"`, three (six if ion mobility is available and `ionmobility = TRUE` (default is `FALSE`); three additional plots with precursor mass against ion mobility) such plots are created side by side: for the identification peptides, the quantitation peptides and the quantitation Pep3D data. If `what` is `"some"`, a subset of the rt/mass space can be defined with `xlim` (default is `c(40, 60)`) and `ylim` (default is `c(1160, 1165)`) and identification peptide, quantitation peptides and EMRTs are presented on the same graph as grey dots, blue dots and red crosses respectively. In addition, rectangles based on the ppm and nsd defined tolerances (see `setPpmError` and `setNsdError`) are drawn and centered at the expected modelled retention time. This last figure allows to visualise the EMRT matching.

plotFragmentMatching

  `signature(object = "Synapter", key = "character", column = "character", verbose = "logical", ...)`: Plots two spectra and fragments against each other. Please see `plotFragmentMatching` for details.

plotFragmentMatchingPerformance

  `signature(object = "Synapter", showAllPeptides = FALSE)`: Creates two plots. The left panel shows the performance of filtering the unique matches of the merged peptides using a different number of common peaks. The right panel shows the performance of filtering the non unique matches of the merged peptides using different differences (delta) in common peaks/fragments. These differences (delta) are calculated between the match with the highest number of common peaks/fragments and the second highest one. Use `filterUniqueMatches` and `filterNonUniqueMatches` to filter the `MatchedEMRT` `data.frame` using one of these thresholds. This function returns a `list` with two named elements (`unqiue` and `nonunqiue` invisibly. These are the same data as return by `fragmentMatchingPerformance`.

  Use `showAllPeptides=TRUE` to add a line for all peptides (not just the merged onces) to both plots.

plotCumulativeNumberOfFragments

  `signature(object = "Synapter", what = c("fragments.ident", "spectra.quant"))`: Plots the cumulative number of the fragments/peaks vs their intensity (log10 scaled). Use the `what` argument to create this plot for the identification fragments (`what = "fragments.quant"`) or the the quantitation spectra (`what = "spectra.quant"`).

##### Exporters

writeMergedPeptides

  `signature(object="Synapter", file = "character", what = "character", ...)`: Exports the merged peptide data to a comma-separated `file` (default name is `"Res-MergedPeptides.csv"`).

writeMatchedEMRTs

  `signature(object="Synapter", file = "character", ...)`: As above, saving the matched EMRT table.

writeIdentPeptides

  `signature(object="Synapter", file = "character", ...)`: As above, exporting the identification peptide data.

writeQuantPeptides

  `signature(object="Synapter", file = "character", ...)`: A above, exporting the quantitation peptide data.

getIdentificationFragments

  `signature(object="Synapter")`: returns the identification fragments as `MSnExp`.

getQuantitationSpectra

  `signature(object="Synapter")`: returns the quantitation spectra as `MSnExp`.

##### Other

as(, "MSnSet")

  `signature(x = "Synapter")`: Coerce object from `Synapter` to `MSnSet` class.

validObject

  `signature(object = "Synapter")`: Test whether a given `Synapter` object is valid.

updateObject

  `signature(object = "Synapter")`: Updates an old `Synapter` object.

### Author(s)

Laurent Gatto lg390@cam.ac.uk

### References

Käll L, Storey JD, MacCoss MJ, Noble WS Posterior error probabilities and false discovery rates: two sides of the same coin. J Proteome Res. 2008a Jan; 7:(1)40-4

Bonferroni single-step adjusted p-values for strong control of the FWER.

Benjamini Y. and Hochberg Y. Controlling the false discovery rate: a practical and powerful approach to multiple testing. J. R. Statist. Soc. B., 1995, Vol. 57: 289-300.

Storey JD and Tibshirani R. Statistical significance for genome-wide experiments. Proceedings of the National Academy of Sciences, 2003, 100: 9440-9445.

Käll, Storey JD, MacCoss MJ, Noble WS Assigning significance to peptides identified by tandem mass spectrometry using decoy databases. J Proteome Res. 2008b Jan; 7:(1)29-34

Improving qualitative and quantitative performance for MSE-based label free proteomics, N.J. Bond, P.V. Shliaha, K.S. Lilley and L. Gatto, Journal of Proteome Research, 2013, in press.

The Effects of Travelling Wave Ion Mobility Separation on Data Independent Acquisition in Proteomics Studies, P.V. Shliaha, N.J. Bond, L. Gatto and K.S. Lilley, Journal of Proteome Research, 2013, in press.

Trypsin cleavage:

Glatter, Timo, et al. Large-scale quantitative assessment of different in-solution protein digestion protocols reveals superior cleavage efficiency of tandem Lys-C/trypsin proteolysis over trypsin digestion. Journal of proteome research 11.11 (2012): 5145-5156. http://dx.doi.org/10.1021/pr300273g

Rodriguez, Jesse, et al. Does trypsin cut before proline?. Journal of proteome research 7.01 (2007): 300-305. http://dx.doi.org/10.1021/pr0705035

Brownridge, Philip, and Robert J. Beynon. The importance of the digest: proteolysis and absolute quantification in proteomics. Methods 54.4 (2011): 351-360. http://dx.doi.org/10.1016/j.ymeth.2011.05.005

cleaver's rules are taken from: http://web.expasy.org/peptide_cutter/peptidecutter_enzymes.html#Tryps

### Examples

    library(synapter) ## always needed

    ## Not run:
    ## (1) Construction - to create your own data objects
    synapterTiny <- Synapter()

    ## End(Not run)

    ## let's use synapterTiny, shipped with the package
    synapterTinyData() ## loads/prepares the data
    synapterTiny ## show object

    ## (2) Filtering
    ## (2.1) Peptide scores and FDR

    ## visualise/explore peptide id scores
    plotPepScores(synapterTiny)
    getPepNumbers(synapterTiny)

    ## filter data
    filterUniqueDbPeptides(synapterTiny) ## keeps unique proteotypic peptides
    filterPeptideLength(synapterTiny, l = 7) ## default length is 7

    ## visualise before FDR filtering
    plotFdr(synapterTiny)

    setPepScoreFdr(synapterTiny, fdr = 0.01) ## optional
    filterQuantPepScore(synapterTiny, fdr = 0.01) ## specifying FDR
    filterIdentPepScore(synapterTiny) ## FDR not specified, using previously set value

    ## (2.2) Mass tolerance
    getPpmErrorQs(synapterTiny)
    plotPpmError(synapterTiny, what="Ident")
    plotPpmError(synapterTiny, what="Quant")

    setIdentPpmError(synapterTiny, ppm = 20) ## optional
    filterQuantPpmError(synapterTiny, ppm = 20)
    ## setQuantPpmError(synapterTiny, ppm = 20) ## set quant ppm threshold below
    filterIdentPpmError(synapterTiny, ppm=20)

    filterIdentProtFpr(synapterTiny, fpr = 0.01)
    filterQuantProtFpr(synapterTiny, fpr = 0.01)

    getPpmErrorQs(synapterTiny) ## to be compared with previous output

    ## (3) Merge peptide sequences
    mergePeptides(synapterTiny)

    ## (4) Retention time modelling
    plotRt(synapterTiny, what="data")
    setLowessSpan(synapterTiny, 0.05)
    modelRt(synapterTiny) ## the actual modelling
    getRtQs(synapterTiny)
    plotRtDiffs(synapterTiny)
    ## plotRtDiffs(synapterTiny, xlim=c(-1, 1), breaks=500) ## pass parameters to hist()
    plotRt(synapterTiny, what="model") ## using default nsd 1, 3, 5
    plotRt(synapterTiny, what="model", nsd=0.5) ## better focus on model

    plotFeatures(synapterTiny, what="all")
    setRtNsd(synapterTiny, 3)     ## RtNsd and PpmError are used for detailed plot
    setPpmError(synapterTiny, 10) ## if not set manually, default values are set automatically
    plotFeatures(synapterTiny, what="some", xlim=c(36,44), ylim=c(1161.4, 1161.7))
    ## best plotting to svg for zooming

    set.seed(1) ## only for reproducibility of this example

    ## (5) Grid search to optimise EMRT matching parameters
    searchGrid(synapterTiny,
               ppms = 7:10,  ## default values are 5, 7, ..., 20
               nsds = 1:3,   ## default values are 0.5, 1,  ..., 5
               subset = 0.2) ## default is 1
    ## alternatively, use 'n = 1000' to use exactly
    ## 1000 randomly selected features for the grid search
    getGrid(synapterTiny)  ## print the grid
    getGridDetails(synapterTiny)  ## grid details
    plotGrid(synapterTiny, what = "total")   ## plot the grid for total matching
    plotGrid(synapterTiny, what = "model")   ## plot the grid for matched modelled feature
    plotGrid(synapterTiny, what = "details") ## plot the detail grid
    getBestGridValue(synapterTiny)  ## return best grid values
    getBestGridParams(synapterTiny) ## return parameters corresponding to best values
    setBestGridParams(synapterTiny, what = "auto") ## sets RtNsd and PpmError according the grid results
    ## 'what' could also be "model", "total" or "details"
    ## setPpmError(synapterTiny, 12) ## to manually set values
    ## setRtNsd(synapterTiny, 2.5)

    ## (6) Matching ident peptides and quant EMRTs
    findEMRTs(synapterTiny)
    plotEMRTtable(synapterTiny)
    getEMRTtable(synapterTiny)
    performance(synapterTiny)
    performance2(synapterTiny)

    ## (7) Exporting data to csv spreadsheets
    writeMergedPeptides(synapterTiny)
    writeMergedPeptides(synapterTiny, file = "myresults.csv")
    writeMatchedEMRTs(synapterTiny)
    writeMatchedEMRTs(synapterTiny, file = "myresults2.csv")
    ## These will export the filter peptide data
    writeIdentPeptides(synapterTiny, file = "myIdentPeptides.csv")
    writeQuantPeptides(synapterTiny, file = "myQuantPeptides.csv")
    ## If used right after loading, the non-filted data will be exported

[Package synapter version 2.4.1 Index]