protViz (version 0.2.06)

PTM_MarkerFinder: PTM MarkerFinder

Description

PTM_MarkerFinder is a fucntion to identify and validate spectra from peptides carrying post-translational modifications.

Usage

PTM_MarkerFinder(data,
    modification,
    modificationName,
    mZmarkerIons,
    minNumberIons=2,
    itol_ppm=10,
    minMarkerIntensityRatio=5,
    mgfFilename=-1,
    PEAKPLOT=TRUE
    )

Arguments

data

A list of spectra where each list element contains a list of mZ, intensity vectors. This object can be received by using the mascotDat2RData.pl perl script.

modification

A double vector containing the mono mass PTMs.

modificationName

A character vector containing the Name of the PTMs. This is the string which will show up in the peptide sequence in square brackets.

mZmarkerIons

The m/z patterns which should be searched.

minNumberIons

Minimal number of marker ions to be found for further analysis.

itol_ppm

The ion tolerance of the marker ions in ppm. default is set to 10ppm.

minMarkerIntensityRatio

The marker ions intencity percentage compared to the sum of all peak intensities.

mgfFilename

if a mgf(mascot generic file) filename is given, the function writes an mgf file containing only the ms2 having the mZmarkerIons.

PEAKPLOT

If this boolean is FALSE the peakplot function is not used.

Details

The functon screens MS2 spectra for the presence of fragment ions specific Post Translational Modifications (PTMs). The function requires an R-object containing the mass spectrometric measurement, the peptide assignments, potential modification information, and a list of marker ions. The R-object can be retrieved right out of the Mascot Server search result dat files using a perl script mascotDat2RData.pl which is include as protViz.

The functions iterates over each spectrum of the mass spectrometric measurement and searches for the as input provided marker ions. If a certain number of marker ions (default is, that two marker ions are required) are found and the maker ion intensity ratio is higher than a given threshold, the tandem mass spectrum is considered as HCD scan type and the corresponding ion series are drawn using the protViz:peakplot methode. Furthermore the function is searching for the corresponding ETD scan having the same peptide mass by screening the succeeding scans for ETD spectra. If such a spectrum is found the peptide spectrum assignment containing the c, z, and y ions is drawn. Note that the PTM MarkerFinder expects the ETD scan right after the HCD scan. If the MS protocol changes the PTM MarkerFinder methode has to be adapted.

For each HCD scan PTM MarkerFinder plots both HCD and ETD scans, a ppm error versus maker ion m/z scatter plot, a intensity versus marker ion m/z plot, and two pie charts where the relative and absolute maker ion intensity are shown.

As a summery report the function returns a table containing the following column attributes: "scans", "mZ", "markerIonMZ", "markerIonIntensity", "markerIonMzError", "markerIonPpmError", and "query" which can be used for statistics.

Furthermore, if mgfFilename is defined, a Mascot Generic File (MGF) is created containig the HCD scans (having the marker ions) and the corresponding ETD scans.

Examples

# some marker ions
    Glykan_MarkerIons <- c(109.02841, 127.03897, 145.04954, 163.06010, 325.11292)
    HexNAc_MarkerIons<-c(126.05495, 138.05495, 144.06552, 168.06552, 186.07608, 204.08665)

    data(HexNAc)

    # prepare modification
    ptm.0<-cbind(AA="-",
        mono=0.0, avg=0.0, desc="unmodified", unimodAccID=NA)
      
    ptm.1<-cbind(AA='N', 
          mono=317.122300, avg=NA, desc="HexNAc",
                  unimodAccID=2)
        
    ptm.2<-cbind(AA='M',
            mono=147.035400, avg=NA, desc="Oxidation",
                    unimodAccID=1)

    m<-as.data.frame(rbind(ptm.0, ptm.1, ptm.2))

    s <- PTM_MarkerFinder(data=HexNAc, modification=m$mono, 
        modificationName=m$desc,
        minMarkerIntensityRatio=3,
        itol_ppm=20,
        mZmarkerIons=HexNAc_MarkerIons) 

    # some statistics
    boxplot(markerIonPpmError~as.factor(scans),
        data=s,
        main='Overview plot: boxplot of marker ion errors (ppm) for each pPTM spectrum',
        xlab='scan number', ylab='ppm error')
    abline(h=0.0,col='grey')

    boxplot(s$markerIonIntensity ~ s$markerIonMZ,
        log='y',
        main='Summary plot: boxplot of marker ion intensities from all pPTM spectra',
        xlab='markerIon m/z', 
        ylab='log10 based marker ion intensity')


    # export
    w<-reshape(s[,c(1,7,3,4)], direction='wide', 
        timevar="markerIonMZ", idvar=c('scans','query'))

    write.table(w, file="HexNAc_PTM_markerFinder.csv",
        sep=',', row.names=FALSE,col.names=TRUE, quote=FALSE)