MsBackendMgf: MS data backend for MGF files

In rformassspectrometry/MsBackendMgf: Mass Spectrometry Data Backend for Mascot Generic Format (mgf) Files

MS data backend for MGF files

Description

The MsBackendMgf class supports import and export of MS/MS spectra data from/to files in Mascot Generic Format (mgf) files. After initial import, the full MS data is kept in memory. MsBackendMgf extends the Spectra::MsBackendDataFrame() backend directly and supports thus the Spectra::applyProcessing() function to make data manipulations persistent.

The MsBackendAnnotatedMgf class supports import of data from MGF files that provide, in addition to the m/z and intensity values, also additional annotations/metadata for each mass peak. For such MGF files it is expected that each line contains information from a single mass peak, separated by a white space (blank). The first two elements are expected to be the peak's m/z and intensity values, while each additional element is considered an annotation for this specific peak. See examples below for the format of a supported MGF file. The backendInitialize() method of MsBackendAnnotatedMgf does not support parameter nlines. Also, import of data can be considerably slower compared to the standard MsBackendMgf backend, because of the additionally required parsing of peak annotations. Peaks information in MGF files are not named, thus, additional peaks annotations are named using the standard naming convention for column named of data frames: the first peaks annotation is called "V1", the second (if available) "V2" and so on.

New objects are created with the MsBackendMgf() or MsBackendAnnotatedMgf() function. The backendInitialize() method has to be subsequently called to initialize the object and import the MS/MS data from (one or more) MGF files.

The MsBackendMgf backend provides an export method that allows to export the data from the Spectra object (parameter x) to a file in mgf format. See the package vignette for details and examples.

Default mappings from fields in the MGF file to spectra variable names are provided by the spectraVariableMapping() function. This function returns a named character vector were names are the spectra variable names and the values the respective field names in the MGF files. This named character vector is submitted to the import and export function with parameter mapping. It is also possible to pass own mappings (e.g. for special MGF dialects) with the mapping parameter.

Usage

## S4 method for signature 'MsBackendMgf'
backendInitialize(
  object,
  files,
  mapping = spectraVariableMapping(object),
  nlines = -1L,
  ...,
  BPPARAM = SerialParam()
)

MsBackendMgf()

## S4 method for signature 'MsBackendMgf'
spectraVariableMapping(object, format = c("mgf"))

## S4 method for signature 'MsBackendMgf'
export(
  object,
  x,
  file = tempfile(),
  mapping = spectraVariableMapping(object),
  exportTitle = TRUE,
  ...
)

## S4 method for signature 'MsBackendAnnotatedMgf'
backendInitialize(
  object,
  files,
  mapping = spectraVariableMapping(object),
  ...,
  BPPARAM = SerialParam()
)

MsBackendAnnotatedMgf()

Arguments

object	Instance of MsBackendMgf class.
files	character with the (full) file name(s) of the mgf file(s) from which MS/MS data should be imported.
mapping	for backendInitialize() and export: named character vector allowing to specify how fields from the MGF file should be renamed. Names are supposed to be the spectra variable name and values of the vector the field names in the MGF file. See output of spectraVariableMapping() for the expected format and examples below or description above for details.
nlines	for backendInitialize() of MsBackendMgf: integer(1) defining the number of lines that should be imported and processed from the MGF file(s). By default (nlines = -1L) the full file is imported and processed at once. If set to a positive integer, the data is imported and processed chunk-wise using readMgfSplit().
...	Currently ignored.
BPPARAM	Parameter object defining the parallel processing setup. If parallel processing is enabled (with BPPARAM different than SerialParam(), the default) and length of files is larger than one, import is performed in parallel on a per-file basis. If data is to be imported from a single file (i.e., length of files is one), parsing of the imported file is performed in parallel. See also BiocParallel::SerialParam() for information on available parallel processing setup options.
format	for spectraVariableMapping(): character(1) defining the format to be used. Currently only format = "mgf" is supported.
x	for export(): an instance of Spectra::Spectra() class with the data that should be exported.
file	character(1) with the (full) file name to which the data should be exported.
exportTitle	logical(1) whether the TITLE field should be included in the exported MGF file. If TRUE (the default) a spectraVariable called "TITLE" will be used, if no such variable is present either the spectraNames(object) will be used or, if they are empty, a title will be generated including the MS level, retention time and acquisition number of the spectrum.

Value

See description above.

Author(s)

Laurent Gatto, Corey Broeckling and Johannes Rainer

Examples

library(BiocParallel)
#' Getting the file names of all example MGF files from MsBackendMgf
fls <- dir(system.file("extdata", package = "MsBackendMgf"),
    full.names = TRUE, pattern = "^spectra(.*).mgf$")

## Create an MsBackendMgf backend and import data from test mgf files.
be <- backendInitialize(MsBackendMgf(), fls)
be

be$msLevel
be$intensity
be$mz

## The spectra variables that are available; note that not all of them
## have been imported from the MGF files.
spectraVariables(be)

## The variable "TITLE" represents the title of the spectrum defined in the
## MGF file
be$TITLE

## The default mapping of MGF fields to spectra variables is provided by
## the spectraVariableMapping function
spectraVariableMapping(MsBackendMgf())

## We can provide our own mapping e.g. to map the MGF field "TITLE" to a
## variable named "spectrumName":
map <- c(spectrumName = "TITLE", spectraVariableMapping(MsBackendMgf()))
map

## We can then pass this mapping with parameter `mapping` to the
## backendInitialize method:
be <- backendInitialize(MsBackendMgf(), fls, mapping = map)

## The title is now available as variable named spectrumName
be$spectrumName

## Next we create a Spectra object with this data
sps <- Spectra(be)

## We can use the 'MsBackendMgf' also to export spectra data in mgf format.
out_file <- tempfile()
export(sps, backend = MsBackendMgf(), file = out_file, map = map)

## The first 20 lines of the generated file:
readLines(out_file, n = 20)

## Next we add a new spectra variable to each spectrum
sps$spectrum_idx <- seq_along(sps)

## This new spectra variable will also be exported to the mgf file:
export(sps, backend = MsBackendMgf(), file = out_file, map = map)
readLines(out_file, n = 20)

####
## Annotated MGF

## An example of a supported annotated MGF file
fl <- system.file("extdata", "xfiora.mgf", package = "MsBackendMgf")

## Lines with peak data start with a numeric and information is
## separated by a whitespace. The first two elements are the peak's m/z
## and intensity while any additional information is considered as
## annotation. Information for each peak is provided in one line.
readLines(fl)

## Importing the data using an `MsBackendAnnotatedMgf`
ba <- backendInitialize(MsBackendAnnotatedMgf(), fl)
ba

## An additional peaks variable is available.
peaksVariables(ba)

ba$V1

## The length of such peaks variables is the same as the length of the
## m/z or intensity values, i.e. each peak has one value (with the value
## being `NA` if missing).
length(ba$V1[[1L]])
length(ba$mz[[1L]])

## Extracting the peaks data from a `Spectra` with a `MsBackendAnnotatedMgf`
s <- Spectra(ba)
pd <- peaksData(s, peaksVariables(ba))[[1L]]
head(pd)
class(pd)

rformassspectrometry/MsBackendMgf documentation built on June 12, 2025, 12:32 a.m.