idAcylCerneg {LipidMS}

Acylceramides (AcylCer) annotation for ESI-

Description

AcylCer identification based on fragmentation patterns for LC-MS/MS DIA or DDA data acquired in negative mode.

Usage

idAcylCerneg(
  msobject,
  ppm_precursor = 5,
  ppm_products = 10,
  rttol = 3,
  rt,
  adducts = c("M-H", "M+CH3COO"),
  clfrags = c(),
  clrequired = c(),
  ftype = c(),
  chainfrags_sn1 = c("cbdiff-cer_M-H"),
  chainfrags_sn2 = c("sph_Mn-62.06001", "sph_M-H-H2O"),
  chainfrags_sn3 = c("fa_Mn-1.9918", "fa_Mn-19.0179"),
  intrules = c("cbdiff-cer_sn1/sph_sn2", "sph_sn2/fa_sn3"),
  rates = c("5/1", "2/1"),
  intrequired = c(T, T),
  coelCutoff = 0.8,
  dbs,
  verbose = TRUE
)

Arguments

msobject

an msobject returned by dataProcessing.

ppm_precursor

mass tolerance for precursor ions. By default, 5 ppm.

ppm_products

mass tolerance for product ions. By default, 10 ppm.

rttol

total rt window for coelution between precursor and product ions. By default, 3 seconds.

rt

rt range where the function will look for candidates. By default, it will search within all RT range in MS1.

adducts

expected adducts for AcylCer in ESI-. Adducts allowed can be modified in adductsTable (dbs argument).

clfrags

vector containing the expected fragments for a given lipid class. See checkClass for details.

clrequired

logical vector indicating if each class fragment is required or not. If any of them is required, at least one of them must be present within the coeluting fragments. See checkClass for details.

ftype

character vector indicating the type of fragments in clfrags. It can be: "F" (fragment), "NL" (neutral loss) or "BB" (building block). See checkClass for details.

chainfrags_sn1

character vector containing the fragmentation rules for the sphingoid base. See chainFrags for details.

chainfrags_sn2

character vector containing the fragmentation rules for the chain fragments in sn2 position. See chainFrags for details. If empty, it will be estimated based on the difference between precursors and sn1 chains.

chainfrags_sn3

character vector containing the fragmentation rules for the acyl chain. See chainFrags for details.

intrules

character vector specifying the fragments to compare. See checkIntensityRules.

rates

character vector with the expected ratesbetween fragments given as a string (e.g. "3/1"). See checkIntensityRules.

intrequired

logical vector indicating if any of the rules is required. If not, at least one must be verified to confirm the structure.

coelCutoff

coelution score threshold between parent and fragment ions. Only applied if rawData info is supplied. By default, 0.8.

dbs

list of data bases required for annotation. By default, dbs contains the required data frames based on the default fragmentation rules. If these rules are modified, dbs may need to be supplied. See createLipidDB and assignDB.

verbose

print information messages.

Details

idAcylCerneg function involves 5 steps. 1) FullMS-based identification of candidate AcylCer as M-H and M+CH3COO. 2) Search of AcylCer class fragments: no class fragments by default. 3) Search of specific fragments that inform about the acyl chain (Cer as M-H), the sphingoid base (neutral loss of 62.0600 of the Sph) and the FA chain (FA as M-H and M-H2O but with a N instead of an O, what results in a mass differences of 1.9918 and 19.0179 respectively). 4) Look for possible chains structure based on the combination of chain fragments. 5) Check intensity rules to confirm chains position. In this case, the fragment coming from the loss of the acyl chain must be at least 5 times more intense the fragment from the sphingoid base and this one, two times more intense than the FA chain from sn3.

Results data frame shows: ID, lipid class, CDB (total number of carbons and double bounds), FA composition (specific chains composition if it has been confirmed), mz, RT (in seconds), I (intensity, which comes directly from de input), Adducts, ppm (mz error), confidenceLevel (Subclass, FA level, where chains are known but not their positions, or FA position level) and Score (parent-fragment coelution score mean in DIA data or relative sum intensity in DDA of all fragments used for the identification).

Value

annotated msobject (list with several elements). The results element is a data frame that shows: ID, lipid class, CDB (total number of carbons and double bounds), FA composition (specific chains composition if it has been confirmed), mz, RT (in seconds), I (intensity), Adducts, ppm (mz error), confidenceLevel (Subclass, FA level, where chains are known but not their positions, or FA position level), peakID, and Score (parent-fragment coelution score mean in DIA data or relative sum intensity in DDA of all fragments used for the identification).

Note

This function has been writen based on fragmentation patterns observed for three different platforms (QTOF 6550 from Agilent, Synapt G2-Si from Waters and Q-exactive from Thermo), but it may need to be customized for other platforms or acquisition settings.

Author(s)

M Isabel Alcoriza-Balaguer <[e-mail]>

Examples

## Not run: 
msobject <- idAcylCerneg(msobject)

## End(Not run)