ICP-MS-assisted nanoHPLC-electrospray Q/time-of-flight MS/MS selenopeptide mapping in Brazil nuts

A method was developed for the identification of selenium-containing peptides issued from proteins of a Se-rich (82.9 mg kg-1) foodstuff, Brazil nut (Bertholletia excelsa). A sample purification procedure was optimized to cope with the 100-fold excess of sulfur analogues and matrix interferences. It was based on the consecutive size-exclusion fractionation of proteins and tryptic peptides, and enrichment of the Se-containing fractions, prior to nanoHPLC-ES-Q/TOF MS/MS. The characteristic isotopic patterns of selenium compounds (always minor peaks) were detected in ESI mass spectra at retention times precisely indicated by the matrix interference-free, sensitive (DL 1.3 fmol) 80Se detection by ICP collision cell MS in the same separation conditions. The potential of the method was demonstrated by the identification of 15 Se-containing peptides, from which all but one were found to originate from the selenised isoforms of the 2S protein.