SELECTIVE DETERMINATION OF PHOSPHOPEPTIDE BETA-CN(1-25) IN A BETA-CASEIN DIGEST BY ADDING IRON - CHARACTERIZATION BY LIQUID-CHROMATOGRAPHY WITH ONLINE ELECTROSPRAY-IONIZATION MASS-SPECTROMETRIC DETECTION

A beta-casein tryptic digest has been analysed by reversed-phase high- performance liquid chromatography (RP-HPLC) with on-line electrospray- ionization mass spectrometry (ESI-MS). Analyses of peptides were carri ed out before and after addition of iron(II) to the peptides in soluti on. In both cases, the majority of peptides were identified by the det ermination of molecular masses by ESI-MS and by prior knowledge of the amino acid sequence of beta-casein, and thus of its corresponding try ptic peptides. In the presence of iron(II), only phosphopeptide beta-C N(1-25) was able to bind iron to form different complexes that have in creased retention times on the RP-HPLC column and that also absorbed a t 280 nm. The method presented here appears to be selective for peptid es containing phosphoseryl cluster(s).