Advantages of derivatization by osmium tetroxide and 2,2'-bipyridine for matrix-assisted laser desorption/ionization post-source decay fragment ion analysis of peptides.

Matrix-assisted laser desorption/ionization--post-source decay (MALDI-PSD) fragment ion analysis is frequently used for peptide sequence determination. PSD fragmentation is often changed or improved in terms of, e.g., sequence coverage, after derivatization. In this work, the influence of modification by an osmium tetroxide-bipyridine reagent (Os,bipy) on the MALDI-PSD behaviour of peptides is studied. The reagent modifies peptides specifically at tryptophan residues and oxidizes methionine to methionine sulfone and cysteine to cysteic acid. As a result the masses of some of the fragments are specifically shifted in case of peptides containing a methionine by +32 Da and, in cases of peptides containing a cysteine residue, by +48 Da. In addition, due to the change in protonation properties of a peptide after oxidation, fragments containing cysteic acid are in most cases totally suppressed. This effect significantly facilitates peptide sequence determination. Improvement of MALDI-TOFMS and PSD analysis after the reaction with Os,bipy is demonstrated for examples involving derivatives of humanin, a novel neuroprotective peptide.