Selective cleavage at internal lysine residues in protonated vs. metalated peptides

The proton, lithium, sodium, potassium and silver ion (Cat(+)) adducts of the peptides GGKAA and SIKVAV are formed by electrospray ionization and the unimolecular chemistry of these ions is examined by low-energy collisionally activated dissociation (CAD) in a quadrupole ion trap mass spectrometer. Comparison of the fragmentation characteristics of [GGKAA+ Cat](+) and [SIKVAV + Cat](+) to those of [YGGFL + Cat](+) reveals that an internal lysine residue enhances the cleavage of the amide bond in C-terminal position (i.e., Lys-Xxx). The amine group of the lysine side chain initiates this reaction, leading to b(n) sequence ions with lactam structures, as corroborated by multiple-stage tandem mass spectrometry (MSn) experiments. The selectivity for forming b-type ions at Lys-Xxx (vs. at other amide bonds) depends on Cat(+), increasing in the order H+ < K+ < Ag+ < Li+ approximate to Na+. Investigation of [PTHIKWGD + Cat](+) further shows that, when both His and Lys residues are present in internal positions, Lys-Xxx bonds are cleaved more selectively than His-Xxx bonds if Cat = Na (or Li), whereas both these bonds break with competitive rates if Cat = Ag. These trends are attributed to the different binding properties of silver vs. alkali metal ions. The enhanced dissociation of Lys-Xxx bonds to b-type fragments should be particularly useful in the sequence analysis of non-tryptic as well as incomplete tryptic digests which may contain internal lysine residues. (C) 2003 Elsevier Science B.V. All rights reserved.