Matrix-Assisted Laser Desorption/Ionization Post-Source Decay Fragmentation of the Cystine-Containing Amphibian Peptides with Novel Cysteine Tags

Long disulfide-containing peptides, brevinins 1E and 2Ec, from the skin secretion of the frog Rana ridibunda were reduced and alkylated with ten novel and three known derivatizing agents. Nine of novel reagents are maleimide derivatives. The peptides were also reduced with dithiothreitol directly on a matrix-assisted laser desorption/ionization (MALDI) target without alkylation. Modified samples were subjected to a MALDI–post-source decay study. Procedures, fragmentation patterns, fragment ion signal abundances and sequence coverage for two peptides modified with 13 tags (or on-plate reduced) are described. The fast on-plate procedure for reduction/alkylation was applied to a crude secretion from Rana ridibunda, providing intense signals of derivatized peptides. The corresponding ions may be used for the tandem mass spectrometry sequencing procedure.