Specifically, we observed a [b4+H2O]+ product ion when the C-terminus had a free carboxyl group (for Orc[Ala11]), and that diagnostic ion was missing when the C-terminus was methyl esterified (for Orc[1-11]-OMe). In contrast, the MS/MS spectra generated on our Q-TOF instrument were insensitive to the structural difference, and this approach could not be used for distinguishing the two peptide sequences. Because MS/MS spectra may not provide the specific,

diagnostic information needed to distinguish the peptide sequences, and because standards are not always available, other measures, such as running extraction solvent EPZ-6438 chemical structure controls with isotopically labeled solvents, may be needed to distinguish this extraction artifact. Protease-catalyzed reactions have been exploited by chemists to carry out a variety of transformations in nonaqueous solvents [2], including C-terminal peptide esterifications [3], [22], [33] and [34]. Most enzymes exploited for this purpose are serine or cysteine proteases, which form reactive acyl-enzyme intermediates that can be attacked Seliciclib purchase by a competing nucleophile, such as methanol. In considering mechanisms that may be responsible for the production of Orc[1-11]-OMe and SSEDMDRLGFG-OMe, we note that the longer precursors to these modified orcokinin family peptides

are not amidated at the C-terminus. Most bioactive neuropeptides are C-terminally amidated to prevent proteolytic degradation; therefore, the orcokinin peptides would be expected to be more susceptible to both Bacterial neuraminidase enzymatic degradation and enzyme-mediated methylation. Additionally, while other C-terminally truncated orcokinins (predominantly Orc[1-12] and Orc[1-11]), have been detected in our investigations

[10] and by other researchers [4], [6], [27] and [40], the C-terminal methylations detected for Orc[1-11]-OMe and SSEDMDRLGFG-OMe have only been associated with Gly11. This implies that there is something unique about this amino acid (G) or the amino acid sequence proximate to this location that, in some way, enhances selectivity toward methanolysis. Finally, the glycine-phenylalanine (GF) motif at positions 11 and 12 are highly conserved elements of crustacean orcokinin sequences, which also may signify that this motif is important to neuropeptide function or processing. Based on this information, we speculate that methanol could participate in either exo- or endopeptidase-mediated pathways leading to the production of Orc[1-11]-OMe, as well as SSEDMDRLGFG-OMe, from full-length orcokinin family peptides. An important element of this mechanism is the acidity of the solvent system, which can promote enzymatic methanolysis over hydrolysis [3]. One hypothesis, pathway A in Fig. 16, would involve C-terminal proteolysis of full-length orcokinin family peptides by an exopeptidase.