Experimental and theoretical investigation of the main fragmentation pathways of protonated H-Gly-Gly-Sar-OH and H-Gly-Sar-Sar-OH

The fragmentation pathways of protonated H-Gly-Gly-Sar-OH and H-Gly-Sar-Sar-OH are investigated by using both computational and experimental techniques. The main goal of these studies is to further investigate which factors determine the branching ratio of the b(2)-y(1) (Paizs, B.; Suhai, S. Rapid Commun. Mass Spectrom. 2002, 16, 375.) and "diketopiperazine" (Cordero, M. M.; Houser, J. J.; Wesdemiotis, C. Anal. Chem. 1993, 65, 1594.) pathways of protonated tripeptides. Protonated H-Gly-Sar-Sar-OH represents a sensitive test for the branching ratio of the b(2)-y(1) and "diketopiperazine" pathways since this ion cannot produce y(1) ions on the b(2)-y(1) channel but only b(2) ions. Protonated H-Gly-Gly-Sar-OH and H-Gly-Sar-Sar-OH exhibit very different fragmentation behavior under the investigated experimental conditions. The former fragments forming mainly y(1) ions (maximum abundance of the b(2) and y(2) ions is similar to15%), while the latter produces mainly b2 ions while at larger internal energies the a(2), y(2), and y(1) ions become also moderately abundant. Theoretical modeling and analysis of the main fragmentation pathways indicate that the majority of the b(2) and y(1) ions of protonated H-Gly-Gly-Sar-OH and the b(2) ions of H-Gly-Sar-Sar-OH are formed on the b(2)-y(1) pathway. (C) 2003 American Society for Mass Spectrometry.