Dissociation of [b5 – H][rad]+ ions composed of one α-methyltryptophan and four alanine residues: The effect of the α-methyl group

Abstract

Collision-induced dissociation (CID) of hexapeptide radical cations consisting of four alanine residues, one α-methyltryptophan and a glycine residue at the C-terminus results predominantly in bond cleavages around the sterically crowded α-methyltryptophan residue, but all generated [b5 – H]+ ions in moderate abundance. Unlike in the dissociations of the [b5 – H]+ ions of the analogous [A4Woxa]+ ions, where regardless of the initial location of the unmodified tryptophan residue the CID spectra were identical, the α-methyltryptophan-containing [b5 – H]+ ions all gave different CID spectra, indicating that there was no evidence of isomerization. Losses of imine-amides and the neutral of mass 141 Da provide evidence that the radical centre is located at the α-carbon of the N-terminal residue. This N-terminal captodative structure inhibits macrocyclization, thereby preventing isomerization and leading to different dissociation products. Density functional theory (DFT) calculations on [AAAWα-MeAoxa]+, the ion that gave the largest variety of dissociation products, established that the captodative structure at the N-terminus is at the global minimum, 27 kJ mol−1 lower in energy than the π-radical cation. Calculations also showed that losses of the dialanine radical from the captodative structure and the side chain of α-methyltryptophan from the π-radical are the two lowest-energy dissociation pathways, consistent with the onsets observed in the experimental energy-resolved CID curves. The overall conclusion is that [b5 – H]+ ions that contain an unmodified tryptophan have the radical centre located on the α-carbon of the tryptophan residue, and this leaves the N-terminal amino group free to initiate macrocyclization leading to isomerization prior to dissociation. When the α-carbon is blocked by a methyl group, as in α-methyltryptophan, then the captodative structure at the N-terminal residue is favoured and this prevents macrocyclization and isomerization.