Carbonyl charge solvation patterns may relate to fragmentation classes in collision-activated dissociation

N-Protonated Isomers and Coulombic Barriers to Dissociation of Doubly Protonated Ala8Arg

Collision-induced dissociation (or tandem mass spectrometry, MS/MS) of a protonated peptide results in a spectrum of fragment ions that is useful for inferring amino acid sequence. This is now commonplace and a foundation of proteomics. The underlying chemical and physical processes are believed to be those familiar from physical organic chemistry and chemical kinetics. However, first-principles predictions remain intractable because of the conflicting necessities for high accuracy (to achieve qualitatively correct kinetics) and computational speed (to compensate for the high cost of reliable calculations on such large molecules). To make progress, shortcuts are needed. Inspired by the popular mobile proton model, we have previously proposed a simplified theoretical model in which the gas-phase fragmentation pattern of protonated peptides reflects the relative stabilities of N-protonated isomers, thus avoiding the need for transition-state information. For singly protonated Ala _n (n = 3-11), the resulting predictions were in qualitative agreement with the results from low-energy MS/MS experiments. Here, the comparison is extended to a model tryptic peptide, doubly protonated Ala₈Arg. This is of interest because doubly protonated tryptic peptides are the most important in proteomics. In comparison with experimental results, our model seriously overpredicts the degree of backbone fragmentation at N₉. We offer an improved model that corrects this deficiency. The principal change is to include Coulombic barriers, which hinder the separation of the product cations from each other. Coulombic barriers may be equally important in MS/MS of all multiply charged peptide ions. Graphical Abstract ᅟ.

N-terminal peptide sequence repetition influences the kinetics of backbone fragmentation: a manifestation of the Jahn-Teller effect?

Analysis of large (>10,000 entries) databases consisting of high-resolution tandem mass spectra of peptide dications revealed with high statistical significance (P < 1[Symbol: see text]10(-3)) that peptides with non-identical first two N-terminal amino acids undergo cleavages of the second peptide bond at higher rates than repetitive sequences composed of the same amino acids (i.e., in general AB- and BA- bonds cleave more often than AA- and BB- bonds). This effect seems to depend upon the collisional energy, being stronger at lower energies. The phenomenon is likely to indicate the presence of the diketopiperazine structure for at least some b2 (+) ions. When consisting of two identical amino acids, these species should form through intermediates that have a symmetric geometry and, thus, must be subject to the Jahn-Teller effect that reduces the stability of such systems.