Peptide cation‐radicals. A computational study of the competition between peptide N-Cα bond cleavage and loss of the side chain in the [GlyPhe‐NH2 + 2H]+• cation‐radical

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Abstract
Cation‐radicals and dications corresponding to hydrogen atom adducts to N‐terminus‐protonated Nα‐glycylphenylalanine amide (Gly‐Phe‐NH2) are studied by combined density functional theory and Møller‐Plesset perturbational computations (B3‐MP2) as models for electron‐capture dissociation of peptide bonds and elimination of side‐chain groups in gas‐phase peptide ions. Several structures are identified as local energy minima including isomeric aminoketyl cation‐radicals, and hydrogen‐bonded ion‐radicals, and ylid‐cation‐radical complexes. The hydrogen‐bonded complexes are substantially more stable than the classical aminoketyl structures. Dissociations of the peptide NCα bonds in aminoketyl cation‐radicals are 18–47 kJ mol−1 exothermic and require low activation energies to produce ion‐radical complexes as stable intermediates. Loss of the side‐chain benzyl group is calculated to be 44 kJ mol−1 endothermic and requires 68 kJ mol−1 activation energy. Rice‐Ramsperger‐Kassel‐Marcus (RRKM) and transition‐state theory (TST) calculations of unimolecular rate constants predict fast preferential NCα bond cleavage resulting in isomerization to ion‐molecule complexes, while dissociation of the CαCH2C6H5 bond is much slower. Because of the very low activation energies, the peptide bond dissociations are predicted to be fast in peptide cation‐radicals that have thermal (298 K) energies and thus behave ergodically. Copyright © 2003 John Wiley & Sons, Ltd.

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