Presolvated Low Energy Electron Attachment to Peptide Methyl Esters in Aqueous Solution: C-O Bond Cleavage at 77 K

In this study, the reactions of presolvated electrons with glycine methyl ester and N-acetylalanylalanine methyl ester (N-aAAMe) are investigated by electron spin resonance (ESR) spectroscopy and DFT calculations. Electrons were produced by gamma-irradiation in neutral 7.5 M LiCl-D2O aqueous glasses at low temperatures. For glycine methyl ester, electron. addition at 77 K results in both N-terminal deamination to form a glycyl radical and C-O ester bond cleavage to form methyl radicals. For samples of N-acetylalanylalanine methyl ester, electrons are found to add to the peptide bonds at 77 K and cleave the carboxyl ester groups to produce methyl radicals. On annealing to 160 K, electron adducts at the peptide links undergo chain scission to produce alanyl radicals and on further annealing to 170 K alpha-carbon peptide backbone radicals are produced by hydrogen abstraction. DFT calculations for electron addition to the methyl ester portion of N-aAAMe show the cleavage reaction is highly favorable (free energy equals to -30.7 kcal/mol) with the kinetic barrier of only 9.9 kcal/mol. A substantial electron affinity of the ester link (38.0 kcal/mol) provides more than sufficient energy to overcome this small barrier. Protonated peptide bond electron adducts also show favorable N-C chain cleavage reactions of -12.7 to -15.5 kcal/mol with a barrier from 7.4 to 10.0 kcal/mol. The substantial adiabatic electron affinity (AEA) of the peptide bond and ester groups provides sufficient energy for the bond dissociation.