Radicals formed by the reaction of electrons with amino acids and peptides in a neutral aqueous glass

The reactions of electrons with a number of amino acids, N-acetylamino acids, N-acetylpeptides, and peptides in a neutral aqueous glass (12 M LiCl-D₂O) have been investigated by electron spin resonance spectroscopy. Electron attachment to the amino acids, glycine and alanine, at 77°K results in immediate deamination. For amino acids with more reactive side groups, e.g., glutamic acid, anion radicals are found with the electron localized on the side group. For all peptides we find immediate N-terminal deamination upon electron attachment at 77°K. Where glycine is at the N terminal, upon warming to the softening point of the glass, deamination is followed by abstraction from an α-carbon position. Stable anions are found for N-acetylamino acids and N-acetylpeptides after electron attachment. These anions undergo deamination of a secondary amine group upon warming to 165°K. Further warming results in hydrogen abstraction to form an α-carbon radical. These results are compared to those found in an alkaline glass (8 M NaOD). It is found that changes in protonation at nitrogen sites greatly affect stability and hyperfine splittings of radicals produced after electron attachment. McLachlan spin density calculations confirm the experimental result that deprotonation at the nitrogen in peptide α-carbon radicals can significantly alter the α-carbon proton splitting. Finally, the results found in this study are discussed in terms of other recent work utilizing pulse radiolysis, product analysis, and esr. Good agreement is found between the various techniques.