The stability of C_α peptide radicals: Why glycyl radical enzymes?

摘要

The conformational space of dipeptide models derived from glycine, alanine, phenylalanine, proline, tyrosine, and cysteine has been searched extensively and compared with the corresponding C_α dipeptide radicals at the G3(MP2)-RAD level of theory. The results indicate that the (least-substituted) glycine dipeptide radical is the thermochemically most stable of these species. Analysis of the structural parameters indicates that this is due to repulsive interactions between the C_α substituents and peptide units in the radical. A comparison of the conformational preferences of dipeptide radicals and their closed-shell parents also indicates that radical stability is a strongly conformation-dependent property.