Structure and reactions of the peroxy radicals of glycine and alanine in peptides: an ab initio study

Abstract Ab initio calculations at the B3LYP/6‐31G(d) and CBS‐RAD level were carried out to investigate the reaction of α C‐centered peroxy radicals of neutral, non‐zwitterionic glycine and alanine (AH), and as residues in model peptides, N ‐formylglycinamide and N ‐formylalaninamide (PH). Bond dissociation enthalpies (BDEs) were calculated for the α C—O ( D   CO α ), O—O ( D OO ) and O—H ( D OH ) bonds of glycine and alanine peroxy radicals (AOO, POO) and hydroperoxides (AOOH, POOH). The predicted BDEs at 298 K, in kJ mol −1 are AOO(Gly), D   CO α  = 70; AOO(Ala), D   CO α  = 69; POO(Gly), D   CO α  = 89; POO(Ala), D   CO α  = 86; AOOH(Gly), D   CO α  = 237, D OO  = 203, D OH  = 371; AOOH(Ala), D   CO α  = 234, D OO  = 195, D OH  = 368; POOH(Gly), D   CO α  = 266, D OO  = 207, D OH  = 380; POOH(Ala), D   CO α  = 264, D OO  = 208, D OH =383. Values of BDE of the peptides in β‐sheet peptide conformations were also estimated by constraining the Ramachandran dihedral angles, Φ and Ψ to, values of −150° and +150°: ( S )‐POO(Gly), D   CO α  = 99; ( R )‐POO(Gly), D   CO α  = 78; ( S )‐POO(Ala), D   CO α  = 88; ( R )‐POO(Ala), D   CO α  = 83; ( S )‐POOH(Gly), D   CO α  = 258, D OO  = 195, D OH  = 362; ( R )‐POOH(Gly), D   CO α  = 278, D OO  = 217, D OH  = 404; ( S )‐ POOH(Ala), D   CO α  = 240, D OO  = 192, D OH  = 355; ( R )‐POOH(Ala), D   CO α  = 270, D OO  = 204, D OH  = 390. The reactions of α C‐peroxy radicals, POO, and α C‐alkoxy radicals, PO, were studied in detail. Copyright © 2004 John Wiley & Sons, Ltd.