Theoretical examination of competitive β-radical-induced cleavages of N–C_α and C_α–C bonds of peptides

Abstract

Selective cleavages of N-C ␣ and C ␣ -C bonds of ␤-radical tautomers of amino acid residues in radical peptides have been examined theoretically by means of the density functional theory at the M06-2X/6-311++G(d,p) level. The majority of the bond cleavages are homolytic via ␤-scission. Their energy barriers depend largely on the ability of the radical being stabilized in the transition structures and the availability of a mobile proton in the vicinity of the ␤-radical center. The N-C ␣ bond is less favorably cleaved than the C ␣ -C bond (except Ser and Thr) for systems without a mobile proton. It is because, firstly, the homolytic cleavage is less favorable for the more polar N-C ␣ bond than for the less polar C ␣ -C bond. Secondly, a less stable -radical localized on the amide nitrogen atom of the incipient N-terminal fragment is formed for the former, while a more stable radical delocalized in a *(CO)-like orbital of the incipient C-terminal fragment is formed for the latter. In the presence of a mobile proton N-terminal to the ␤-radical center, some degrees of heterolytic cleavage character, as preferred by the polar N-C ␣ bond, are observed. Consequently, its barrier is reduced. If the mobile proton is located at the C-terminal amide oxygen of the ␤-radical center, the C ␣ -C bond cleavage will be significantly suppressed. It is because the radical in the incipient C-terminal fragment becomes more localized as a -radical on the carbon atom of its protonated amide group. With basic amino acid residues, the C ␣ -C bond cleavage can be reactivated. Heterolytic cleavage of the polar N-C ␣ bond can be largely facilitated if a mobile proton N-terminal to the ␤-radical center is available and the radical in the incipient C-terminal fragment is sufficiently stabilized, for instance, by the aromatic side chain of Trp and Tyr. Therefore, cleavages of the N-C ␣ bond induced by the ␤-radical tautomer of Trp and Tyr are often preferred as compared with cleavages of the C ␣ -C bond in peptide radical cations containing mobile protons.Résumé : Nous avons modélisé les clivages sélectifs des liaisons N-C ␣ et C ␣ -C des tautomères ␤-radicalaires de résidus d'acides aminés dans des peptides radicalaires au niveau M06-2X/6-311++G(d,p) de la théorie de la fonctionnelle de la densité. La majorité des clivages de liaison sont homolytiques et se produisent par scission ␤. Leurs barrières énergétiques dépendent fortement de l'aptitude du radical à être stabilisé dans les structures de transition et de la présence d'un proton labile à proximité du centre ␤-radicalaire. Le clivage de liaison N-C ␣ est moins favorable que celui de la liaison C ␣ -C (sauf dans les résidus Ser et Thr) dans les systèmes sans proton labile. Ce phénomène s'explique premièrement par le fait que le clivage homolytique des liaisons N-C ␣ plus polaires est moins favorable que celui des liaisons C ␣ -C moins polaires. Deuxièmement, un radical moins stable, localisé sur l'atome d'azote de l'amide du fragment N-terminal naissant, se forme dans le premier cas, alor...

Formation of n → π+ interaction facilitating dissociative electron transfer in isolated tyrosine-containing molecular peptide radical cations

“…An another high-lying structure is the b-radical FY b GG-1 (41 kJ mol À1 ), 52 in which a proton from the b-carbon of Tyr has been transferred to the N-terminal amide oxygen and the delocalization of the unpaired electron of the phenol side chain is extended to its b-carbon. Transferring the proton of the a-carbon of the N-terminal Phe to its amide oxygen can give F a YGG-1, known as the a-radical with the spin density delocalized among the N-terminal NH 2 , the a-carbon of Phe and the N-terminal amide group.…”

Tautomerization and Dissociation of Molecular Peptide Radical Cations

“…Consequently, the β‐radical–driven, rather than the charge‐driven, pathway occurred selectively upon the CID of the [RG i –2 FG 7– i ] •+ species. A DFT analysis of small peptide models suggested that the unpaired electron of the C‐terminal [x m ] • fragment yielded from the β‐radical–induced C α –C bond cleavage is located in a π*(CO)‐like orbital, which can be destabilized by a proton on the peptide backbone. This theoretical prediction is consistent with experimental observations of the [a n ] + ions (from C α –C bond cleavage), instead of the [c n + 2H] + ions (from N–C α bond cleavage) being produced from the CID of [RG i –2 FG 7– i ] •+ species.…”