Oxidative damage to biomolecules such as lipids, proteins, nucleotides, and sugars has been implicated in the pathogenesis of various diseases. Superoxide radical anion (O2•-) addition to nitrones bearing an amide N-H has been shown to be more favored as compared to other nitrones [Villamena, F. A., (2007) J. Am. Chem. Soc. 129, 8177-8191]. It has also been demonstrated by others [Winterbourn, C. C., (2004) Biochem. J. 381, 241-248] that O•2- addition to tyrosine to form hydroperoxide is favored in the presence of basic amino groups, but the mechanism for this observation remains obscure. We, therefore, hypothesized that the α-effect resulting from the interaction of O2•- with N-H can play a crucial role in the enhancement of hydroperoxide formation. Understanding this phenomenon is important in the elucidation of mechanisms leading to oxidative stress in cellular systems. Computational (at the PCM/B3LYP/6-31+G **//B3LYP/6-31G level of theory) as well as experimental studies were carried out to shed insights into the effect of amide or amino N-H on the enhancement (or stabilization) of hydroperoxide formation in tyrosine. H-bond interaction of amino acid group with O2•- results in the perturbation of the spin and charge densities of O2 •-. A similar phenomenon has been predicted for non-amino acids bearing H-bond donor groups. Using the FOX assay, tyrosyl hydroperoxide formation was enhanced in the presence of H-bond donors from amino acids and non-amino acids. The role of H-bonding in either stabilizing the hydroperoxide adduct or facilitating O2•- addition via an α-effect was further theoretically investigated, and results show that the latter mechanism is more thermodynamically preferred. This study provides new mechanistic insights in the initiation of oxidative modification to tyrosyl radical.
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