Theoretical and experimental studies of tyrosyl hydroperoxide formation in the presence of H-bond donors

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.