Theoretical Study of the Reactivity and Selectivity of Various Free Radicals with Cysteine Residues

Radicals in biochemical environments can lead to protein damage. Theoretical studies can help us to understand the observed radical selectivity. In this work, the kinetics and thermodynamics of the hydrogen-transfer (HT) and single-electron transfer (SET) reactions between a cysteine derivative and 17 free radicals of biological significance have been theoretically investigated in aqueous and lipid media. With the exception of the reaction with ^•OCCl₃, all SET reactions in aqueous medium have rate constants in the diffusion-limited regime. The γ site of cysteine was found to be the most reactive for the HT reactions with all the radicals, with rate constants in the diffusion limit for ^•OH, ^•OCHCl₂, and ^•OCCl₃. The HT reactions from the α and γ positions have very similar ΔG° values and even though the β position is the least thermodynamically favored, when the HT from β is exergonic it is a more reactive site than α. The results obtained confirm that the Bell-Evans-Polanyi principle does not apply to the reactions between amino acid residues and free radicals and that reactivity comparisons demand proper kinetic calculations.