H-Atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates

Abstract

Using the pulse radiolysis and other radiation chemical techniques, the reaction kinetics of ˙CH₃ and ˙CH₂OH radicals with alanine anhydride, glycine anhydride and sarcosine anhydride were examined in aqueous solution. The second order rate constant of reaction of ˙CH₃ with alanine anhydride has been determined at 2 × 10⁵ dm³ mol⁻¹ s⁻¹. A very similar value (8 × 10⁴ dm³ mol⁻¹ s⁻¹) has been obtained by measuring the methane and ethane yields as a function of the alanine anhydride concentration at different dose rates and evaluation of these data by computer analysis. Similar experiments carried out with glycine anhydride and sarcosine anhydride yielded identical rate constants of 4 × 10⁴ dm³ mol⁻¹ s⁻¹ for these compounds. It was found that ˙CH₂OH radical does not react with alanine anhydride at an appreciable rate (k < 2 × 10² dm³ mol⁻¹ s⁻¹). The reactions of ˙CH₃, ˙CH₂OH, and ˙C(CH₃)₂OH with both cyclic anhydrides and acyclic glycine and alanine peptides were studied by means of theoretical calculations at the B3LYP/6-311+G(d,p) level of theory. Free energies in the gaseous phase were determined in the classical harmonic oscillator–rigid rotator model, and used to estimate rates of H-transfer reactions at the ^αC-site of the peptides. The effects of aqueous solution on the activation enthalpies were estimated by the SCIPCM procedure. Using the experimental A-factor for the methyl radical + D,L-alanine anhydride reaction, modified experimental Arrhenius A-factors were obtained for the reactions of the other species, and used to calculate solution-phase rate constants. The reactions are discussed in terms of the charge and spin polarisation in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory. Rate constants, calculated by transition state theory are in good agreement with the available experimental data.