Ab initio study of the energetics of protonation, deprotonation and homocomplexed cations and anions formation in systems modeling side chains of biomolecules

Abstract

Energy calculations of acid–base interactions in systems modeling side chains of polypeptides and proteins have been carried out using ab initio methods based on the restricted Hartree–Fock (RHF) and Møller–Plesset (MP)calculations in the 6-31G* and 6-31+G** basis sets in the gaseous phase. Both the protonation and deprotonation energies of basic and acidic groups, as well as the formation energies of acid–base systems stabilized by symmetric N–H–N and O–H–O type hydrogen bonds were estimated. The effect of a solvent on the acid–base processes was accounted employing a polarizable continuum model (PCM) in the calculations. Results of the calculations suggested that among the model systems studied, the following pairs formed the most stable systems: 1-methylguanidine (modeling the guanidine group of arginine) and protonated 1-methylguanidine in the class of cationic complexes and phenol (modeling the phenolic group of tyrosine) and phenolate ion in the class of anionic complexes. Furthermore, it has been found that the calculation at the RHF level and in PCM model energies, Δ E homo, and Gibbs free energies, Δ G homo, of the homocomplexed cations formation correlate well with respective calculated energies, Δ E prot, and Gibbs free energies, Δ G prot, of protonation of the amines studied.