Quantum Chemical Simulations of the Proton Transfer in Water Wires Attached to Molecular Walls

Abstract

Quantum chemical methods are applied to study the stages of the proton transfer in water wires attached to molecular walls containing the side chains of His and Asp residues. Several molecular models of variable complexity are considered by using different techniques, and in every case, the most important reactions of cleavage and formation of chemical bonds in proton wires are treated at the ab initio level. The largest molecular model, which mimics structural elements of the M2 ion channel, is investigated with the help of a quantum mechanical−molecular mechanical (QM/MM) method with flexible effective fragments. Smaller models are considered at nonempirical levels in order to purify conclusions of the QM/MM approach. The results of calculations show that the only transition state structure found on the proton-transfer route refers to the stage of proton detachment from the Nε atom of the imidazole ring to the neighboring water molecule. The corresponding energy barriers are estimated.