Quantum chemical studies of proton transport through biomembranes

A proposed mechanism of proton transport through biological membranes was investigated with the aid of ab initio molecular orbital methods. It requires the existence within a transmembrane protein of a hydrogen-bonded chain of residues. The full transport process was broken down into a number of simpler steps, each of which involves the transfer of a proton from one residue to the next along the chain. The hydroxyl-containing residues were modeled by water molecules. Linear hydrogen-bonded systems H+ (H2O)n, n = 2, 3, 4, 5 were studied using the 4-31G basis set. The dimer was shown to furnish an excellent model for study of proton transfer in larger systems. Deformations of the hydrogen bond lead to large increases in the energy barrier to proton transfer. Certain properties of the electron density distribution show a close correspondence with transfer energetics; consequently, they may have some predictive use. The transition state to double proton transfer in the trimer and pentamer involve partial transfer of only one proton. Implications of the above observations upon the proposed mechanism were discussed.