Understanding Proton Transfer in Non-aqueous Biopolymers based on
Helical Peptides: A Quantum Mechanical Study
Abstract
Histidine (an imidazole-based amino acid) is a promising building block
for short aromatic peptides containing a proton donor/acceptor moiety.
Previous studies have shown that polyalanine helical peptides
substituted at regular intervals with histidine residues exhibit both
structural stability as well as high proton affinity and high
conductivity. Here, we present first-principle calculations of
non-aqueous histidine-containing 310-, and -helices and show that
they are able to form hydrogen-bonded networks mimicking proton wires
that have the ability to shuttle protons via the Grotthuss shuttling
mechanism. The formation of these wires enhances the stability of the
helices, and our structural characterizations confirm that the secondary
structures are conserved despite distortions of the backbones. In all
cases, the helices exhibit high proton affinity and proton transfer
barriers on the order of 1~4 kcal/mol. Zero-point energy
calculations suggest that for these systems, ground state vibrational
energy can provide enough energy to cross the proton transport energy
barrier. Additionally, ab initio molecular dynamics results suggests
that the protons are transported unidirectionally through the wire at a
rate of approximately 2 Å every 20 fs. These results demonstrate that
efficient deprotonation-controlled proton wires can be formed using
non-aqueous histidine-containing helical peptides.