Thermal dependence of the hydrated proton and optimal proton transfer

TL;DR

This study investigates how temperature affects the structure and proton transfer mechanisms in hydrated protons using advanced quantum simulation methods, revealing a temperature range optimal for proton transfer relevant to biological processes.

Contribution

It combines state-of-the-art quantum Monte Carlo and path-integral molecular dynamics to elucidate the thermal behavior of the hydrated proton at a fundamental level.

Findings

Low thermal expansion of hydrogen bonds up to 300 K due to proton delocalization.

Weakening of hydrogen bonds above 300 K with localized configurations.

250-300 K identified as optimal for proton transfer phenomena.

Abstract

Water is a key ingredient for life and plays a central role as solvent in many biochemical reactions. However, the intrinsically quantum nature of the hydrogen nucleus, revealing itself in a large variety of physical manifestations, including proton transfer, gives rise to unexpected phenomena whose description is still elusive. Here we study, by an unprecedented combination of state-of-the-art quantum Monte Carlo methods and path-integral molecular dynamics, the structure and hydrogen-bond dynamics of the protonated water hexamer, the fundamental unit for the hydrated proton. We report a remarkably low thermal expansion of the hydrogen bond from zero temperature up to 300 K, owing to the presence of short-Zundel configurations, characterised by proton delocalisation and favoured by the synergy of nuclear quantum effects and thermal activation. The hydrogen bond strength progressively weakens above 300 K, when localised Eigen-like configurations become relevant. Our analysis, supported by the instanton statistics of shuttling protons, reveals that the near-room-temperature range from 250 K to 300 K is a ``sweet spot'' for proton transfer, and thus for many phenomena depending on it, including life.