Kinetic pathways of water exchange in the first hydration shell of magnesium.

Nadine Schwierz

doi:10.1063/1.5144258

Abstract

Water exchange between the coordination shells of metal cations in aqueous solutions is fundamental in understanding their role in biochemical processes. Despite the importance, the microscopic mechanism of water exchange in the first hydration shell of Mg2+ has not been resolved since the exchange dynamics is out of reach for conventional all-atom simulations. To overcome this challenge, transition path sampling is applied to resolve the kinetic pathways, to characterize the reaction mechanism and to provide an accurate estimate of the exchange rate. The results reveal that water exchange involves the concerted motion of two exchanging water molecules and the collective rearrangement of all water molecules in the first hydration shell. Using a recently developed atomistic model for Mg2+, water molecules remain in the first hydration shell for about 40 ms, a time considerably longer compared to the 0.1 ms predicted by transition state theory based on the coordinates of a single water molecule. The discrepancy between these timescales arises from the neglected degrees of freedom of the second exchanging water molecule that plays a decisive role in the reaction mechanism. The approach presented here contributes molecular insights into the dynamics of water around metal cations and provides the basis for developing accurate atomistic models or for understanding complex biological processes involving metal cations.

Highlights

The present study focuses on water exchange in the first hydration shell of Mg2+ as an intriguing example for exchange dynamics on the micro- to millisecond timescale
Transition path sampling is applied to gain insight into the reaction mechanism and to identify the solvent molecules, which play a decisive role in the exchange reaction
Based on the reaction mechanism obtained from transition path sampling, a reaction coordinate is defined, which incorporates the concerted motion of the two exchanging water molecules

Summary

Introduction

Water exchange between the first and second hydration shell around metal ions is fundamental for a large variety of processes ranging from simple chemical reactions in aqueous solutions to the structure and function of biomolecules. In particular, the mechanism governs any type of reaction involving the replacement of strongly bound hydration water from the first hydration shell, which is essential, for instance, in metalloenzyme catalyzed reactions, regulatory biochemical processes, and the transport of metal ions across cell membranes.3,5–7The molecular nature of water gives rise to an intriguing interplay of hydrogen bonding, packing, and orientational effects leading to complex dynamics in which the concerted motion of solvent molecules becomes important. It is, not surprising that water relaxation times in the first hydration shell surrounding ions can be significantly altered compared to bulk. A striking example is the mean lifetime of water molecules in the first coordination shell of metal cations. The molecular nature of water gives rise to an intriguing interplay of hydrogen bonding, packing, and orientational effects leading to complex dynamics in which the concerted motion of solvent molecules becomes important.. The molecular nature of water gives rise to an intriguing interplay of hydrogen bonding, packing, and orientational effects leading to complex dynamics in which the concerted motion of solvent molecules becomes important.2,8–10 It is, not surprising that water relaxation times in the first hydration shell surrounding ions can be significantly altered compared to bulk.. The lifetimes span more than 18 orders of magnitude, ranging from a few picoseconds for Cs+ to hundreds of years for Ir3+.7 Given this tremendously broad timescale, the question arises how mechanistic insights can be obtained at the molecular level The lifetimes span more than 18 orders of magnitude, ranging from a few picoseconds for Cs+ to hundreds of years for Ir3+.7 Given this tremendously broad timescale, the question arises how mechanistic insights can be obtained at the molecular level

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Conclusion