The hydrated excess proton (aka “hydronium cation”) is critical in many areas of chemistry, biology, and materials science. Despite playing a central role in fundamental chemical (e.g., acid-base) and biological (e.g., bioenergetics) processes, the nature of the excess proton remains mysterious, surprising, and sometimes misunderstood. In this presentation our longstanding efforts to characterize proton solvation and transport in biomolecular systems will be described.  These studies employ a novel, accurate, and computationally efficient multiscale reactive molecular dynamics method combined with large scale computer simulation. The methodology allows for the treatment of explicit (Grotthuss) proton shuttling and charge defect delocalization, which strongly influences proton solvation and transport in proteins such as transmembrane proton channels, pumps, and transporters/antiporters. The unique electrostatics related to the dynamic delocalization of the excess proton charge defect in water chains and amino acid residues will be elaborated, as well as the effects of these complex electrostatics on the proton transport and selectivity properties. The often opposing and asymptotic viewpoints related to electrostatics on one hand and Grotthuss proton shuttling on the other will be reconciled and unified into a single conceptual framework. The intrinsically coupled nature of the excess proton translocation and the water hydration can also be elaborated through these computer simulations. It is found that a prior existing “water wire”, e.g., one seen in an x-ray crystal structure, is not necessary for excess protons to transport through hydrophobic spaces in proteins via water mediated Grotthuss shuttling. The proton translocation process can sometimes create its own transient water wire as needed. Specific simulation results will be given for the enzyme and proton pump cytochrome c oxidase (CcO) as well as the ClC Cl-/H+ antiporter. A comparison to experimental results will be provided where possible.

Proton transport in biomolecular systems: A remarkably complex and collective phenomenon

251st National Meeting (2016)

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