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Center for Nonlinear Studies |
The hydrated excess proton plays a critical role in many areas of chemistry, biology, and materials science. Despite playing the 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 seminar my group’s longstanding efforts to characterize proton solvation and transport will be described. Theses studies employ a novel multiscale reactive molecular dynamics method combined with large scale computer simulation. The method allows for the treatment of explicit (Grotthuss) proton shuttling and charge defect delocalization, which strongly influences proton solvation and transport in numerous environments including bulk water, water interfaces, and biomolecular systems. One particular focus of my talk will be on the process of protons passing into and through transmembrane biological proton channels. 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 channel 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. Specific simulation results will be given for the important M2 proton channel of influenza A and a comparison to experimental results will be discussed where possible. Another example will be given for a remarkable process that has been recently observed in our computer simulations of proton transport through a hydrophobic carbon nanotube. Surprisingly, before the hydrated proton enters the tube, it starts shuttling water molecules into the otherwise dry tube via Grotthuss shuttling, effectively creating its own water wire where none existed before. As the proton enters the nanotube (2 ~ 3 Ã… in), the tube transitions to being fully wet. Water molecules enter the nanotube by passing “through†the proton charge defect via the Grotthuss-like bonding topology rearrangement. Interestingly, other monoatomic cations (e.g., K+) have just the opposite effect - blocking the wetting process and making the nanotube even drier. As the dry nanotube gradually becomes wet when the proton charge defect enters it, the free energy barrier of proton permeation through the tube drops significantly. This finding suggests that an important wetting mechanism may influence proton translocation in more complex systems, one in which protons “create†their own water structures in hydrophobic spaces (e.g., protein cavities) before migrating through them. If this is true, then a prior existing “water wireâ€, e.g., one seen in an x-ray crystal structure, may not be necessary for excess protons to transport through hydrophobic spaces via water mediated Grotthuss shuttling. They can create their own water wires as needed.
Host: Robert Ecke