The Grotthuss hopping mechanism for the hydrated excess proton, which is the process of the net positive charge defect translocation via the rearrangement of the covalent bonds, is the key to interpreting many unique characteristics of proton transport in aqueous systems. For many years, Grotthuss proton transport through hydrogen bonded water networks has been the focus of theoretical and experimental discussions. However, is there a related process in which water can be transported through a proton charge defect in order to �wet� the �path ahead� for the charge defect to then migrate? This process has been recently observed in our computer simulations of explicit proton transport through a hydrophobic carbon nanotube. A 30 � dry nanotube was assembled between two graphene sheets separating bulk water. 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 � 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 biomolecular 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 wire�, e.g., one seen in an x-ray crystal structure or molecular dynamics simulations without an explicit excess proton, 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.

Wetting of hydrophobic spaces by hydrated excess protons

248th National Meeting (2014)

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