Ah initio and density functional theory (DFT) calculations are used to inve
stigate a (NH3)(3)Zn2+. . . (OH2)(n). . NH3 system that models the proton c
onduction, via a chain of water molecules, from a Zn2+ ion to a His residue
some distance removed. The optimal configuration of this chain, with n = 3
, contains a number of fairly short H-bonds separating the water molecules.
The conduction of a proton from the Zn-bound water to the terminal N-accep
tor is energetically favored by a concerted process wherein all protons are
in flight from one molecule to the next along the chain, at approximately
the same time. This optimal process also includes the shortening of each H-
bond as the donor and acceptor move toward one another at the midpoint of t
he transfer as well as small alterations in the distance between Zn and its
various ligands. The barrier for the conduction process is not adversely a
ffected by a lengthening of the chain to include as many as five waters. Ho
wever, the process is slowed considerably if additional H-bonds are formed
between members of the chain and peripheral molecules. The reorientation of
the chain that places the ultimate N-acceptor at an angle of greater than
some 30 degrees from the Zn- -O axis of the Zn-bound water can also slow th
e conduction process.