The gramicidin A channel: energetics and structural characteristics of the progression of a sodium ion in the presence of water.

The distribution of water molecules in the Gramicidin A (GA) channel is determined by theoretical computations, and the role of this water on the energetics of the system upon progression of a sodium cation through the channel is investigated. In the absence of the ion, water molecules form a chain along the channel, hydrogen bonded to one another and to the L carbonyl oxygens, while others stay at the entrances of the channel, hydrogen-bonded to the free carbonyl oxygens of the L-Tryptophan residues. According to the definition adopted for the "inside" and the "outside" of the channel, it is found to contain at most 7 or 9 water molecules. When a hydrated sodium cation approaches and enters the channel, the structural properties corresponding to the minimized total energy of the system GA-water-Na+ indicate a reorganization, but not a destruction, of the chain of water molecules. The "energy profile" for the system GA-Na+-(22 waters) is analyzed in terms of its components and in comparison to the corresponding intrinsic profile computed earlier in vacuo. It appears that the presence of water does not unduely modify the pathway or the qualitative features of the energetics of the cation passage, except at the entrance, where the partial and progressive dehydration of the cation plays an important role. The presence and characteristics of the minimum found earlier at 10.5 A from the center are conserved.