FIGURE 1.

K⁺ ion coordination in selectivity filters of K-channels is markedly different from that in bulk water. KcsA, for example, offers three binding sites for K⁺ ions (2), S1–S3, where it coordinates with the ion using eight of its backbone carbonyl oxygen atoms (highlighted in red) at an average distance of 2.8 Å. In contrast, in a 40-ps-long AIMD (PW91) simulation of K⁺ ion in bulk water (generated by extending the previously reported (8) 14-ps-long trajectory), only four water molecules are seen most tightly bound to the K⁺ ion at an average distance of 2.8 Å. These four water molecules correspond to the maximum number that contribute to the principle maxima of the radial distribution function (g(r)); the fifth and the sixth nearest waters statistically do not contribute to the peak of the principal maxima; whereas the seventh and the eighth nearest waters are seldom seen within the canonical inner coordination shell (gray area) of the ion.