Figure 4.

Structural and energetic effects of turning off the six intra-molecular hydrogen bonds in valinomycin. The hydrogen bonds, represented as dashed lines, were turned off using chemical substitutions involving replacement of proton acceptor atoms =O with =CH₂ groups. A representative substitution is highlighted inside blue circles. QC optimization following these substitutions resulted in a K⁺ complexed structure K⁺.mVal with a backbone atom RMSD of 0.65 Å, and a K⁺-O distance of 2.88 Å. In the case of Na⁺ complex, Na⁺.mVal, these substitutions resulted in 4 of the 6 carbonyl oxygens to coordinate Na⁺ ions at smaller distances of 2.33 Å, as compared to 2.64 Å in the native Na⁺.Val structure. The remaining two carbonyl oxygens are at a distance of 5.12 Å from the Na⁺ ion. The absence of hydrogen bonds also resulted in increasing the free energy difference between the Na⁺ and K⁺ complexes from 12.3 to 17.8 kcal/mol.