CAPTURE OF POTASSIUM-IONS BY VALINOMYCIN - A MOLECULAR-DYNAMICS SIMULATION STUDY

Molecular dynamics simulations of the capture of both hydrated and unh ydrated potassium ions by the antibiotic valinomycin are reported, The re are no explicit solvent interactions although a stochastic bath is used to simulate thermal equilibrium at ambient temperature, An ''open ring'' conformer has been found and, in agreement with common interpr etations of experimental data, is strongly implicated in the capture p rocess. The distant attraction of the cation to the biopolymer is domi nated by the dipole-charge interaction, and as the cation approaches, conformational changes are induced in the biopolymer to enhance its di pole moment. These changes involve both amide and ester carbonyl group s aligning toward the approaching cation, Initial coordination is via amide carbonyls, although this is eventually overtaken by ester carbon yl coordination on a time scale of about 30 ps. When cation water of h ydration is included in the simulations, the time scale of the capture process is lengthened by approximately 3 orders of magnitude, The pre sence of the hydrated cation is sufficient to induce the conformationa l change from the twisted-bracket form of valinomycin to the open ring structure. The valinomycin rapidly changes shape in the early part of the capture, but displacement of water molecules by (mainly) eater ca rbonyls is a slow process. One water molecule remained firmly attached to the complex after 18 ns, This raises the question as to whether wa ter is transported with the complex through membranes in vivo.