Host/guest conformations of biological systems: valinomycin/alkali ions

Collision cross sections of gas phase valinomycin-alkali ion complexes were measured in helium using the ion mobility based ion chromatography techniq ue. For the lithiated and sodiated species a value of 267 Angstrom(2) was m easured whereas the cross sections for the potassiated, rubidiated, and ces iated complexes were larger 272, 277, and 279 Angstrom(2) respectively. The systematic increase with ion size indicates that the backbone folding of t he cyclic valinomycin molecule is dependent on the choice of alkali ion. Th is result is in good agreement with theoretical cross sections of model str uctures obtained by molecular mechanics simulations. The model structures d emonstrate that the valinomycin host completely encapsulates the alkali ion with five or six of the polar carbonyl groups in the first solvation spher e of the alkali ion. The polar core of the complex is shielded by the aliph atic valinomycin side chains, which were found to be predominant on the com plex surface. The lithium ion is solvated by a fivefold carbonyl coordinati on sphere with at least four of the five carbonyls belonging to valine unit s. The sodiated species exhibits a five- to sixfold carbonyl coordination w ith highly excited O ... Na+ vibrations at 300 K. In the potassiated and ce siated complexes the alkali ion is coordinated by six valine carbonyl group s in a near octahedral arrangement causing the valinomycin backbone to fold in a quasi-S-6 symmetric fashion. These results demonstrate that the overa ll size and shape of the complex is not quite the same for different alkali ions, in contrast to conclusions made from solution salt extraction experi ments and assumptions made in previous molecular mechanics calculations. Ho wever, our results were found to be in good agreement with earlier spectros copic studies carried out on alkali salt valinomycin crystals and solutions thereof in organic solvents. Relative alkali ion-valinomycin binding energ ies extracted from the molecular mechanics data were able to qualitatively explain the experimentally observed preference of valinomycin for hosting p otassium over lithium and sodium. (C) 1999 Elsevier Science B.V.