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

Abstract Collision cross sections of gas phase valinomycin–alkali ion complexes were measured in helium using the ion mobility based ion chromatography technique. For the lithiated and sodiated species a value of 267 A 2 was measured whereas the cross sections for the potassiated, rubidiated, and cesiated complexes were larger 272, 277, and 279 A 2 , respectively. The systematic increase with ion size indicates that the backbone folding of the cyclic valinomycin molecule is dependent on the choice of alkali ion. This result is in good agreement with theoretical cross sections of model structures obtained by molecular mechanics simulations. The model structures demonstrate that the valinomycin host completely encapsulates the alkali ion with five or six of the polar carbonyl groups in the first solvation sphere of the alkali ion. The polar core of the complex is shielded by the aliphatic valinomycin side chains, which were found to be predominant on the complex surface. The lithium ion is solvated by a fivefold carbonyl coordination sphere with at least four of the five carbonyls belonging to valine units. The sodiated species exhibits a five- to sixfold carbonyl coordination with highly excited O…Na + vibrations at 300 K. In the potassiated and cesiated complexes the alkali ion is coordinated by six valine carbonyl groups in a near octahedral arrangement causing the valinomycin backbone to fold in a quasi- S 6 symmetric fashion. These results demonstrate that the overall size and shape of the complex is not quite the same for different alkali ions, in contrast to conclusions made from solution salt extraction experiments and assumptions made in previous molecular mechanics calculations. However, our results were found to be in good agreement with earlier spectroscopic studies carried out on alkali salt valinomycin crystals and solutions thereof in organic solvents. Relative alkali ion–valinomycin binding energies extracted from the molecular mechanics data were able to qualitatively explain the experimentally observed preference of valinomycin for hosting potassium over lithium and sodium.