Refinement of the solution conformation of valinomycin with the acid coupling constants from the 13C-nuclear-magnetic-resonance spectra.

Abstract

The C′= O and Cα signals in the 13C nuclear magnetic resonance (NMR) spectra of valinomycin have been assigned and the vicinal 1H…13C coupling constants have been determined by double and triple heteronuclear resonance. In conjunction with the vicinal H-NCα-H and H-CαCβ-H proton-proton constants, the results led to unequivocal determination of the torsion angles φ and of the population distribution of the Cα-Cβ rotational states. The Φ torsion angles for the hydroxy acid residues were estimated from the vicinal 1H-CαC′-15N constants. The combined data permitted refinement of the conformational states of valinomycin in different solvents. For the KS+ complex of valinomycin the observed couplings are in complete accord with the conformations we had earlier proposed for solutions and that had also been established by X-ray analysis. In the 13C spectra of the valinomycin-Tl+ complex 13C…203,205Tl+ spin-spin couplings were observed for the l and d-valine carbonyls, unequivocal proof of the donor-acceptor interaction with the cation. In media of weak polarity (cyclohexane, chloroform) the conformation of the valinomycin molecule is similar to that of the K+ complex. In such a ‘bracelet’ structure formed by six fused β-turns of type II and II′, the amino acid carbonyls are axial with certain inclination towards the symmetry axis. On formation of a 1:1 complex with a cation the carbonyl orientation changes, now bending towards the center of the molecular cavity. In the ‘propeller’ conformation, predominant in solvents of medium polarity (for instance CCl4/(C2H3)2SO, 3/1) the three β-turns are of type II. The 1H and 13C chemical shifts are interpreted in terms of conformational changes in the valinomycin molecule, intermolecular and intramolecular hydrogen bonds and interaction with the metal cation.