Refinement of the Solution Conformation of Valinomycin with the Aid of 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.

Monitoring the Backbone Conformation of Valinomycin by Raman Optical Activity

“…Nevertheless, as shown below, comparison of the spectra obtained with different solvents and coupling of quantum modeling and molecular dynamics make prediction of the backbone conformation more reliable. Previous works [2,19,21,22] suggest that complexed valinomycin in solution has almost the same backbone structure as in the crystal. [23] The complex adopts a symmetric bracelet-like form in which all six ester carbonyl groups are strongly bound to the potassium ion in a central cavity.…”

“…[19] For the symmetric bracelet, two isopropyl conformers (sB1,2) dominate (with calculated populations of 69 and 27 %; see [a] Ref. [21].…”

Valinomycin and its interaction with ions in organic solvents, detergents, and lipids studied by Fourier transform IR spectroscopy

“…* Numerous biophysical studies have been undertaken in attempts to elucidate the molecular mechanism by which valinomycin transports K + across membranes. Such studies include crystal log rap hi^^^^ and spectroscopic [7][8][9] determinations of the structure of noncomplexed and K + -complexed valinomycin.…”

Improved Measurement of 3J(Hαi, Ni+1) Coupling Constants in H2O Dissolved Proteins

“…Remaining phases are cycled according to φ 2 = y; φ 3 = 2(x), 2(−x); φ 4 = x, −x; φ 5 = y; φ 6 = x; φ receiver = x, 2(−x), x. Gradients are sine-bell shaped and have the following durations, peak amplitudes, and directions: G 1 , 0.5 ms, 7.5 G/cm, x; G 2 , 0.5 ms, 6 G/cm, y; G 3 , 0.5 ms, 5 G/cm, z; G 4 , 0.4 ms, 39.45 G/cm, xyz; G 5 , 0.3 ms, −4 G/cm, xy; G 6 , 0.3 ms, 5.5 G/cm, xy; G 7 , 0.2 ms, 16 G/cm, xyz. For each t 1 increment N-and P-type transients are collected alternately by inverting the polarity of G 4 along with pulse phases φ 5 and φ 6 . The two transients are stored separately and then added and subtracted to form the real and imaginary parts of a complex FID with a 90 • zero-order phase shift being added to one of the components.…”