Formation and structure of the potassium complex of valinomycin in solution studied by Raman optical activity spectroscopy

Abstract

The formation and structure of the potassium complex with valinomycin in solution were studied by means of Raman and Raman optical activity (ROA) spectroscopy. The complexation caused significant spectral changes, particularly in the region 1200-1400 cm(-1). The experimental spectra were interpreted using first principles computations. A complete computational conformational search combined with the spectral analysis revealed the arrangement of the isopropyl side chains in the complex. From a total of 6579 unique conformers two predominant ones were confirmed in the solution by ROA. A third one was predicted theoretically, but its population in the experiment could be estimated only roughly. The most populated conformer does not exhibit C(3) symmetry, and is different from that present in the crystal and the NMR-derived structure. Molecular dynamics techniques were used to estimate the molecular flexibility and its effect on the spectra. Density functional computations and Cartesian coordinate transfer (CCT) techniques provided the ROA and Raman spectral shapes and intensities well comparable with the experiment. The polar solvent (methanol) environment modeled with a polarizable continuum model (PCM) leads to rather minor changes in the conformer populations and vibrational properties as compared to vacuum computations, due to the hydrophobic character of the complex. Additional computational experiments suggest that the vibrational interactions determining the ROA spectra are quite local, which contributes to the good spatial resolution of the method. A reduction of the noise in the experimental spectra as well as increased precision of the simulations is desirable for the further exploration of the potential of the ROA spectroscopy for biomolecular studies in the future.

Monitoring the Backbone Conformation of Valinomycin by Raman Optical Activity

“…In a previous study, [19] we investigated the extreme sensitivity of ROA spectroscopy to the isopropyl side-chain conformation in the potassium-valinomycin complex, confirmed by comparison of the experimental and calculated data. Characteristic ROA peaks of the complex, in particular a sharp + /À ROA couplet around 1325 cm…”

“…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.…”

“…Conformation Search on Isopropyl Side Chains: As shown before [19] the ROA spectrum of valinomycin strongly depends not only on the backbone, but also on the side-chain isopropyl torsion angles. The torsion angle was defined as ffH a C a C b H b , as in the previous study.…”

“…A similar procedure as in reference [19] was used for annealing and propagation of valinomycin, with a cubic periodic solvent box with 3 nm a side. The X-ray geometry of free valinomycin [26] was used as starting structure.…”

The early development and application of FTIR difference spectroscopy to membrane proteins: A personal perspective

“…Our Raman studies confirmed the existence of β-type hydrogen bonding when gramicidin A was solubilized in a hydrophobic solvent similar to the hydrophobic interior of lipid bilayers. Many subsequent vibrational studies have been made on both VM and Gramicin A [60,169,244,248].…”