Orientation of the Valine-1 Side-Chain of the Gramicidin Transmembrane Channel and Implications for Channel Functioning - a H-2 Nmr-Study
by Killian, Josephine Antoinette; Taylor, Martin Jeffrey; Koeppe II, R. Erdman
The orientation of the valine-1 side chain of gramicidin was determined by solid-state H-2 NMR using valine-1-deuterated (d8) gramicidin. The peptide was incorporated into DMPC bilayers that were oriented between glass plates. When the plates were oriented with their normal perpendicular to the magnetic field, four quadrupolar splittings were observed of 106, 68, 9.7, and 2.0 kHz. These resonances were assigned to C(alpha)D, C(beta)D, and the deuterons of each of the C(gamma)D3 methyl groups, respectively. The average orientation of the various C-D bonds was calculated with respect to the helix axis. The angle obtained for the C(alpha)-D resonance was consistent with a single-stranded beta6.3-helical model for the backbone but not with double-helical models. The angles of the side chain were then fitted to a model for the right-handed beta6.3-helix. Rotation of the valine-1 side chain yielded a set of torsion angles that matched the angles as determined from the H-2 NMR measurements. The corresponding orientation of the valine-1 side chain (chi1 = -5-degrees) was found to be quite unusual, but it explains well the importance of a branched side chain at position 1 for channel formation and stability. A van der Waals interaction between valine-1 of one monomer and alanine-5 of the other helps to stabilize the gramicidin dimer.
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