DIRECT DETERMINATION OF A MOLECULAR TORSIONAL ANGLE IN THE MEMBRANE-PROTEIN RHODOPSIN BY SOLID-STATE NMR

A solid-state NMR method (double-quantum heteronuclear local field NMR ) is applied to a C-13(2) labeled sample of the 41 kD integral membran e protein rhodopsin. The technique operates under magic-angle-spinning conditions, with good sensitivity and resolution, and allows a divert determination of molecular torsional angles, without estimating inter nuclear distances. In rhodopsin, we determine the H-C10-C11-H torsiona l angle of the retinylidene chromophore to be 160 +/- 10 degrees, indi cating a significant deviation from the planar 10-11-s-trans conformat ion. Double-quantum heteronuclear local field NMR is shown to be a fea sible method for the accurate determination of local molecular conform ation in large molecular systems which are unsuitable for crystallogra phy.