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A solid-state NMR method (double-quantum heteronuclear local field NMR) is applied to a 13C2 labeled sample of the 41 kD integral membrane protein rhodopsin. The technique operates under magic-angle-spinning conditions, with good sensitivity and resolution, and allows a direct determination of molecular torsional angles, without estimating intemuclear distances. In rhodopsin, we determine the H-C1CHC11-H torsional angle of the retinylidene chromophore to be 160 dt 10°, indicating a significant deviation from the planar 10-11-s-trans conformation. Double-quantum heteronuclear local field NMR is shown to be a feasible method for the accurate determination of local molecular conformation in large molecular systems which are unsuitable for crystallography.i

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