Abstract
We have used a new solid-state NMR technique--rotational resonance--to determine both internuclear distances and the relative orientations of chemical groups (dihedral angles) in retinal bound to bacteriorhodopsin (bR) and in retinoic acid model compounds. By matching the rotational resonance condition (delta = n omega r/2 pi, where delta is the difference in isotropic chemical shifts for two dipolar coupled spins, omega r/2 pi is the mechanical rotational frequency of the sample in the MAS experiment, and n is a small integer denoting the order of the resonance), we selectively reintroduce the dipolar coupling and enhance the rate of magnetization exchange. Spectroscopic data and theoretical simulations of the magnetization exchange trajectories for the 8,18-13C dipolar coupled pair in retinoic acid model compounds, crystallized in both the 6-s-cis and 6-s-trans forms, indicate that an accurate determination of the internuclear distance is possible. For the n = 1 resonance we find the distance determination to be reasonably independent of the relative orientation of the groups. In contrast, for the n = 2 resonance, there is a more pronounced dependence on the relative orientation of the groups which permits an estimate of the angle around the 6-s bond for the cis and trans forms to be 42 +/- 5 degrees and 90 +/- 10 degrees, respectively, in good agreement with crystallography. In bR we demonstrate that the 8-13C-18-13C distance is 4.1 A and the average 8-13C-16-13C/8-13C-17-13C distance is 3.3-3.5 A.