I3C NMR spectra of a large number of crystalline retinal derivatives have been obtained by using cross-polarization and magic-angle sample spinning. Most derivatives yield spectra with narrow lines (width around 20 Hz) which can be assigned via their 13C-'H dipolar coupling, their chemical shift tensors, comparison with solution spectra, or specific labeling. Measurement of the rotational sideband intensities in the spectra permit calculation of the chemical shielding tensors, and these data have allowed us to analyze variations in isotropic shifts of these compounds in more detail. We show that the tensors exhibit an odd/even effect which results from the steric crowding on one side of the polyene chain, that *-electron perturbations affect primarily the in-plane elements of the tensor, and, conversely, that strong steric interactions due to cis-trans isomerization affect the out-of-plane element. Finally, we observe a downfield shift at the C-5 position on isomerization about the 6-7 bond, and thus deduce from the observed shifts that retinal derivatives are 25-30% 6-s-trans in solution. The polyene aldehyde all-trans-retinal and its geometrical isomers play a central role in rhodospin and other visual and in bacteriorhodopsin and other light-driven membrane pump^.^^^ In both types of systems, photon-induced isomerization is the initial event of light tran~duction.~~~ However, in neither case are the spectroscopic properties of the intact protein similar to those of the free aldehydes, or of simple model compounds derived from it. Clearly, therefore, the conformational and electronic properties of the chromophores are strongly modulated by the approtein, and an understanding of the differences between bacteriorhodospin and rhodospin on the one hand, and retinal and its derivatives on the other, is crucial to an understanding of the entire system. For some years, I3C NMR has been used sporadically to in- vestigate both retinal derivatives and retinal-containing proteins. Initially, solution NMR techniques were applied to detergent- solubiIized rhodospin and bacteriorhodop~in,~~~ but the spectra obtained by using this approach were both of poor quality and prone to artifacts. The problems resulted from the extreme measures necessary to achieve short rotational correlation times in solution for what are both intrinsic membrane proteins. A more logical approach is to use the methodology of high-resolution dilute-spin NMR of solids, namely cross-polarization combined with magic-angle sample spinning (MASS),9 to study the intact membranes. This not only provides higher quality data but also permits examination of the anisotropic NMR interactions which