Determination of the solution conformation of retinals by using lanthanoid shift reagents

Abstract

The conformations of all-trans-, 9-cis-, 11-cis-, and 13-cis-retinals and 8′-apo-β-carotenal in solution have been examined by observation of lanthanoid-induced shifts (LIS) in their ¹H n.m.r. spectra. Analysis has been carried out with the aid of a computer program, LISCA, in which rotamer averaging and consideration of more than one position for the bound lanthanoid can be tried. When this is done several structures may be found equally good as far as agreement between the experimental and calculated LIS ratios is concerned. In this case the choice becomes dependent upon other evidence such as gadolinium-induced broadening or nuclear Overhauser enhancements (NOEs). We have shown that the lanthanoid binds to both lone pairs of the carbonyl oxygen atom for all four isomers. However, the metal occupies the cis-site for less than one fifth of the time spent in the trans-site. The angle between the ionone ring and the side chain, D_R, ranges from 108 to 120° for the retinals. This reflects a real, average ring orientation with the skewed s-cis-form predominating. This angle is in agreement with the value determined by other techniques. For the angle. D_12–13, about the single bond C12–C13 in 11-cis-retinal, a value of 90 ± 20° has been found. This agrees with determinations by the NOE technique and represents an average of at least two conformers probably with values of D_12–13 equal to +30 and –150°. However, the LIS data appear to exclude mixtures of conformers which are s-trans and s-cis about this bond.