CONFORMATIONAL-ANALYSIS OF ALL-E-RETINAL, ALL-E-8,16-METHANORETINAL, (1S)-ALL-E-8,16-METHANO-18-NORRETINAL, AND ALL-E-17,18-DINOR-8,16-METHANORETINAL BY MEANS OF NMR-SPECTROSCOPY AND MOLECULAR-DYNAMICS

The conformations of all-E-retinal (1), (1S)-all-E-8,16-methano-18-nor retinal [(1S)-2], all-E-8,16-methanoretinal (3) and all-E-17,18-dinor- 8,16-methanoretinal (4) in solution were studied by H-1-NMR spectrosco py and molecular-mechanics and molecular-dynamics simulations. The H-1 -NMR spectrum of 1 is characterized by fast inversion of two half-chai r rings and by a skewed 6-s-cis geometry of the polyene chain. Molecul ar-dynamics simulations at 1000K on 1 show the existence of two, dynam ically stable, skewed 6-s-cis conformers (torsion angle C5-C6-C7-C8: 8 4-degrees and -83-degrees), separated by a 6-s-trans transition state. The conformationally locked structures of 2, 3 and 4 have similar con formations and are, in a geometrical sense, ideal model compounds to s tudy rhodopsins with a well-defined 6-s-trans orientation of the chrom ophore.