Three bacteriorhodopsins with ring‐didemethylated 6‐s‐locked chromophores and their properties

Three novel, 6‐ s ‐locked rigidified retinals, racemic all‐ E 1,5‐didemethyl‐8,16‐methanoretinal, all‐ E 1,1‐didemethyl‐8,18‐methanoretinal and all‐ E 8a, 18‐didehydro‐1, 1‐didemethyl‐8, 18‐ methanoretinal were prepared in good yield in high purity on a 100‐mg scale. For the preparation of the key intermediate in the synthesis of 1,5‐didemethyl‐8,16‐methanoretinal, reductive cyanation of an unsaturated cyanohydrin to the corresponding conjugated nitrile was accomplished using triethylsilane and trifluoroacetic acid. The three locked retinals interact with bacterioopsin to form bacteriorhodopsin with about the same rate as the native chromophore. This work proves that steric interaction of the 1,1‐dimethyl group in the chromophore is an important factor in binding to bacterioopsin.