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Organisms utilize light as energy sources and as signals. Rhodopsins, which have seven transmembrane ␣-helices with retinal covalently linked to a conserved Lys residue, are found in various organisms as distant in evolution as bacteria, archaea, and eukarya. One of the most notable properties of rhodopsin molecules is the large variation in their absorption spectrum. Sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII) function as photosensors and have similar properties (retinal composition, photocycle, structure, and function) except for their max (SRI, ϳ560 nm; SRII, ϳ500 nm). An expression system utilizing Escherichia coli and the high protein stability of a newly found SRI-like protein, SrSRI, enables studies of mutant proteins. To determine the residue contributing to the spectral shift from SRI to SRII, we constructed various SRI mutants, in which individual residues were substituted with the corresponding residues of SRII. Three such mutants of SrSRI showed a large spectral blue-shift (>14 nm) without a large alteration of their retinal composition. Two of them, A136Y and A200T, are newly discovered color tuning residues. In the triple mutant, the max was 525 nm. The inverse mutation of SRII (F134H/Y139A/ T204A) generated a spectral-shifted SRII toward longer wavelengths, although the effect is smaller than in the case of SRI, which is probably due to the lack of anion binding in the SRII mutant. Thus, half of the spectral shift from SRI to SRII could be explained by only those three residues taking into account the effect of Cl ؊ binding.

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An effective method for the preparation of 8-membered ring enone, (4S,5R,7R,8R)-4,8-bis(benzyloxy)-7-(t-butyldimethylsiloxy)-5-(4-methoxybenzyloxy)-2,6,6-trimethylcyclooct-2-enone (1), in sufficient quantities was developed. Optically active trioxy aldehyde, (3R,4S)-4-benzyloxy-5-(t-butyldimethylsiloxy)-3-(4-methoxybenzyloxy)-2,2-dimethylpentanal (3), was prepared first by diastereoselective dihydroxylation of (2R,4S)-2-(4-methoxyphenyl)-5,5-dimethyl-4-vinyl-1,3-dioxane derived from D-pantolactone. Next, 8-chloro-7-oxoaldehyde, (2R,3R,5R,6R)-2,6-bis(benzyloxy)-5-(t-butyldimethylsiloxy)-8-chloro-3-(4-methoxybenzyloxy)-4,4-dimethyl-7-oxononanal (30), was newly synthesized by the following reactions: i) MgBr2•OEt2-mediated diastereoselective aldol reaction of the aldehyde 3 with (Z)-2-benzyloxy-1-methoxy-1-(trimethylsiloxy)ethene and ii) reaction of methyl (2R,3R,5R,6S)-2,6-bis(benzyloxy)-3,7-bis(t-butyldimethylsiloxy)-5-(4-methoxybenzyloxy)-4,4-dimethylheptanoate with 1,1-dichloroethyllithium, followed by successive reductive dehalogenation of thus formed 1,1-dichloroethyl ketone with 1,1-dichloroethyllithium. Then, the chiral 8-chloro-7-oxoaldehyde 30 was converted to the 8-membered ring enone 1 by SmI2-mediated aldol cyclization.

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