X-ray crystal structures of diacetates of 6-s-cis and 6-s-trans astaxanthin and of 7,8-didehydroastaxanthin and 7,8,7′,8′-tetradehydroastaxanthin: comparison with free and protein-bound astaxanthins

The crystal structures of the 6‐s‐cis [s‐cis‐(1)] and 6‐s‐trans [s‐trans‐(1)] conformers of the diacetates of astaxanthin (AXT) and those of (3S,3′S)‐7,8‐didehydroastaxanthin [(3S,3′S)‐3,3′‐dihydroxy‐7,8‐didehydro‐β,β‐carotene‐4,4′‐dione (2)] and (3S,3′S)‐7,8,7′,8′‐tetradehydroastaxanthin [(3S,3′S)‐3,3′‐dihydroxy‐7,8,7′,8′‐tetradehydro‐β,β‐carotene‐4, 4′‐dione (3)] are reported. The conformations of these four molecules vary in particular with the angle of twist of the end rings out of the plane of the polyene chain; for s‐cis‐(1), the end rings are twisted out of the plane of the polyene chain by an angle of −49.0 (5)°, and the conformation is therefore similar to that found for unesterified AXT as well as for the carotenoids, canthaxanthin and β,β‐carotene. For s‐trans‐(1), the end rings are coplanar with the polyene chain and its conformation is much more similar to that of the protein‐bound AXT in the blue protein, crustacyanin, which is found in the shell of lobsters, although s‐trans‐(1) shows much less bowing of the polyene chain. In (2) and (3) the end rings are also almost coplanar with the polyene chain with the end rings in (2) in the s‐cis conformation, and in (3) in the s‐trans conformation. Thus, an extensive ensemble of the possible β end‐ring conformations has been determined. These structures are compared with one another as well as unbound, unesterified AXT and protein‐bound AXT. Also, the effect of the end‐ring conformations on the colour and UV–vis spectra of the crystals was established.