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-beta,beta-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-beta,beta-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)degrees , and the conformation is therefore similar to that found for unesterified AXT as well as for the carotenoids, canthaxanthin and beta,beta-carotene.

13-cis-Beta,beta-carotene and 15-cis-beta,beta-carotene.

13-cis-Beta,beta-carotene, C(40)H(56), crystallizes with a complete molecule in the asymmetric unit, whereas 15-cis-beta,beta-carotene, also C(40)H(56), has twofold symmetry about an axis through the central bond of the polyene chain. The polyene methyl groups are arranged on one side of the polyene chains for each molecule and the 6-s-cis beta end groups, with the cyclohexene rings in half-chair conformations, are twisted out of the planes of the polyene chains by angles ranging from 41.37 (17) to 52.

Unravelling the chemical basis of the bathochromic shift in the lobster carapace; new crystal structures of unbound astaxanthin, canthaxanthin and zeaxanthin.

The crystal structures of the unbound carotenoids, synthetic astaxanthin (3S,3'S:3R,3'S:3R,3'R in a 1:2:1 ratio), canthaxanthin and (3R,3'S, meso)-zeaxanthin are compared with each other and the protein bound astaxanthin molecule in the carotenoprotein, beta-crustacyanin. Three new crystal forms of astaxanthin have been obtained, using different crystallization conditions, comprising a chloroform solvate, a pyridine solvate and an unsolvated form. In each structure, the astaxanthin molecules, which are similar to one another, are centrosymmetric and adopt the 6-s-cis conformation; the end rings are bent out of the plane of the polyene chain by angles of -42.