TY - JOUR
T1 - Unravelling the chemical basis of the bathochromic shift in the lobster carapace; new crystal structures of unbound astaxanthin, canthaxanthin and zeaxanthin
AU - Bartalucci, Giuditta
AU - Coppin, Jennifer
AU - Fisher, Stuart
AU - Hall, Gillian
AU - Helliwell, John R.
AU - Helliwell, Madeleine
AU - Liaaen-Jensen, Synnøve
PY - 2007/3/16
Y1 - 2007/3/16
N2 - 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, Β-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 -42.6 (5), -48.9 (5) and -50.4 (3)°, respectively, and are disordered, showing the presence of both R and S configurations (in a 1:1 ratio). In the crystal packing of the chloroform and pyridine solvates, the astaxanthin molecules show pair-wise end-to-end intermolecular hydrogen bonding of the adjacent 3-hydroxyl and 4-keto oxygens, whereas in the unsolvated crystal form, the hydrogen-bonding interaction is intermolecular. In addition, there are intermolecular C - H hydrogen bonds in all three structures. The canthaxanthin structure, measured at 100 and 293 K, also adopts the 6-s-cis conformation, but with disorder of one end ring only. The rotation of the end rings out of the plane of the polyene chains (ca -50 ° for each structure) is similar to that of astaxanthin. A number of possible C - H hydrogen bonds to the keto O atoms are also observed. (3R,3′S, meso)-zeaxanthin is centrosymmetric with a C5 - C6 - C7 - C8 torsion angle of -74.9 (3)°; the molecules show pair-wise hydrogen bonding between the hydroxyl O atoms. In addition, for all the crystal structures the polyene chains were arranged one above the other, with intermolecular distances of 3.61-3.79 Å, indicating the presence of π-stacking interactions. Overall, these six crystal structures provide an ensemble of experimentally derived results that allow several key parameters, thought to influence colour tuning of the bathochromic shift of astaxanthin in crustacyanin, to be varied. The fact that the colour of each of the six crystals remains red, rather than turning blue, is therefore especially significant. © International Union of Crystallography 2007.
AB - 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, Β-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 -42.6 (5), -48.9 (5) and -50.4 (3)°, respectively, and are disordered, showing the presence of both R and S configurations (in a 1:1 ratio). In the crystal packing of the chloroform and pyridine solvates, the astaxanthin molecules show pair-wise end-to-end intermolecular hydrogen bonding of the adjacent 3-hydroxyl and 4-keto oxygens, whereas in the unsolvated crystal form, the hydrogen-bonding interaction is intermolecular. In addition, there are intermolecular C - H hydrogen bonds in all three structures. The canthaxanthin structure, measured at 100 and 293 K, also adopts the 6-s-cis conformation, but with disorder of one end ring only. The rotation of the end rings out of the plane of the polyene chains (ca -50 ° for each structure) is similar to that of astaxanthin. A number of possible C - H hydrogen bonds to the keto O atoms are also observed. (3R,3′S, meso)-zeaxanthin is centrosymmetric with a C5 - C6 - C7 - C8 torsion angle of -74.9 (3)°; the molecules show pair-wise hydrogen bonding between the hydroxyl O atoms. In addition, for all the crystal structures the polyene chains were arranged one above the other, with intermolecular distances of 3.61-3.79 Å, indicating the presence of π-stacking interactions. Overall, these six crystal structures provide an ensemble of experimentally derived results that allow several key parameters, thought to influence colour tuning of the bathochromic shift of astaxanthin in crustacyanin, to be varied. The fact that the colour of each of the six crystals remains red, rather than turning blue, is therefore especially significant. © International Union of Crystallography 2007.
KW - Bathochromic shift
KW - Carotenoids
KW - Hydrogen bonding
U2 - 10.1107/S0108768106052633
DO - 10.1107/S0108768106052633
M3 - Article
SN - 0108-7681
VL - 63
SP - 328
EP - 337
JO - Acta Crystallographica Section B: Structural Science
JF - Acta Crystallographica Section B: Structural Science
IS - 2
ER -