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

Abstract

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 -42.6 (5), -48.9 (5) and -50.4 (3) degrees , 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 degrees 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) degrees ; 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 A, indicating the presence of pi-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.

Polymorphism and solvates of 3,3'‐dihydroxy‐ß,ß‐carotene‐4,4'‐dione: Screening and their thermodynamics

“…The powder pattern of polymorph II is plotted together with the calculated data of Bartalucci et al [16] and Hashimoto et al [16]. The pattern of the crystals prepared by the thermal treatment in this work shows a good agreement with calculated pattern in the literature.…”

“…Several crystal structures of DCD solved are protein-bound [13,14] except the work of Hashimoto et al [15] and Bartalucci et al [16] which gave the solved crystal structures of DCD. Especially, in the work of Bartalucci et al, two solvates, a pyridine solvate and a chloroform solvate were generated and studied on their crystallography.…”

Variation in carotenoid–protein interaction in bird feathers produces novel plumage coloration

“…Unlike structurally coloured feathers with spongy b-keratin nanostructures [43][44][45][46], the azimuthal average of the SAXS data from I. isabellae shows no peaks for any optically relevant spatial frequencies, or q-values (see electronic supplementary material, figure S2). Rather, the azimuthal average has a slope that is closely congruent to Porod's Law (q 24 ), the expectation for a random heterogeneous mixture of two materials with well-defined interface [47]. In the absence of any nanostructure that could enhance the coherent scattering of blue light, the purple feathers of male I. isabellae are produced by pigmentary absorption.…”

Origin of the colour of lobsters; crystal structures of unbound carotenoids

“…[1]). X-ray structure determination of pure crystalline polymer samples is usually of low accuracy and reliability because of a low structural order in the polymer crystals [2]. The inconvenience caused namely by high flexibility and polydispersity of polymer samples disappears when the polymer molecule is fixed in a regular three-dimensional scaffold of suitable protein crystals with high solvent content.…”

“…The packing of the molecules is quite different in each case involving hydrogen bonding and π-π stacking interactions. Theses studies are described in [2]. In addition, most recently, scis and s-trans isomers of an astaxanthin ester have been crystallised, where the s-trans isomer has a similar conformation to that found in the protein bound astaxanthin molecules in β-crustacyanin.…”