Crystal structure of fucoxanthin: resolving a long-standing crystallographic challenge

Fucoxanthin (C₄₂H₅₈O₆) is a highly functionalized allenic epoxy xanthophyll containing hydroxy, acetoxy, ketone and conjugated polyene units. Although its constitution and stereochemistry had been established by chemical, spectroscopic and synthetic studies, its small-molecule single-crystal X-ray structure has remained unavailable. Moss reported in 1979 [Pure Appl. Chem. (1979), 51, 507-514] that an early X-ray study by G. M. Sheldrick had reached only a partial solution of the fucoxanthin structure and that refinement had stalled at R ≃ 25%. The first small-molecule single-crystal X-ray structure of fucoxanthin has now been determined as the diethyl ether monosolvate, C₄₂H₅₈O₆·C₄H₁₀O, at 100 K in the space group P1 using synchrotron radiation (λ = 0.4132 Å). The asymmetric unit contains two crystallographically independent fucoxanthin molecules and two diethyl ether molecules. The structure defines the relative stereochemistry, allene geometry, epoxide geometry, polyene bond-length alternation and solvate-assisted packing of fucoxanthin, and provides an independent small-molecule metrical reference for comparison with protein-bound fucoxanthin ligands.