X-ray Crystal Structure and Time-Resolved Spectroscopy of the Blue Carotenoid Violerythrin

Violerythrin, a blue-colored carotenoid, has been investigated by X-ray crystallography and steady-state and ultrafast time-resolved absorption spectroscopy. The X-ray crystal structure of violerythrin shows that the molecule is nearly planar with the terminal rings positioned in the s-trans conformation. The steady-state and time-resolved spectroscopic data of violerythrin do not differ significantly from those of other carbonyl carotenoids with long (N > 10) π-electron conjugated chains. This indicates that while the four carbonyl groups in violerythrin are critical for generating the bathochromic shift that leads to the blue color of the molecule, no dramatic changes attributable to a charge-transfer state known to affect the excited-state properties of carotenoids with short polyene chains occur. This may be due to the symmetric distribution of the carbonyl groups, which would preclude such an effect. The structural requirements for a blue, neutral, carotenoid are a planar, symmetric, cross-conjugated chromophore, containing at least 30 π-electrons, a central polyene chain with 9 or 10 conjugated carbon−carbon double bonds connected at each end by an s-trans or trans bond to two identical, cyclic end groups, each possessing a conjugated keto group further cross-conjugated to another keto group, or a double bond in a quinoid type structure.