Conformational dependence of the electronic absorption by astaxanthin and its implications for the bathochromic shift in crustacyanin

The conformational dependence of the electronic absorption by astaxanthin is believed to be of relevance for the bathochromic shift that this carotenoid assumes upon binding to crustacyanin, the protein macromolecular complex responsible for the slate-blue colouration of lobster shell. Here, we report quantum chemical calculations suggesting that the bathochromic shift that can be attributed to changes in astaxanthin conformation brought about by binding to the protein in fact is rather small. In particular, by subjecting an exhaustive set of different astaxanthin conformations to time-dependent density functional theory (TD-DFT) and semiempirical configuration interaction singles (ZINDO/S) calculations, it is found that the bathochromic shift due to the protein-enforced coplanarity of the beta-ionone rings with the polyene chain is considerably smaller (TD-DFT: 22-37 nm; ZINDO/S: 11 - 19 nm) than the 4100 nm shift that recently [ B. Durbeej and L. A. Eriksson, Chem. Phys. Lett., 2003, 375, 30] ( see ref. 16) was predicted to arise from a hydrogen-bond mediated interaction involving one of the astaxanthin C4 keto groups and a histidine residue of the surrounding protein. Moreover, the calculations suggest that the protein-induced bowing of astaxanthin about the center of the polyene chain is of no relevance for the observed bathochromic shift.