Spectroscopy and quantum chemical modeling reveal a predominant contribution of excitonic interactions to the bathochromic shift in alpha-crustacyanin, the blue carotenoprotein in the carapace of the lobster Homarus gammarus.
To resolve the molecular basis of the coloration mechanism of alpha-crustacyanin, we used (13)C-labeled astaxanthins as chromophores for solid-state (13)C NMR and resonance Raman spectroscopy of [6,6',7,7']-(13)C(4) alpha-crustacyanin and [8,8',9,9',10,10',11,11',20,20']-(13)C(10) alpha-crustacyanin. We complement the experimental data with time-dependent density functional theory calculations on several models based on the structural information available for beta-crustacyanin. The data rule out major changes and strong polarization effects in the ground-state electron density of astaxanthin upon binding to the protein. Conformational changes in the chromophore and hydrogen-bond interactions between the astaxanthin and the protein can account only for about one-third of the total bathochromic shift in alpha-crustacyanin. The exciton coupling due to the proximity of two astaxanthin chromophores is found to be large, suggesting that aggregation effects in the protein represent the primary source of the color change.
van Wijk
Arjan A C
AA
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, NL-2300 RA Leiden, The Netherlands.
Spaans
Arnold
A
Uzunbajakava
Natallia
N
Otto
Cees
C
de Groot
Huub J M
HJ
Lugtenburg
Johan
J
Buda
Francesco
F
D000818:Animals; D002247:Carbon Isotopes; D002352:Carrier Proteins; D008958:Models, Molecular; D008121:Nephropidae / Q000737:chemistry; D019906:Nuclear Magnetic Resonance, Biomolecular; D010860:Pigments, Biological / Q000737:chemistry*; D011506:Proteins / Q000737:chemistry*; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D024341:Xanthophylls; D019207:beta Carotene / Q000031:analogs & derivatives* / Q000737:chemistry
D002247:Carbon Isotopes; D002352:Carrier Proteins; D010860:Pigments, Biological; D011506:Proteins; D024341:Xanthophylls; C018494:crustacyanins; D019207:beta Carotene; C005948:astaxanthine