Web Release Date: January 15,
Spectroscopy and Quantum Chemical Modeling Reveal a
Predominant Contribution of Excitonic Interactions to the
Bathochromic Shift in 
-Crustacyanin, the Blue
Carotenoprotein in the Carapace of the Lobster Homarus
gammarus
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and
Contribution from the Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, NL-2300 RA Leiden, The Netherlands, Faculty of Science and Technology, University of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands
Received August 17, 2004
Abstract:
To resolve the molecular basis of the coloration mechanism of -crustacyanin, we used 13C-labeled astaxanthins as chromophores for solid-state 13C NMR and resonance Raman spectroscopy of
[6,6',7,7']-13C4 -crustacyanin and [8,8',9,9',10,10',11,11',20,20']-13C10 -crustacyanin. We complement the
experimental data with time-dependent density functional theory calculations on several models based on
the structural information available for 
-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 -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.
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