Spectroscopy and quantum chemical modeling reveal a predominant contribution of excitonic interactions to the bathochromic shift in α-crustacyanin, the blue carotenoprotein in the carapace of the lobster Homarus gammarus

To resolve the molecular basis of the coloration mechanism of α-crustacyanin, we used ¹³C-labeled astaxanthins as chromophores for solid-state ¹³C NMR and resonance Raman spectroscopy of [6,6′,7,7′]-¹³C₄ α-crustacyanin and [8,8′,9,9′,10,10′,11,11′,20,20′]- ¹³C₁₀ α-crustacyanin. We complement the experimental data with time-dependent density functional theory calculations on several models based on the structural information available for β-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 α-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.