Resonance Raman spectroscopy and quantum chemical modeling studies of protein–astaxanthin interactions in α-crustacyanin (major blue carotenoprotein complex in carapace of lobster, Homarus gammarus)

Abstract

Resonance Raman spectroscopy and quantum chemical calculations were used to investigate the molecular origin of the large redshift assumed by the electronic absorption spectrum of astaxanthin in α-crustacyanin, the major blue carotenoprotein from the carapace of the lobster, Homarus gammarus. Resonance Raman spectra of α-crustacyanin reconstituted with specifically ¹³C-labeled astaxanthins at the positions 15, 15,15′, 14,14′, 13,13′, 12,12′, or 20,20′ were recorded. This approach enabled us to obtain information about the effect of the ligand–protein interactions on the geometry of the astaxanthin chromophore in the ground electronic state. The magnitude of the downshifts of the CC stretching modes for each labeled compound indicate that the main perturbation on the central part of the polyene chain is not homogeneous. In addition, changes in the 1250–1400 cm⁻¹ spectral range indicate that the geometry of the astaxanthin polyene chain is moderately changed upon binding to the protein. Semiempirical quantum chemical modeling studies (Austin method 1) show that the geometry change cannot be solely responsible for the bathochromic shift from 480 to 632 nm of protein-bound astaxanthin. The calculations are consistent with a polarization mechanism that involves the protonation or another interaction with a positive ionic species of comparable magnitude with both ketofunctionalities of the astaxanthin-chromophore and support the changes observed in the resonance Raman and visible absorption spectra. The results are in good agreement with the conclusions that were drawn on the basis of a study of the charge densities in the chromophore in α-crustacyanin by solid-state NMR spectroscopy. From the results the dramatic bathochromic shift can be explained not only from a change in the ground electronic state conformation but also from an interaction in the excited electronic state that significantly decreases the energy of the π-antibonding CO orbitals and the HOMO–LUMO gap. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 358–370, 1999