Protein‐chromophore interactions in α‐crustacyanin, the major blue carotenoprotein from the carapace of the lobster, Homarus gammarus a study by 13C magic angle spinning NMR

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Abstract
MAS (magic angle spinning) 13C NMR has been used to study protein‐chromophore interactions in α‐crustacyanin, the blue astaxanthin‐binding carotenoprotein of the lobster, Homarus gammarus, reconstituted with astaxanthins labelled with 13C at the 14,14′ or 15,15′ positions. Two signals are seen for α‐crustacyanin containing [14,14′‐13C2]astaxanthin, shifted 6.9 and 4.0 ppm downfield from the 134.1 ppm signal of uncomplexed astaxanthin in the solid state. With α‐crustacyanin containing [15,15′‐13C2]astaxanthin, one essentially unshifted broad signal is seen. Hence binding to the protein causes a decrease in electronic charge density, providing the first experimental evidence that a charge redistribution mechanism contributes to the bathochromic shift of the astaxanthin in α‐crustacyanin, in agreement with inferences based on resonance Raman data [Salares, et al. (1979) Biochim. Biophys. Acta 576, 176–191]. The splitting of the 14 and 14′ signals provides evidence for asymmetric binding of each astaxanthin molecule by the protein.

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