The dynamic origin of color tuning in proteins revealed by a carotenoid pigment

Understanding the microscopic origin of the color tuning in pigment-protein complexes is a challenging yet fundamental issue in photoactive biological systems. Here, we propose a possible interpretation by using a state-of-the-art multiscale strategy based on the integration of quantum chemistry and polarizable atomistic embeddings into a dynamic description. By means of such a strategy we are able to resolve the long-standing dispute over the coloration mechanism in the crustacyanin protein. It is shown that the combination of the dynamical flexibility of the carotenoid pigments (astaxanthin) with the responsive protein environment is essential to obtain quantitative predictions of the spectral tuning. The strong linear correlation between the excitation energies and the bond length alternation in the long-chain carotenoids modulated by the dynamical protein environment is a novel finding explaining the high color tunability in crustacyanin.