Solvent effects on the optical properties of photosynthetic pigments evaluated by evolutionary optimization

Abstract

With the development of computational technologies, it became possible to do numerical simulations of the optical properties of molecules and organic crystals taking into account the modern sophisticated theoretical approaches. Our work is devoted to the study of carotenoids in different solvents. Carotenoids, being photosynthetic pigments, are located inside pigment-protein complexes and are covalently bound to the proteins. They have a wide variety and despite their relative simple structure, their optical properties remain poorly understood. Thus, their role in photosynthetic machinery is still an open question. In this study, we investigate the properties of carotenoid electronic excitations by fitting their absorption spectra considering pigments in polar and nonpolar solvents. As it is known, carotenoids have four main nuclear vibration modes. Each mode is characterized by three parameters: frequency, Huang-Rhys factor and damping factor. In particular, the Huang-Rhys factor cannot be measured experimentally. To do the modeling, we developed special software to fit experimental data using differential evolution (DE) algorithm, which allows obtaining an unambiguous solution for applied quantum models. With the help of DE, it was possible to estimate the influence of the solvent on the excited states of a pigment. Examining astaxanthin and lycopene spectra in three solvents (chloroform, hexane, ethanol), we have shown that some quantum parameters are very sensitive to the type of solvent and can be considered as markers of polarity.