Understanding Solvent/Bixin Interactions by Raman Spectroscopy

In this work, Raman spectroscopy and density functional theory (DFT) calculations have been used in order to understand the nature of the interaction between the carotenoid bixin and organic solvents with dielectric constant varying from ca. 2 (toluene) to ca. 47 (dimethyl sulfoxide). Raman shifts registered for the main carotenoid marker band, the C=C stretching mode (1525-1533 cm-1), were used as a probe to monitor spectral changes due to chemical interaction between bixin in the solid state with the analogous solvated species. Raman spectra of bixin in solution showed significant bathochromic shifts in the wavenumber position of ν(C=C) when compared with the solid state. Among the solutions, subtle differences were observed and the polarizability rather than dielectric constant of the solvents seems to be a more appropriate parameter to explain the differences in Raman shifts which could be correlated with the solvation degree of bixin. Correlation between DFT analysis and molecular dynamics simulations obtained for cis-bixin in DMSO and CHCl3 demonstrated that the interaction of the solute with the respective solvents occurs in specific portions of the molecule, however, this result was not confirmed by the experimental data since bixin is solvated by larger numbers of solvent molecules.