The carotenoid bixin: Optical studies of aggregation in polar/water solvents

In contrast to the H-aggregation that results in a 100 nm blue shift in the optical absorption maximum for all trans zeaxanthin and astaxanthin in methanol/water solutions, the carotenoid bixin in methanol water solution does not form an H-aggregate. Instead, it exhibits a 4 nm red shift in a 50% water-methanol solution, suggesting possible J-aggregation. The lack of H aggregation can be traced to the fact that the ground state geometry for bixin is cis and not trans, resulting in an unfavorable geometry for H-aggregate formation. However, bixin is one carotenoid that exhibits enhanced water solubility compared to other common carotenoids. The optical spectrum is reported for bixin in chloroform, dioxane, dimethylformamide, tetrahydrofuran, acetonitrile, and methanol. The optical absorption maximum is blue shifted relative to chloroform as the polarizability of the solvent decreases and the polarity increases. A blue shift (0-4 nm) is observed when water is added to these solvents, the largest shift found for 5% water in acetonitrile, the most polar of the solvents besides methanol.