Enhanced carotenoid solubility in organic solvents via trans-cis photoisomerization induced by nanosecond-pulsed laser irradiation

This study presents a rapid photosensitizer-free method for trans-cis photoisomerization of carotenoids using nanosecond-pulsed laser irradiation. Pulsed laser irradiation enhanced the apparent dissolved fraction of β-carotene, astaxanthin, and lycopene suspended in organic Solvents more significantly than continuous-wave irradiation. This was due to the formation of cis isomers, as evidenced by the appearance of multiple peaks in the chromatograms. The isomerization efficiency exhibited a quadratic dependence on light intensity, indicating a two-step photon absorption process. Optimal wavelengths for photoisomerization were 58-82 nm longer than the all-trans absorption maxima, suggesting the involvement of triplet states. Temporal analysis revealed a rapid decrease in the all-trans ratio, plateauing within 5 min under high-intensity conditions, and the cis-isomer content reached 89.3 ± 0.5% for β-carotene. These findings provide mechanistic insights into trans-cis photoisomerization and demonstrate a clean and efficient approach for producing cis-isomer-rich carotenoids, offering potential applications in functional foods, cosmetics, and pharmaceuticals.