A Fourier-transform infrared (FTIR) spectrum of the radical cation of β-carotene photoinduced in photosystem II (PSII) membranes was obtained at 80K under oxidizing conditions, by utilizing the light-induced FTIR difference technique. Formation of the β-carotene cation was monitored with the electronic absorption band at 993 nm. An FTIR spectrum of a chemically-generated β-carotene cation in chloroform was also measured and compared with the spectrum of PSII. Since the FTIR bands of carotenoid cation have characteristic features with strong intensities, they can be useful markers in studying the reaction of carotenoid in PSII.
