Detection of the D0→D1 transition of β-carotene radical cation photoinduced in photosystem II

Abstract

The D₀→D₁ absorption band of a β-carotene radical cation in the near-infrared (NIR) region was detected for the first time in photosystem II (PSII). PSII-enriched membranes and isolated reaction center (RC) complexes (D1/D2/Cytb₅₅₉) from spinach were illuminated at 80 and 150 K, respectively, in the presence of electron acceptors. In both preparations, UV-Vis-NIR difference spectra upon illumination exhibited a medium-intensity band at ∼1450 nm along with a strong band at ∼990 nm. The latter band has been assigned to the D₀→D₂ transition of the radical cation of the β-carotene in the secondary electron transfer pathway in PSII. These NIR bands disappeared at 210 K in the PSII membranes, and diminished their intensities in the RC complexes partially depleted of carotenoid. The absence or diminish of the β-carotene cation with little change in the formation of chlorophyll cations under these conditions were also confirmed by detecting light-induced FTIR difference spectra in the mid-IR region. From these results, it was concluded that the NIR band observed at ∼1450 nm arose from the D₀→D₁ transition of the β-carotene radical cation. It was shown that the observed band in the RC complexes was a mixture of the band of one β-carotene cation (Car₅₀₇⁺) at 1464 nm and that of the other cation (Car₄₈₉⁺) at a wavelength shorter than 1434 nm, indicating that the D₀→D₁ transition is sensitive to the protein environment. It is proposed that the position and the relative intensity of the D₀→D₁ band together with the well-known D₀→D₂ band can be useful monitors to investigate the properties of the radical cation and the molecular interaction of β-carotene in the PSII proteins.