Two Redox-Active β-Carotene Molecules in Photosystem II^†

Photosystem 11 (PS 11) contains secondary electron-transfer paths involving cytochrome b(559) (Cyt b(559)), chlorophyll (Chl), and beta-carotene (Car) that are active under conditions when oxygen evolution is blocked such as in inhibited samples or at low temperature. Intermediates of the secondary electron-transfer pathways of PS 11 core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS 11 membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redox-active carotenoids are formed upon low-temperature illumination. The Car(+) near-IR absorption peak varies in wavelength and width as a function of illumination temperature. Also, the rate of decay during dark incubation of the Car(+) peak varies as a function of wavelength. Factor analysis indicates that there are two spectral forms of Car(+) (Car(A)(+) has an absorbance maximum of 982 run, and Car(B)(+) has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS 11, we observe a shift of the Car(+) peak to shorter wavelength when oxidized tyrosine D (Y-D.) is present in the sample that is explained by an electrostatic interaction between Y-D. and a nearby beta-carotene that disfavors oxidation of Car(B). The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS 11 is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.