Two redox-active β-carotene molecules in photosystem II

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b₅₅₉ (Cyt b₅₅₉), chlorophyll (Chl), and β-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 II core complexes from Synechocystis PCC 6803 and Synechococcus sp. and spinach PS II membranes have been investigated using low temperature near-IR spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. We present evidence that two spectroscopically distinct redoxactive 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 nm, and Car_B⁺ has an absorbance maximum of 1027 nm) that decay at different rates. In Synechocystis PS II, 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 β-carotene that disfavors oxidation of Car_B. The sequence of electron-transfer reactions in the secondary electron-transfer pathways of PS II is discussed in terms of a hole-hopping mechanism to attain the equilibrated state of the charge separation at low temperatures.