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+ (CarA+ has an absorbance maximum of 982 nm, and CarB+ 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 (YD.) is present in the sample that is explained by an electrostatic interaction between YD. and a nearby β-carotene that disfavors oxidation of CarB. 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.
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