Two Redox-Active β-Carotene Molecules in Photosystem II (original) (raw)

Photosystem II (PS II) contains secondary electron-transfer paths involving cytochrome b 559 (Cyt b 559), chlorophyll (Chl), and-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 electrontransfer 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-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.