title: Two redox-active beta-carotene molecules in photosystem II.
abstract: Photosystem II (PS II) 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 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 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 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 beta-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.
authors.affiliation: Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
authors.forename: Cara A
authors.initials: CA
authors.lastname: Tracewell
authors.forename: Gary W
authors.initials: GW
authors.lastname: Brudvig
mesh_terms: D000222:Adaptation, Physiological; D000458:Cyanobacteria; D003624:Darkness; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D005615:Freezing; D045342:Light-Harvesting Protein Complexes; D016011:Normal Distribution; D010084:Oxidation-Reduction; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D019265:Spectroscopy, Near-Infrared / Q000379:methods / Q000706:statistics & numerical data; D018724:Spinacia oleracea; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D019207:beta Carotene / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; C066447:tyrosyl radical Y(D); D019207:beta Carotene; D014443:Tyrosine