Chlorophyll and carotenoid radicals in photosystem II studied by pulsed ENDOR

Faller, P., Maly, T., Rutherford, A. W. and Macmillan, F. (2001) Chlorophyll and carotenoid radicals in photosystem II studied by pulsed ENDOR. Biochemistry, 40 (2). pp. 320-326. ISSN 0006-2960

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The stable carotenoid cation radical (Car(.+)) and chlorophyll cation radical (Chl(z)(.+)) in photosystem II (PS II) have been studied by pulsed electron nuclear double resonance (ENDOR) spectroscopy. The spectra were essentially the same for oxygen-evolving PS II and Mn-depleted PS II. The radicals were generated by illumination given at low temperatures, and the ENDOR spectra were attributed to Car(.+) and Chl(z)(.+) on the basis of their characteristic behavior with temperature as demonstrated earlier [Hanley et al. (1999) Biochemistry 38, 8189-8195]: i.e., (a) the Car(.+) alone was generated by illumination at less than or equal to 20 K, while Chl(z)(.+) alone was generated at 200 K, and (b) warming of the sample containing the Car(.+) to 200 K resulted in the loss of the signal attributable to Car(.+) and its replacement by a spectrum attributable to the Chl(z)(.+). A map of the hyperfine structure of Car(.+) in PS II and in organic solvent was obtained. The largest observed hyperfine splitting for Car(.+) in either environment was in the order of 8-9 MHz. Thus, the spin density on the cation is proposed to be delocalized over the carotenoid molecule. The pulsed ENDOR spectrum of Chl(z)(.+) was compared to that obtained from a Chl a cation in frozen organic solvent. The hyperfine coupling constants attributed to the beta -protons at position 17 and 18 are well resolved from Chl(z)(.+) in PS II (10.8 and 14.9 MHz) but not in Chl a(.+) in organic solvent (12.5 MHz). This suggests a more defined conformation of ring IV with respect to the rest of the tetrapyrrole ring plane of Chl(z)(.+) than Chl a(.+) probably induced by the protein matrix.

Item Type: Article
Faculty \ School: Faculty of Science > School of Chemistry
University of East Anglia > Faculty of Science > Research Groups > Biophysical Chemistry
Depositing User: Rachel Smith
Date Deposited: 19 Jul 2011 12:09
Last Modified: 25 Jul 2018 07:42

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