Chlorophyll and carotenoid radicals in photosystem II studied by pulsed ENDOR.

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 < or =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.