FOURIER-TRANSFORM INFRARED STUDY OF THE CATION-RADICAL OF P680 IN THEPHOTOSYSTEM-II REACTION-CENTER - EVIDENCE FOR CHARGE DELOCALIZATION ON THE CHLOROPHYLL DIMER

A Fourier transform infrared (FTIR) difference spectrum of the primary electron donor (P680) of photosystem II upon its photooxidation (P680 (+)/P680) was obtained in the frequency region of 1000-3000 cm(-1). Th e reaction center (RC) complex (D1-D2-Cytb559) was used for the measur ements in the presence of ferricyanide as an exogenous electron accept or. Control measurements of electronic absorption (300-1200 nm) showed that illumination of the RC complex at 150 K induced major oxidation of P680 concomitant with oxidation of a carotenoid and an accessory ch lorophyll (Chl). Illumination at 250 K also specifically bleached one of the two beta-carotene molecules bound to the RC complex, and the sa mple thus treated exhibited little formation of a carotenoid cation on subsequent illumination at 150 K. The P680(+)/P680 FTIR difference sp ectrum (with minor contamination of Chl(+)/Chl) was measured at 150 K using this partially carotenoid-deficient RC complex. The spectrum sho wed a broad positive band centered at similar to 1940 cm(-1), which co uld be ascribed to an infrared electronic transition of P680(+) analog ous to that previously observed in various bacterial P+. This finding indicates that a positive charge is delocalized over (or hopping betwe en) the two Chl molecules in P680(+). The low intensity of this electr onic band compared with that of the bacterial band could have three po ssible explanations: weak resonance interaction between the constituen t Chl molecules, an asymmetric structure of P680(+), and the differenc e in Chl species. Bands in the C=O stretching region (1600-1750 cm(-1) ) were interpreted in comparison with resonance Raman spectra of the R C complex. The negative peaks at 1704 and 1679 cm(-1) were proposed as candidates for the keto C-9=O bands of P680. The observation that nei ther of these bands agreed with the main keto C-9=O band at 1669 cm(-1 ) in the previous (3)P680/P680 FTIR spectrum [Noguchi et al. (1993) Bi ochemistry 32, 7186-7195] led to the idea that the triplet state migra tes to a Chl (designated as Chl(T)) different from P680 at low tempera tures. Based on these results, structural models of Chl molecules incl uding P680 and Chl(T) and their coupling in the cation, triplet, and Q y singlet states are discussed.