ON THE NATURE OF THE INTERMEDIATE ELECTRON ACCEPTOR (A₁) IN THE PHOTOSYSTEM I REACTION CENTER

Abstract

A sodium dodecyl sulfate-Photosystem I (PSI) complex has been prepared and characterized with respect to its electron acceptors. Component X and iron-sulfur centers A and B are absent from this preparation but the intermediate electron acceptor (A,) is present. Flash-induced absorbance changes at 25°C show charge separation, followed by a back-reaction with a half-time of 5 ps. The spectrum of the flash-induced change from 350 to 550 nm indicates a contribution from the intermediate electron acceptor, A;, as well as from P700'. EPR studies show that A; is associated with a free-radical signal having a g-value of 2.0025 and a linewidth of 12 gauss. A, would appear to be associated with a monomeric form of either Chl a or pheophytin a. INTRODUCIION Recent kinetic studies utilizing optical and electron paramagnetic resonance (EPR) spectroscopic tech-

Evidence for photoreduction of monomeric bacteriochlorophyll a as an electron acceptor in the reaction center of the green photosynthetic bacterium Prosthecochloris aestuarii

“…In photosystem I particles in which the iron-sulfur center A2 was reduced,g-values of 2.0033 [19] and 2.004 [12] have been reported for A i-. However, in photosystem I preparations which did not contain A2, the g-value of A[ [20] resembled that of Chl a-in vitro [21 ].…”

Radical pair interactions in spinach chloroplasts

“…Our choice of sample ~~' ,: .., co~ditions for this study was based on the current model of the photosystem I reaction center organization, which may be represented as [3,4] for algae and subchloroplast particles. However, if the photosystem I reaction center is poised in the redox state P700 A 1 x-Fd 8 -FdA-prior to the low temperature pulsed-light EPR measurement, a substantial change in both dynamics and spectral lineshape of the observed transient species is observed~ It is possible to interpret these new findings to distinguish between two candidates for the primary acceptor A 1 , which has been attributed to a chlorophyll a monomer [15,16] or a chlorophyll a dimer [17]. We also raise questions concerning the validity of g-value assignments that ignore interactions between the primary photoreactants, and we describe the use of a quantum mechanical model that permits a quantitative analysis of the experimental lineshapes.…”

Photoaccumulation of reduced primary electron acceptors of photosystem I of photosynthesis

“…Thus, neither the optical nor ESR evidence [4,6,7,24] indicates that the acceptor is a Chl a dimer as earlier proposed [4,8], and in fact it seems much more likely that the acceptor is a monomer as in green bacteria [18]. The absorption difference spectrum would not exclude that the acceptor is pheophytin a [25], but this seems unlikely because of the Em of pheophytin a in vitro [26] and because of its reported absence in photosystem I particles I27].…”