MODEL STUDIES ON THE EXCITED-STATE EQUILIBRIUM PERTURBATION DUE TO REACTION-CENTER TRAPPING IN PHOTOSYSTEM-I

The fluorescence yield for 694 nm excitation in a Photosystem I-200 pa rticle is significantly lower than that for 665 nm excitation. This su pports the previous suggestion, based on a thermodynamic analysis of a bsorption and emission spectra, that thermal equilibration in the 690- 700 nm spectral interval is perturbed, presumably by primary photochem istry [Croce et al. (1996) Biochem 35: 8572-8579]. This equilibrium pe rturbation was used in the present study as a novel fit parameter in n umerical simulations aimed at describing the kinetic/thermodynamic pro perties of exciton flow and primary photochemistry in PS I. To this en d a four energy level scheme was developed which satisfactorily descri bed all the fit parameters, including that of the equilibrium perturba tion. An important characteristic which distinguished this model from other model studies is the presence of a number of chlorophyll molecul es with absorption maximum near 695 nm, tightly coupled to P700. The m ain conclusions are: (I) about six chlorophyll molecules absorbing nea r 695 nm are tightly coupled to P700, in close agreement with the rece nt crystallographic structure for the Photosystem I core [Krauss et al . (1996); Nature Struct Biol 3: 965-973]; (II) energy transfer from th e bulk pigments to the P700 core pigments is slow; (III) analysis of t he most physically straightforward model indicates that the primary ph otochemical charge separation rate is very high (k(pc) greater than or equal to 2.5 ps(-1)), though it is possible to simulate the equilibri um perturbation with lower k(pc) Values assuming a large free energy d ecrease in the excited state of P700; (IV) the red spectral forms slow down reaction centre trapping by a 2-3 fold factor.