Model studies on the excited state equilibrium perturbation due to reaction centre trapping in Photosystem I

The fluorescence yield for 694 nm excitation in a Photosystem I-200 particle is significantly lower than that for 665 nm excitation. This supports the previous suggestion, based on thermodynamic analysis of absorption and emission spectra, that thermal equilibration in the 690-700 nm spectral interval is perturbed, presumably by primary photochemistry [Croce et al. (1996) Biochem 35: 8572-8579]. This equilibrium perturbation was used in the present study as a novel fit parameter in numerical simulations aimed at describing the kinetic/thermodynamic properties of exciton flow and primary photochemistry in PSI. To this end a four energy level scheme was developed which satisfactorily described all the fit parameters, including that of the equilibrium perturbation. An important characteristic which distinguished this model from other model studies is the presence of a number of chlorophyll molecules with absorption maximum near 695 nm, tightly coupled to P700. The main conclusions are: (I) about six chlorophyll molecules absorbing near 695 nm are tightly coupled to P700, in close agreement with the recent crystallographic structure for the Photosystem I core [Krauss et al. (1996); Nature Struct Biol 3: 965-973]; (II) energy transfer from the bulk pigments to the P700 core pigments is slow; (III) analysis of the most physically straightforward model indicates that the primary photochemical charge separation rate is very high (k(pc) ≤ 2.5 ps^-1), thought it is possible to simulate the equilibrium perturbation with lower k(pc) values assuming a large free energy decrease in the excited state of P700; (IV) the red spectral forms slow down reaction centre trapping by a 2-3 fold factor.