Fluorescence Spectroscopy of Excitation Transfer in Photosystem I (original) (raw)

This thesis centers on the study of excitation transfer in a photosynthetic antenna array. The spectroscopic properties of two pigment-protein complexes were investigated. These complexes, isolated from higher plants, display an unusual temperature dependent fluorescence behavior. We have chosen to study this fluorescence behavior with respect to energy transfer to the reaction center and in an isolated intact antenna preparation. A Photosytem I complex, PSI-200, was isolated from spinach. This complex contains the reaction center, primary electron acceptors and a 200 ChI antenna complement. We have characterized this system by both steady state and time-resolved fluorescence spectroscopy. At room temperature the main emission peak occurs at 690 nm with a large shoulder extending from 710 to 740 nm. Selective excitation of ChI b enhances fluorescence emission at longer wavelengths (F735). Additionally, the intensity of F735 increases dramatically as the temperature is lowered to 77K, while F690 remains relatively constant. Fluorescence polarization measurements indicate that this emission arises from pigments which absorb in the long wavelength region of the spectrum and comprise a relatively small portion of the antenna population. Comparison of spectral characteristics were made with a PSI complex isolated from the thermophilic cyanobacterium, Synechococcus, sp. The thermophilic complex has a smaller antenna complement relative to PSI-200 and is more stable with respect to thermal degradation. To address the role of ChI b in stimulating long wavelength fluorescence and the temperature dependence of the system, we have studied the energy transfer dynamics in an antenna complex, LHC-I isolated from PSI-200. This intact antenna preparation has an emission maximum at 685 nm (F685) with a large shoulder ranging from 710 to 740 nm. We observe that both emission bands increase as the temperature is lowered; at 77K F735 completely dominates the emission spectrum. Kinetic measurements indicate that initially absorbed excitation is rapidly redistributed to longer wavelength emitting pigments within 40 ps. The temperature dependence of F685 results from increased back transfer from long wavelength emitters to F685. We suggest that changes in excitation transfer between the various emitting species and a non-radiative fluorescence quenching mechanism account for the temperature dependence of the system.