Chemical platinization and its effect on excitation transfer dynamics and P700 photooxidation kinetics in isolated photosystem I.

Isolated photosystem I (PSI) reaction center/core antenna complexes (PSI-40) were platinized by reduction of [PtCl6]2- at 20 degrees C and neutral pH. PSI particles were visualized directly on a gold surface by scanning tunneling microscopy (STM) before and after platinization. STM results showed that PSI particles were monomeric and roughly ellipsoidal with major and minor axes of 6 and 5 nm, respectively. Platinization deposited approximately 1000 platinum atoms on each PSI particle and made the average size significantly larger (9 x 7 nm). In addition to direct STM visualization, the presence of metallic platinum on the PSI complexes was detected by its effect of actinic shading and electrostatic shielding on P700 photooxidation and P700+ reduction. The reaction centers (P700) in both platinized and nonplatinized PSI-40 were photooxidized by light and reduced by ascorbate repeatedly, although at somewhat slower rates in platinized PSI because of the presence of platinum. The effect of platinization on excitation transfer and trapping dynamics was examined by measuring picosecond fluorescence decay kinetics in PSI-40. The fluorescence decay kinetics in both platinized and control samples can be described as a sum of three exponential components. The dominant (amplitude 0.98) and photochemically limited excitation lifetime remained the same (16 ps) before and after platinization. The excitation transfer and trapping in platinized PSI-40 was essentially as efficient as that in the control (without platinization) PSI. The platinization also did not affect the intermediate-lifetime (400-600 ps) and long-lifetime (> 2500 ps) components, which likely are related to intrinsic electron transport and to functionally uncoupled chlorophylls, respectively. The amplitudes of these two components were exceptionally small in both of the samples. These results provide direct evidence that although platinization dramatically alters the photocatalytic properties of PSI, it does not alter the intrinsic excitation dynamics and initial electron transfer reactions in PSI.