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

Abstract

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.

Enhanced Photocurrent Production by Photosystem I Multilayer Assemblies

“…where d is the film thickness, V dep is the initial volume of the PSI solution applied to the substrate, [PSI] soln is the PSI concentration in solution prior to vacuum assembly, N A is Avogadro’s constant, A film is the area covered by the film on the substrate, and V PSI is the volume attributed to single PSI complex in the film, including any residual salt, solvent, and surfactant molecules still associated with the protein after deposition. From (1), we obtain a packing density, V PSI , of 954 nm 3 per PSI complex, which is comparable to volumes obtained from previous reports of the dimensions of PSI as measured by transmission electron microscopy (TEM),26 atomic force microscopy (AFM),27 and scanning tunnelling microscopy (STM) 28…”

Fabrication of Oriented Multilayers of Photosystem I Proteins on Solid Surfaces by Auto‐Metallization

“…This result is also in harmony with earlier measurements done by scanning tunneling microscopy, [16] that indicated a change in the voltage current pattern of platinizated single PS I molecules. Sequential assembly of PS I monolayer and platinization decreased and increased the CPD at approximately similar magnitudes, as was observed in the first monolayer (Table 1).…”

Nanoscale Photosynthesis: Photocatalytic Production of Hydrogen by Platinized Photosystem I Reaction Centers¶

“…Moreover, previous studies have indicated that PSI reaction centers, like thylakoid membranes, can be platinized. A study of the chemical platinization of PSI indicates that the platinization process has no inhibitory effect on excitation-transfer dynamics and/or reaction-center-pigment (P700) photooxidation kinetics (8). Golbeck and Bryant (9) have presented a model of the PSI complex.…”