Jw. Lee et al., CHEMICAL PLATINIZATION AND ITS EFFECT ON EXCITATION TRANSFER DYNAMICSAND P700 PHOTOOXIDATION KINETICS IN ISOLATED PHOTOSYSTEM-I, Biophysical journal, 69(2), 1995, pp. 652-659
Isolated photosystem I (PSI) reaction center/core antenna complexes (P
SI-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 elli
psoidal with major and minor axes of 6 and 5 nm, respectively. Platini
zation deposited similar to 1000 platinum atoms on each PSI particle a
nd 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 electr
ostatic shielding on P700 photooxidation and P700(+) reduction. The re
action 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 trapp
ing dynamics was examined by measuring picosecond fluorescence decay k
inetics in PSI-40. The fluorescence decay kinetics in both platinized
and control samples can be described as a sum of three exponential com
ponents. The dominant (amplitude 0.98) and photochemically limited exc
itation lifetime remained the same (16 ps) before and after platinizat
ion. The excitation transfer and trapping in platinized PSI-40 was ess
entially as efficient as that in the control (without platinization) P
SI. The platinization also did not affect the intermediate-lifetime (4
00-600 ps) and long-lifetime (>2500 ps) components, which likely are r
elated to intrinsic electron transport and to functionally uncoupled c
hlorophylls, respectively. The amplitudes of these two components were
exceptionally small in both of the samples. These results provide dir
ect evidence that although platinization dramatically alters the photo
catalytic properties of PSI, it does not alter the intrinsic excitatio
n dynamics and initial electron transfer reactions in PSI.