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A SEMI-CLASSICAL MODELING OF REVERSIBLE CHARGE SEPARATION IN REACTION CENTERS OF PHOTOSYNTHESIS. A.G. YAKOVLEV1, V.A. SHUVALOV1
1

,2

Belozersky Institute of Chemical and Physical Biology of Moscow State University, Moscow 119991, Russian Federation. E-mail: yakov@genebee.msu.su 2 Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region 142290, Russian Federation. E-mail: shuvalov@issp.serpukhov.su

In photosynthesis, the primary charge separation occurs in react ion centers (RCs) which are pigment-protein complexes of a photosynthet ic membrane. In RCs of purple bacteria Rhodobacter (Rba.) sphaeroides this process consists in electron transfer fro m excited dimer o f bacteriochlorophyll P* to bacteriopheophyt in HA wit h a part icipat ion o f mo no meric bacteriochlorophyll BA. Charge separation is acco mpanied by oscillat ions o f a populat ion of RC states when RCs are excited by femtosecond (<30 fsec) light pulses. In the present work the charge separation dynamics of nat ive, modified and mutant Rba. sphaeroides RCs is modeled in terms o f the kinetics equat ions for the populat ions wit h rate constants depended on t ime through the dependence on "slow" coordinates. This approach is based on the Marcus theory and on its applicat ion to charge separat ion in RCs. A goal o f the work was to build up a more realist ic model o f incoherent charge separat ion for qualitat ive and quant itative explanat ion o f oscillatory phenomena at early t imes o f this process. A success in this way would mean that the observed femtosecond oscillat ions of the populat ions of the RC states may have incoherent nature too. The simplest case o f regular changes of the "slow" coordinates with t ime was studied in the model. It was found that the five states model P*
905

BAHA P*

940

BAHA I P+BAHA P+BAH

A

adequately explains the
A

experimental kinet ics o f at t > 150 fsec. In the five states model two different excited states of P* and three charge separated states I, P+BAHA and P+BAH were used. The new
A A

intermediate state I was assumed to be populated earlier than the P+BAHA and P+BAH states. The simplest three states model P*BAHA P+BAHA P+BAH

does not provide

the correct form o f the oscillat ions and does not explain the out-of-phase oscillat ions of the P* stimulated emissio n at 905 and 940 nm. The four states model P* P
+ 905

BAHA P*

940

BAH

A

BA

HA P BAH

+

A

well explains the oscillatory features of the P* st imulated emissio n

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kinet ics. A drawback of the four states model is the unacceptably large amplitude o f the oscillat ions o f the react ion energet ics and the wrong posit io n o f the P+B the P*
905 A

level higher than

BA and P*

940

BA levels. In the model the oscillat ions in the populat ion kinetics arise

fro m the external modulat ion o f the react ion energetics leading to the modulat ion o f the rate constants. The qualitat ive agreement of our model with the experimental data can be reached in the wide range of the parameters. This indicates the stabilit y of the so lut ion founded in the model against the parameter fluctuations. In our opinio n, the most interesting result of the modeling is a conclusio n about a possibilit y o f an existence of the new intermediate states participat ing in primary charge separation. We signed these states as *
+ A B 940

and I. The

first state is presumably associated with the P* stimulated emissio n band at 940 nm. The 940nm band might be ascribed to the P P state with partial charge separation between P
A

and PB. This hypothesis is indirect ly confirmed by the quantum-mechanical calculat ions o f the electron-spin densit y shift fro m PA to PB in P* and by the IR spectroscopy data. The intermediate state I was introduced in the model in order to co-ordinate the posit ion o f the P+BA level below the P*B level by several hundreds of cm1 wit h the presence of the deep oscillat ions in the populat ion of the P+B
A

state. The dynamics of the I state populat ion in the
A

model mainly co incide wit h the oscillatory co mponent of the experimental B band dynamics. These means that the state I may be the P+B
A

absorption

state with part ial charge
940

separation between P* and BA. The main advantage of the presence of the P*

BAHA and I
940

intermediates seems to consist in the very high rates o f the primary react ions. The P*

BAH

A

and I states act as mediators which quickly receive an electron from the previous state and quickly pass it to the next state. Based on the numerous data one can conclude that shortly after the excitation o f P by <30-fsec pulse the nuclear motion inside P has a coherent character. If the coherent nuclear wavepacket is quickly damped then the nuclei would continues the incoherent motion. The model presented here describes the influence of this incoherent component of the nuclear motion on primary charge separat ion in nat ive and mutant RCs of Rba. sphaeroides. The closeness between the calculated and measured kinet ics at t >150 fsec demo nstrates a possibilit y o f incoherent nature of the observed oscillat ions. We acknowledge the partial financial support of the Russian Foundat ion for Basic Research (grant N 11-04-00312a).

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