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Theoretical investigation of the rates of electron transfer processes Q(I)(-)+Q(II)->Q(I)+Q(II)(-) and Q(I)(-)+Q(II)(-)->Q(I)+Q(II)(2-) in photosynthesis

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Title Theoretical investigation of the rates of electron transfer processes Q(I)(-)+Q(II)->Q(I)+Q(II)(-) and Q(I)(-)+Q(II)(-)->Q(I)+Q(II)(2-) in photosynthesis
 
Creator DATTA, SN
MALLIK, B
 
Subject primary-charge separation
photosystem-ii particles
reaction centers
rhodopseudomonas-sphaeroides
bacterial photosynthesis
spinach-chloroplasts
chlorophyll-alpha
acceptor
reduction
kinetics
 
Description A theoretical investigation on the rates of electron-transfer processes Q(I)(-) + Q(II) --> Q(I)(-) + Q(II)(-) and Q(I)(-) + Q(II)(-) --> Q(II)(-) + Q(II)(2-) was carried out by using the Marcus theory of long-range electron transfer in solution. The molecular reorganizational parameter lambda, the free-energy change Delta G(0) for the overall reaction, and the electronic matrix element H-DA for these two processes were calculated from the INDO-optimized geometries of molecules Q(I), Q(II), and histidine. Q(I) and Q(II) are plastoquinones (PQ) which are hydrogen-bonded to a histidine each, and the two histidines may or may not be coordinated to a Fe2+ ion. The plastoquinone representing Q(I) is additionally flanked by two peptide fragments. Each of the species (Pep)(2)Q(I) . His and His . Q(II) has been considered to be immersed in a dielectric continuum that represents the surrounding molecules and protein folds. INDO calculations confirm the standard reduction potential for the first process (calculated 0.127 V; observed 0.13 V) and predict a midpoint potential of 0.174 V for the second process at 300 K at pH 7 (experimental value remains uncertain but is known to be close to 0.13 V). The plastoquinone fragment carries almost all the net charge (about 95.7%) in [PQ . His](-) and the net charge in [PQH . His](-). The electron is transferred effectively from the plastoquinone part of [(Pep)(2)Q(I) . His](-) to the plastoquinone moiety of Q(II). His in the first step and to the plastoquinone fragment of HisH(+). Q(II)(-) in the second step. Therefore, we made use of the formula for the rate of through-space electron transfer from Q(I) to Q(II) (and to Q(II)(-)). The plastoquinones are, of course, electronically coupled to histidines, and the transfer is, in reality, through the molecular bridge consisting of histidines and also Fe2+. The through-bridge effect is inherent in our calculation of Delta G(0), H-DA, and the reorganization parameter A. We investigated the correlation between half-times for the transfer and (D-op(-1) - D-s(-1)), where D-op and D-s are, respectively, optical and static dielectric constants of the condensed phase in the vicinity of the plastoquinones. We found that with reasonable values of D-op (2.6) and D-s (8.5) the experimental rates are adequately explained in terms of transfers from the plastoquinone moiety of Q(I) to that of Q(II). The t(1/2) values calculated for the two processes are 247 and 472 mu s in the absence of Fe2+ and 134 and 181 mu s in the presence of Fe2+. These are in good agreement with the observed values which are approximate to 100 and approximate to 200 mu s when Fe2+ is present in the matrix and which are known to be almost twice as large when the Fe2+ is evicted from the matrix. The present work also shows that the Marcus-Hush theory of long-range electron transfers can be successfully applied to the investigation of processes occurring in a semirigid condensed phase like the thylakoid membrane region. (C) 1997 , Inc.
 
Publisher JOHN WILEY & SONS INC
 
Date 2011-08-16T16:16:22Z
2011-12-26T12:55:02Z
2011-12-27T05:43:33Z
2011-08-16T16:16:22Z
2011-12-26T12:55:02Z
2011-12-27T05:43:33Z
1997
 
Type Article
 
Identifier INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 61(5), 865-879
0020-7608
http://dx.doi.org/10.1002/(SICI)1097-461X(1997)61:5<865::AID-QUA12>3.0.CO;2-2
http://dspace.library.iitb.ac.in/xmlui/handle/10054/9551
http://hdl.handle.net/10054/9551
 
Language en