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Transport of excitation energy in a three-dimensional doped molecular aggregate. VII. Physical chemistry of exciton processes in thylakoid membrane

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Title Transport of excitation energy in a three-dimensional doped molecular aggregate. VII. Physical chemistry of exciton processes in thylakoid membrane
 
Creator DATTA, SN
SHAH, D
 
Subject coupled coherent
higher-plants
crystals
motion
model
chlorophyll
chloroplast
absorption
migration
kinetics
 
Description We numerically investigated the process of exciton transfer in thylakoid membranes. Recent knowledge of the structure and shape of these membranes was used to formulate four prototypes for which the numerical simulations were carried out. The intensity of the solar spectrum was chosen so as to account for the actual process of photosynthesis occurring in nature at sea level at an average latitude. As the most important point of departure from previous studies on the simulation of exciton dynamics in a molecular aggregate, we considered the exciton generation to be a continuous process. Thus, excitations of the chlorophylls and the special pairs, exciton transfer from host pigments to other hosts, trapping of excitons by reaction centers, host and trap fluorescence, and exciton utilization in the reaction centers were ail treated as simultaneous events. The rate of generation of excitons was determined for each chlorophyll molecule and each special pair individually by following standard expressions from the semiclassical treatment of the interaction of radiation with matter. The excitonic interaction energy was taken as the energy of interaction of two transition dipoles. The exciton hopping rate was determined from the expressions derived in our previous theoretical work. The simulation was based on a numerical solution of the generalized master equation describing the exciton dynamics that include exciton creation, exciton decay by fluorescence, exciton transfer from one site to another, exciton trapping by special pairs, and exciton utilization by chemical reaction at the traps. The following quantities were empirically selected, determined, or estimated: the thylakoid size, shape, and constitution; chlorophyll and dimer excitation energies and dipole strengths; the solar energy spectrum; exciton relaxation time; fluorescence rate constants; and the rate constant of the chemical reaction of the excited special pairs. We found that each closed membrane typically utilizes about 4% of the incident photons to generate chlorophyll excitons. The fraction of excitons used up in chemical reactions leading to the Z-scheme varies from 11 to 26%. The main observation is that the continuous nature of exciton generation quickly leads to a steady rate of flow to the reaction centers, which vindicates the diffusion model proposed by Pearlstein in 1967. This observation can never be made from a simulation that is based on the initial generation of all the excitons. Furthermore, we worked out the precise kinetic equations describing the physical chemistry of exciton processes. The most important point is that the rate constant for exciton utilization is determined by the rate constants for exciton generation, host molecular fluorescence, coherent exciton migration, and incoherent exciton transfer. While the fluorescence rate constant can be taken from experiment, all the other rate constants are given by quantum mechanical expressions and they can be evaluated for any organization of the host molecules and traps on the surface of a thylakoid membrane. This information can be used to formulate the rate of growth of plants. Besides, as in our earlier investigations, we found that the second scale of molecular excitations, the millisecond scale of the Z-cycle, the near nanosecond scale of exciton fluorescence, the nanosecond scale of exciton transfer, the nanosecond-to-picosecond scale of the chemical reaction of the excited reaction center, and the picosecond scale of phonon dynamics in thylakoid membranes are consistent with one another. (C) 1999 , Inc.
 
Publisher JOHN WILEY & SONS INC
 
Date 2011-08-16T16:50:04Z
2011-12-26T12:55:03Z
2011-12-27T05:43:35Z
2011-08-16T16:50:04Z
2011-12-26T12:55:03Z
2011-12-27T05:43:35Z
1999
 
Type Article
 
Identifier INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 74(3), 357-370
0020-7608
http://dx.doi.org/10.1002/(SICI)1097-461X(1999)74:3<357::AID-QUA9>3.3.CO;2-0
http://dspace.library.iitb.ac.in/xmlui/handle/10054/9560
http://hdl.handle.net/10054/9560
 
Language en