IN VITRO MODELING OF THE P680 AND P700 REACTION CENTER CHLOROPHYLL-A COMPLEXES

MICHAEL STANFORD SHOWELL, Purdue University

Abstract

In this work the photochemical and photophysical properties of the green plant reaction center chlorophyll-a (Chl a) complexes, P680 and P700, are modeled in vitro by photoreactive aggregates of hydrated chlorophyll derived from the stereospecific interactions in coordination and hydrogen bonds involving the electrophilic Mg atom and nucleophilic cyclopentanone carbonyl groups at C9 and C10 of the Chl a molecules. The 700-nm absorbing monohydrate dimer, (Chl a(.)H(,2)O)(,2), is shown to exhibit optical properties compatible with those attributed to P700. The participation of the C10 carbomethoxy group in the reciprocal hydrogen bonding interactions characteristic of (Chl a(.)H(,2)O)(,2) is established through variable temperature IR studies. The observed doubling of the radiative rate upon dimer formation is shown to be consistent with exciton formation in the Chl a S(,1) state. A quantum-mechanical description of triplet localization is given. The photophysical and photochemical properties of the dihydrate polymeric aggregate, (Chl a(.)2H(,2)O)(,n), are elucidated by a combination of optical, electron spin resonance, and photogalvanic measurements. A general kinetic model of electron tunneling between the primary electron donor and acceptor is derived and applied to a quantitative analysis of the long-lived component observed in the dark decay of (Chl a(.)2H(,2)O)(,n(GREATERTHEQ)2)('+). The reduction potential of photooxidized (Chl a(.)2H(,2)O)(,n) is measured and found to be sufficient to promote water oxidation. It is shown that visible light illumination of (Chl a(.)2H(,2)O)(,n) films on metal electrodes results in water photolysis and the reduction of exogenous substrates such as pheophytin and ferredoxin. The biphotonic origin of (Chl a(.)2H(,2)O)(,2) photochemistry is established. It is concluded that (Chl a(.)2H(,2)O)(,n(GREATERTHEQ)2) affords a reasonable in vitro model of P680. A quantitative analysis of kinetic data derived from ('14)CO(,2) tracer studies serves to establish the light path of carbon reduction in vivo. The relevant results and observations from the in vitro modeling experiments are combined with those obtained from recent in vivo studies into a revised scheme for green plant photosynthesis.

Degree

Ph.D.

Subject Area

Chemistry

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