The kinetic behavior of a typical Hill reaction catalyzed by thylakoids and using the oxidized form of 2,6-dichloroindophenol (DCPIPox) as the artificial clectron acceptor, is considered. Here, the light absorption process and the reduction of DCPlPox are autocatalytically coupled, leading to the occurrence of rnultiplc steady states with respect to either the acceptor concentration or the incident light intensity. Experimental evidence is prescntcd for both cases and the emergence of autocatalysis is discussed. The effect of the spatial arrangment on the global behavior of the system described is emphasized.Biocatalytic reactions have bcen studied almost enlerely from the viewpoint of their time-dependent behavior. Rcccntly, a more relevant and realistic approach has consisted of coupling purely catalytic functions with various phenomena intrinsically linked to space, such as diffusion, partition of metabolites, vectorial processes and convection. When autocatalytic reactions are considered, qualitatively different behavior of the system has been predicted theoretically, for example multistability and/or spatiotcniporal structure formation [I]. Experimental evidence of multistability for either ki nctic or electrochemical memory in diffusion-reaction coupled and reconstitutcd enzyme systems has been reported [2 -51. Nevertheless, in the framework of this spatiotemporal concept, littlc attention has been paid to radiation phenomena which, likc diffusion. are also essentially space-dependent and whose role is fundamental in numerous chemical and biochemical processes. A thorough theoretical analysis considcring a simple photodependent isomerization reaction in which one of the isomers absorbs light and restores this energy to the system in the form of heat, was published by Nitzan et al. 16, 71. Here, the light absorption process and the increase of the system's temperature become autocatalytically coupled, leading to the occurrence of multiple steady states and hysteresis. The latter predictions have been experimentally confirmed in the case of the N 0 2 / N 2 0 4 dissociation reaction [8] and more recently in the S03F/S206F2 system [9]. As far as we are aware, no experimental examination of the phenomena pertaining the biological photoreactions has been yet attemped.In the present work, chloroplast thylakoids are immobilized [lo-121 in an artificial membrane lying on the bottom of a reactor fed with DCPIPox as the electron acceptor. The DCPl Pox species adsorbs strongly within the range of illumi- Ahhrc~viutions. DCPIPox, oxidized form of the 2,6-dichloroindophenol. nation wavelengths. Under such conditions we show that thc association between the purely photoassisted reduction of the DCPIPox, and its absorption by light intensity in the range of 600 -800 nm, leads to the appearance of an autocatalytic effect in the rate of reaction. More precisely, the yield of the photoreaction first increases and then decreases. as the DCPIPox concentration increases. The coupling of this nonlinear reaction term with a tra...

It has been shown that the coupling between the photoreduction of the oxidized form of dicliloroindophenol (an artificial electron acceptor) by thylakoids and the incident light intensity can lead to the appearance of multiple steady states when the system is operated under open conditions. In the present work, a numerical study and experimental evidence are presented on the occurrence of dissipative structures in an arrangement of two continuously stirred tank reactors with mutual mass exchange of dichloroindophenol through an inert membrane. The stable spatial structures are generated by the creation of transient internal and external asymmetries. A nontrivial hysteresis effect between symmetric and asymmetric stable steady states has been observed. (4)(5)(6). Scriven and co-workers (7-9) and Martinez and Baer (10, 11) have extended and generalized the work of Turing and thus far have analyzed the onset to instability in arbitrary networks of compartments. The coexistence of multiple stationary solutions with properties similar to those described by Martinez and Scriven has been found also in distributed reaction-diffusion systems (12-16).Few models are concerned with enzyme kinetics that of- MATERIALS AND METHODSPreparation of Thylakoids. Thylakoids were prepared from fresh lettuce (Latuca sativa, var. romaine) in a sorbitol buffer (pH 7.6) by the procedure of Epel and Neumann (23).Immobilization Procedure. The immobilization process, based on a method using a calcium alginate gel (24), was modified to obtain films rather than beads. The thylakoid suspension was mixed with an equal volume of a 4% sodium alginate solution in sorbitol buffer (330 mM, pH 7.6 Under our experimental conditions, the DCIPox exhibits an absorTtion peak centered at 600 nm (E600 = 17.9 mM--cm-'). No detectable absorbance of the reduced form (DCIPred) is measured for wavelengths between 500 and 800 nm. All measurements were carried out in a modified extraction buffer with no phosphate, 50 mM CaCl2, and 5 mM MgCl2-Abbreviations: DCIP, dichloroindophenol; CSTR, continuously stirred tank reactor; bp, bifurcation points; lp, limit points. 551The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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