IMMOBILIZATION OF BIOPHOTOLYTIC MEMBRANES: SIMPLIFIED KINETIC MODEL OF PHOTOSYNTHETIC ELECTRON TRANSPORT

The ability to immobilize and stabilize thylakoid membranes from photosynthetic organisms (cyanobacteria, green algae, or higher plants) could facilitate their potential use as photocatalysts in processes for the production of hydrogen from water using solar energy (biophotolysis). The feasibility of immobilizing photosynthetic membranes with retention of their electron transport capability was explored in this study.; The method of immobilizing chloroplast thylakoid membranes which was observed to best preserve their photochemical activity involved the following steps: (1) mixing an aqueous suspension of thylakoid membranes (isolated from Spinacia oleracea) with an aqueous suspension of 1.5% (w/v) collagen; (2) adding glutaraldehyde to a concentration of 0.01% (w/v) and crosslinking for 20 minutes at 0(DEGREES)C; (3) casting the collagen-thylakoid mixture on a flat surface; and (4) freeze-drying the cast mixture to form a macroporous collagen-thylakoid film composite. Films prepared by this immobilization technique had a very open, sponge-like fibrous structure with a bulk density of 0.049 g/cm('3) of film.; Photosynthetic electron transport activities of free and immobilized thylakoid membranes were determined by measuring the photoproduction of oxygen with a Clark-type polarographic probe using potassium ferricyanide as the electron acceptor (Hill reaction).; A simplified kinetic model of the photosynthetic electron transport system (PETS) was formulated based on a reaction scheme which is consistent with current concepts of photosynthesis. The kinetic model relating electron transport rate (O(,2) evolution) to incident light intensity is a rectangular hyperbolic function, in agreement with the observed behavior of both immobilized and non-immobilized thylakoid membranes. Initial-rate data from batch assays were used to evaluate the two parameters of the kinetic model.; The operational stability of immobilized thylakoids during continuous exposure to incident light intensities approximating that of full sunlight (ca. 100 mW/cm('2)) was studied in a continuous-flow stirred tank reactor and a tubular batch recycle reactor. The immobilized thylakoids were observed to function for no more than forty minutes of continuous illumination (110 mW/cm('2)), with complete inactivation occurring during this period of operation. A reactor performance model was developed which can adequately simulate the complete transient behavior of the continuous-flow photocatalytic reactor in response to light.; While immobilization was found to improve the storage stability of thylakoid membranes in the dark, no evidence was found for an increase in operational stability during continuous illumination as a consequence of the immobilization.; Glutaraldehyde fixation of the cyanobacterium Anacystis nidulans was found to be much more effective in preserving the photochemical activity of cells when the fixation was conducted under conditions (0.75 M potassium phosphate buffer) which limit the dissociation of accessory light-harvesting pigments (phycobiliproteins) from the thylakoid membrane.; The results of the study demonstrate that thylakoid membranes isolated from higher plant cells can be immobilized while retaining their capacity to catalyze light-dependent electron transport reactions. The physical form of the immobilized thylakoid preparation (collagen-thylakoid film) was demonstrated to be suitable for use in continuous-flow reactor systems. However, the short operational lifetime of these photocatalytic films during continuous illumination prevents serious consideration of their use in applied process for solar energy conversion by the biophotolysis of water.