Student Theses and Dissertations

Date of Award

1965

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Abstract

One of the major problems in the study of photosynthesis is understanding the role which the structure of the photosynthetic apparatus has in influencing the nature and efficiency of the photochemical electron-transfer reactions of chlorophyll. This research has involved the study of a simple model for the chloroplast with the purpose of gaining some insight into this problem. It is known that chlorophyll and related molecules will photosensitize oxidation-reduction reactions both in solution and in the adsorbed or colloidal state. The model chosen as being most closely related to chlorophyll in vivo has the pigment adsorbed to the surface of small particles. Pheophytin was used in these studies because chlorophyll was sometimes degraded by the particle surface. A method was developed to study quantitatively the photosensitized reduction of an azo dye in the highly scattering particle suspensions. Measurements of the absorption and fluorescence properties of the coated particles were made to learn more about the state of the pheophytin on the particle surface. The effect of the surface was differentiated from the effect of interactions among the pigment molecules by varying both the type of surface and the concentration of the pheophytin on the particle surface. At less than 0.01 surface coverage of pheophytin on nonpolar polystyrene particles, the quantum yield of photosensitization is the same as that with the pigment in 90% methanol solution, within a factor of three. The quantum yield falls to 1/2 this maximum value only at 0.4 coverage. In this region the absorption spectra are broadened toward longer wavelengths, indicating aggregation of the pheophytin. The fluorescence also decreases with increasing surface concentration and is 50% quenched at 0.03 coverage. The quenching of monomer fluorescence is accounted for by energy transfer to the very weakly fluorescent aggregates, the number of which is estimated by a one dimensional Ising model. Analysis of these results shows that small aggregates are photoreactivev but probably not as active as the monomers. On a polar surface (zeolite particles) the absorption spectra are always broadened and the fluorescence yield is very low for all surface concentrations of pheophytin. The photochemical activity increases with increasing coverage, but at best only reaches the low yield of the 1.0 coverage polystyrene. The type of surface thus has a profound effect on the adsorbed pigment. This model shows that adsorption to an interface does not necessarily affect the photochemical activity of the adsorbed pigment and that photochemical reactions can occur at relatively high pigment concentrations. Comparison shows that the particle model has some properties which are strikingly similar to those of the photosynthetic apparatus. Further studies of this model, e.g., the stepwise adsorption of additional reactants known to be present in the chloroplast, should lead to a greater understanding of the photochemical reactions in photosynthesis.

Comments

A thesis submitted to the Faculty of The Rockefeller Institute in partial fulfillment of the requirements for the degree of Doctor of Philosophy

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