CHLOROPHYLLS IN POLYMERS .1. STATE OF CHLOROPHYLL-A IN UNSTRETCHED POLYMER SYSTEMS

Model systems for the study of energy transfer processes are useful fo r the elucidation of the various factors governing the mechanism of en ergy transfer in photosynthetic systems. Here we describe the characte rization of two systems, consisting of chlorophyll a incorporated in a nhydrous nitrocellulose and polyvinylalcohol films. First, optical spe ctroscopy and time-resolved fluorescence techniques are used to charac terize the state of the chlorophyl:l molecules in the films. We find t hat in nitrocellulose films the state of chlorophyll a depends strongl y on the ratio of nitrocellulose to dimethylsulfoxide in the solutions from which the films are cast. The state of chlorophyll a in polyviny lalcohol films does not depend on the amount of polymer originally dis solved in dimethylsulfoxide. In these films the pigment is monomeric a t low concentrations of chlorophyll a, but aggregates are formed at mu ch lower concentrations than in nitrocellulose. The latter fact is exp lained by the existence of peckers in polyvinylalcohol, leading to hig h local concentrations. To further test the suitability of the nitroce llulose polymer films as model systems for energy transfer processes, time-resolved fluorescence anisotropy profiles are measured in depende nce of the concentration of pigments in the matrix. Fits of the observ ed decay profiles to the predicted decay show good correspondence, as long as no traps are present. Furthermore, the fitted decay times yiel d the correct value of the Forster radius R(0) as compared to the valu e obtained spectroscopically. We thus conclude that the chlorophyll a- nitrocellulose system can be very appropriate for the study of energy transfer processes between photosynthetic pigment, since the pigments are uniformally distributed in the matrix.