Structure of the Red Fluorescence Band in Chloroplasts

Using Weber's method of "matrix analysis" for the estimation of the number of fluorescent species contributing to the emission of a sample, it is shown that the fluorescence¹ band in spinach chloroplast fragments at room temperature originates in two species of chlorophyll a. Emission spectra obtained upon excitation with different wavelengths of light (preferentially absorbed in chlorophyll a or b) are presented. Upon cooling to - 196°C, the fluorescence efficiency increases about twentyfold. Two additional bands, that now appear at 696 and 735 mµ, suggest the participation of four molecular species. Emission spectra observed at different concentrations of chloroplast fragments with excitation in chlorophyll a and b and excitation spectra for different concentrations of chloroplast fragments and measurements at 685 and 760 mµ are presented. Two of the four emission bands may belong to pigment system I and two to system II. The 685, 696, and 738 mµ bands respond differently to temperature changes. In the -196°C to -150°C range, the intensity of the 685 mµ band remains constant, and that of the 696 mµ band decreases twice as fast as that of the 738 mµ band.