Energy transfer from protochlorophyllide to chlorophyllide during photoconversion of etiolated bean holochrome

The photoconversion of protochlorophyllide to chlorophyllide in etiolated bean leaves or leaf extracts exhibits complicated kinetics that are neither simple first-order nor second-order with respect to the reactant. By comparing the chlorophyllide absorbance with the intensity of chlorophyllide fluorescence excited at wavelengths where both pigments absorb, we demonstrate that the kinetic complexity results from the transfer of electronic excitation from protochlorophyllide to chlorophyllide. Measurements of the polarization of chlorophyllide fluorescence indicate that efficient excitation transfer occurs at room temperature over pigment aggregates containing at least four molecules. The relative quantum efficiency of chlorophyllide-excited chlorophyllide fluorescence remains constant during photoconversion of holochrome or etioplast preparations. This result does not support the proposal of increasing exciton interaction between chlorophyllides during the course of photoconversion.