Photochemical Properties of Chlorophyllin a

Chlorophyllin a can be photoreduced and can sensitize photoreductions and photooxidations in a manner similar to that of chlorophyll a. Neither the cyclopentanone ring nor magnesium is essential for photochemical activity. Chlorophyllin a (or chlorin a) undergoes a reversible photoreduction in the presence of ascorbic acid to give a product which can in a subsequent reaction either complex with pyridine to give a stable pink substance (the Krasnovsky intermediate) or reduce an azo dye. The kinetics of these reactions are presented and require the participation of a long-lived metastable state of chlorophyllin. The quantum yield of the Krasnovsky reaction is greater for red light excitation than for blue light. In sensitized photooxidation the metastable species react with oxygen to give an unstable intermediate which oxidizes the substrate. Binding of chlorophyllin to a high polymer enhances its photochemical activity.