Photoreduction of Chlorophyll a in the Presence of Ascorbic Acid in Pyridine Solutions.

Abstract

Illumination with red light brings about the photoreduction of chlorophyll a in O2-free, aqueous pyridine solutions containing ascorbic acid. The quantum yield of the photoreduction is 0 in dry pyridine and rises to about 5% in 30% aqueous pyridine. Water is apparently required to ionize ascorbic acid (the ascorbate anion being the effective electron donor). The 1st recognizable product of the light reaction is pink reduced chlorophyll, described by Krasnovsky, which can be oxidized back to chlorophyll by safranine T. Hydrogen ions, formed by the ionization of ascorbic acid, react rapidly in the dark with pink reduced chlorophyll to form blue-gray reduced pheophytin, the absorption spectrum of which was measured. The blue-gray derivative also arises directly when an aqueous solution of ascorbic acid and pheophytin is irradiated with red light. In the dark, pheophytin is formed from reduced pheophytin. In aqueous pyridine solutions, the photoreduction of chlorophyll is not reversible because of the conversion of reduced chlorophyll to reduced pheophytin; the photoreduction of pheophytin is, however, entirely reversible, i.e., photoreduction in the light and back reaction in the dark can be carried out several times without attendant, irreversible changes.