Photoreduction of Protochlorophyll and Its Derivatives

Abstract—Photoreduction of protochlorophyll and a series of its derivatives (with gradual simplification of the structure) was studied in ascorbic acid‐propanol‐pyridine mixtures. Additions were introduced into the solutions to suppress some side processes. Conditions were found for the photoreduction of pigment on the 7,8‐linkage, with the formation of corresponding chlorins. Chlorin yields depended on the nature of the metal in the centre of the pigment molecule. The yield of chlorophyll upon photoreduction of protochlorophyll was 30 per cent, and the yields of Zn‐derivatives of protochphyll was about 35–80 per cent. Photochemically prepared chlorins were isolated by chromatography. Some differences were discovered between their electronic, infrared, and nuclear magnetic resonance spectra and those of natural chlorins. These differences result from chlorins with a cis arrangement of hydrogen atoms on the 7,8‐linkage being selectively formed upon photoreduction, while natural chlorins have the trans arrangement. The mechanism of the photoreduction reaction is discussed.