Light-Induced Valence Change of a Manganese-Containing Chlorophyll Derivative

Mn-(III)-chlorin, Photosynthesis, Photoreduction, Photooxidation, Hydroxylamine K3(Mn-(III)-chlorin e6)acetate, a water-soluble chlorophyll derivative with manganese as central atom was prepared. The influence of reducing agents, light, and bicarbonate on the oxida­ tion states of manganese in the chlorin complex was investigated. In alkaline solutions the central Mn3+ was reducible to Mn2+. The illumination of the partially reduced complex first accelerated the reduction (light reduction: Mn3+ -> Mn2+), which was then followed by a strong reoxidation (light oxidation: Mn2+ -* Mn3+) continuing after switching off the light. This redox effect of light was obtained only in presence of hydroxylamine as reducing agent. Illumination time, intensity, and frequency had an influence on this effect. In the light/dark change the process was reversible. In regard to the requirement of manganese for the photosynthetic oxygen evolution the observed valence change of manganese in the chlorin complex was compared to the supposed reaction mechanism on the donor side of photosystem II.