Pathways of Chemical Evolution of Photosynthesis

Abstract

The primary metabolism of protobionts was probably based on the electron transfer reactions regulated by catalysts or photosensitizing pigments. The action of photoreceptive pigments was inevitable in the case of electron transfer leading to light energy storage in the reaction products. The primitive tetrapyrrolic pigments formed abiogenically (porphin, chlorin) as well as their more complicated biogenic analogs (chlorophylls) are capable to photosensitize electron transfer in systems, having various degree of molecular complexity. The inorganic photosensitizers (titanim dioxide, zinc oxide, etc.). being excited in near UV are able to perform the same reactions as porphyrins — electron transfer from donor to acceptor molecule (including photoreduction of viologens) or water molecule photooxidation (oxygen liberation), coupled with reduction of ferric compounds and quinones. The inorganic photosensitizers are not used in biological evolution; actually the inorganic ions entered into tetrapyrrolic cycle, forming effective photocatalysts. Inclusion of pigments into primary membranes led to elaborated coupling between pigments and enzymatic systems. The involvement of the excited pigments into the biocatalytic electron transfer chain served as prerequisite of effective function of photosynthetic organisms.