The Origin of Life and Evolutionary Biochemistry : Springer : 9781468421170 : 1468421174 : 14 Mar 2012 : The primary metabolism of protobionts was probably based on the electron transfer reactions regulated by catalysts or photo­ sensitizing 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 pig­ ments formed abiogenically (porphin, chlorin) as well as their more complicated biogenic analogs (chlorophylls) are capable of photo­ sensitizing electron transfer in systems, having various degrees of molecular complexity. The inorganic photosensitizers (titanium dioxide. zinc oxide. etc.). being excited in the near ultraviolet. 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 compound