Singlet oxygen and biological systems

Singlet oxygen is receiving increasing attention as a reactive species in many chemical and biological systems. It can be generated easily by microwave discharge, chemically or by photosensitized irradiation and also is found to be a common reactive atmospheric pollutant generated by solar radiation. Singlet oxygen reacts with a host of materials which are unaffected by “normal” oxygen. From a biological standpoint, there is strong evidence that living systems have mechanisms for the protection of molecules such as lipids and nucleic acids which would otherwise be damaged by exogenous singlet oxygen. Interestingly, other constituents such as chlorophyll and riboflavin found in living systems, are in turn excellent singlet oxygen sensitizers. Furthermore, there are indications that living systems may produce singlet oxygen internally, for example in the enzyme peroxidase-catalase system. The presence of singlet oxygen in biological systems has led to new theories on carcinogenesis. In view of these biological implications, a selective survey has been made of the current literature on singlet oxygen concentrating on the involvement of this species in the aforementioned processes. Examples from the chemical literature are presented which illustrate the scope of singlet oxygen reactions and/or serve as models for biochemical processes.