Detection of singlet oxygen production by ESR

Abstract

SINGLET molecular oxygen is a very powerful oxidant. Its action is important in a variety of chemical and biological processes^1–4, for examples dye-sensitised photooxidation of lipids, proteins and nucleic acids⁴, photodynamic inactivation of viruses⁵ and cells⁴, phototherapy of cancer^6,7, carcinogenesis⁸, haemolysis of erythocytes⁹, sensitisation of the human skin⁴ and degradation of food⁴. The methods used to detect singlet oxygen are unspecific, of low sensitivity or laborious. Photooxidation of 1,3-diphenylisobenzofuran seems to be the most widely used diagnostic test for ¹O₂. However, in the absence of additional control experiments this test does not prove the intermediacy of ¹O₂ (ref. 4) and 1,3-diphenylisobenzofuran has very low solubility and dimerises in aqueous solutions. Lion et al.¹⁰ have proposed a new method to detect ¹O₂ involving the generation of stable nitroxide radicals when ¹O₂ reacts with the sterically hindered amine 2,2,6,6,-tetramethylpiperidin. When using this method to detect ¹O₂ in neutral aqueous solutions, we found no radical production. We report here our investigation of this problem, as it is biologically important to be able to detect ¹O₂ production in such solutions.