AZIDOETHIDIUM DERIVATIVES AS PHOTOAFFINITY LABELS - STUDY OF THE PHOTOCHEMISTRY OF THE MONOAZIDOETHIDIUM DERIVATIVES IN WATER

Photolysis of the 2-azido-7-N-(3-aminopropionyl)ethidium (1a,b) and 7- azido-2-N-(3-aminopropionyl)ethidium (2a,b) derivatives has been studi ed in water. It has been established that both the product distributio n and the quantum yield of photodecomposition are independent of excit ation wavelength (313, 365, 436 nm). The quantum yields are also indep endent of dissolved oxygen concentration and are equal to 0.92+/-0.18 for 2-azidoethidium and 0.57+/-0.12 for 7-azidoethidium derivatives. U nder irradiation of 2-azidoethidium derivatives (la,b) two main produc ts have been isolated: 7-N-(3-aminopropionyl)ethidium (5a,b) and subst ituted thyl-3-oxo-3,5-dihydroazepino[3,4-c]isoquinolinium (6a,b). Unde r irradiation of 7-azidoethidium derivatives (2a,b) more than ten prod ucts have been detected by high performance liquid chromatography and two of them, 2-N-(3-aminopropionyl)ethidium (7a,b) and substituted 7-h ydroxy-8-aminophenanthredinium (8a,b), have been isolated. The yields of the corresponding amines 5a,b and 7a,b substantially decrease in th e presence of oxygen and these products are formed in the reaction of triplet nitrenoethidium derivatives with water. The yields of the comp ounds 6a,b, 8a,b and other products are oxygen independent and they ar e formed in the reaction of singlet intermediates. Quantum chemical ca lculations of structure and formation enthalpy of intermediates have b een performed by the AM1 method. The difference in the photochemistry of the 2- and 7-nitrenoethidium derivatives is likely to be caused by the different thermodynamics of the isomerization of 2- and 7-nitrenoe thidium derivatives into potential intermediates.