Azidoethidium derivatives as photoaffinity labels: study of the photochemistry of the monoazidoethidium derivatives in water

Abstract

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 studied in water. It has been established that both the product distribution and the quantum yield of photodecomposition are independent of excitation wavelength (313, 365, 436 nm). The quantum yields are also independent of dissolved oxygen concentration and are equal to 0.92 ± 0.18 for 2-azidoethidium and 0.57 ± 0.12 for 7-azidoethidium derivatives. Under irradiation of 2-azidoethidium derivatives ( 1a,b) two main products have been isolated: 7- N-(3-aminopropionyl)ethidium ( 5a,b) and substituted 6-ethyl-3-oxo-3,5-dihydroazepino [3,4 c] isoquinolinium ( 6a,b). Under irradiation of 7-azidoethidium derivatives ( 2a,b) more than ten products have been detected by high performance liquid chromatography and two of them, 2- N-(3-aminopropionyl)ethidium ( 7a,b) and substituted 7-hydroxy-8-aminophenanthredinium ( 8a,b), have been isolated. The yields of the corresponding amines 5a,b and 7a,b substantially decrease in the presence of oxygen and these products are formed in the reaction of triplet nitrenoethidium derivatives with water. The yields of the compounds 6a,b,8a,b and other products are oxygen independent and they are formed in the reaction of singlet intermediates. Quantum chemical calculations of structure and formation enthalpy of intermediates have been performed by the AMI method. The difference in the photochemistry of the 2- and 7-azidoethidium derivatives is likely to be caused by the different thermodynamics of the isomerization of 2- and 7-nitrenoethidium derivatives into potential intermediates.