Ring opening photoreactions of cytosine and uracil with ethylamine

The photochemical reactions of cytosine (Cyt) and uracil (Ura) with ethylam ine, an analog of the side chain of the amino acid lysine, have been studie d. After irradiation of Cyt in aqueous ethylamine at lambda = 254 nm, N-(N' -ethylcarbamoyl)-3-aminoacrylamidine (Ia) and N-(N'-ethylcarbamoyl)-3-ethyl aminoacrylamidine (Ib) were isolated as products, while irradiation of Ura gave N-(N'-ethylcarbamoyl)-3-aminoacrylamide (IIa) and N-(N'-ethylcarbamoyl )-3-ethylaminoacrylamide (IIb) as products. Studies in which Ia and IIa wer e incubated with ethylamine at various pH values indicate that Ib and IIb a re secondary products produced via thermal reactions of Ia and IIa with eth ylamine. Heating of Ia and Ib leads to ring closure with the resultant form ation of 1-ethylcytosine; small amounts of 1-ethyluracil are also produced. Heating of IIa and IIb produces 1-ethyluracil as the sole product. Spectro scopic properties were determined for each of these opened ring products, a s well as for N-(N'-ethylcarbamoyl)-3-amino-2-methylacrylamidine (III) and N-(N'-ethylcarbamoyl)-3-amino-2-methylacrylamide (IV). Quantum yield measur ements showed that Ia was formed with a Phi of 1.6 x 10(-4) at pH 9.8, whil e Phi for formation of IIa was 7.2 x 10(-4) at pH 11.5. A profile of the re lative quantum yield for formation of Ia, determined as a function of pH, s howed that the maximum quantum yield occurs at around pH 9.5; the analogous profile for IIa shows a maximum quantum yield at pH 11.3 and above. Aceton e sensitization does not produce Ia in the Cyt-ethylamine system, which ind icates that the known triplet state of Cyt is not involved in reactions lea ding to this opened ring product.