Ring opening photoreactions of cytosine and uracil with ethylamine

The photochemical reactions of cytosine (Cyt) and uracil (Ura) with ethylamine, an analog of the side chain of the amino acid lysine, have been studied. After irradiation of Cyt in aqueous ethylamine at lambda = 254 nm, N-(N'-ethylcarbamoyl)-3-aminoacrylamidine (Ia) and N-(N'-ethylcarbamoyl)-3-ethylaminoacrylamidine (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 were incubated with ethylamine at various pH values indicate that Ib and IIb are secondary products produced via thermal reactions of Ia and IIa with ethylamine. Heating of Ia and Ib leads to ring closure with the resultant formation of 1-ethylcytosine; small amounts of 1-ethyluracil are also produced. Heating of IIa and IIb produces 1-ethyluracil as the sole product. Spectroscopic properties were determined for each of these opened ring products, as well as for N-(N'-ethylcarbamoyl)-3-amino-2-methylacrylamidine (III) and N-(N'-ethylcarbamoyl)-3-amino-2-methylacrylamide (IV). Quantum yield measurements showed that Ia was formed with a phi of 1.6 x 10(-4) at pH 9.8, while phi for formation of IIa was 7.2 x 10(-4) at pH 11.5. A profile of the relative quantum yield for formation of Ia, determined as a function of pH, showed 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. Acetone sensitization does not produce Ia in the Cyt-ethylamine system, which indicates that the known triplet state of Cyt is not involved in reactions leading to this opened ring product.