Photoexchange products of cytosine and 5-methylcytosine with Nα-acetyl-l-lysine and l-lysine

Abstract The photoinduced exchange reactions of cytosine ( Ia ) and 5-methylcytosine ( IIa ) with N α-acetyl- l -lysine, a derivative of the common amino acid l -lysine, were studied. These reactions of Ia and IIa at pH 7.5 produce, respectively, 2- N -acetylamino-6-(1-cytosinyl)hexanoic acid ( Ib ) and 2- N -acetylamino-6-(1-(5-methylcytosinyl))hexanoic acid ( IIb ) as major final products. In addition, small amounts of the corresponding deamination products were formed in the 5-methylcytosine- N α-acetyl- l -lysine and cytosine- N α-acetyl- l -lysine systems, namely 2- N -acetylamino-6-(1-thyminyl)-hexanoic acid and 2- N -acetylamino-6-(1-uracilyl)hexanoic acid. The compounds Ib and IIb were deacetylated by acid hydrolysis to yield the corresponding lysine products: 2-amino-6-(1-cytosinyl)hexanoic acid ( Ic ) and 2-amino-6-(1-(5-methylcytosinyl))hexanoic acid ( IIc ). The compound Ic was identified as a product in the photoreaction of cytosine with l -lysine at near neutral pH, while IIc is found as a product in the corresponding reaction of 5-methylcytosine. The occurrence of the above photoexchange reactions at, pH values near those found in physiological systems could have biological implications; in particular, our observations suggest that cytosine and 5-methylcytosine residues, contained in DNA, might react with the ϵ-amino groups of lysine residues in proteins upon UV irradiation of nucleosomes and other DNA-protein complexes under physiological conditions.