The photoinduced exchange reactions of cytosine (Ia) and 5-methylcytosine (IIa) with N alpha-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 alpha-acetyl-L-lysine and cytosine-N alpha-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 epsilon-amino groups of lysine residues in proteins upon UV irradiation of nucleosomes and other DNA-protein complexes under physiological conditions.