The cyclobutane dimers of 5-methylcytosine and their deamination products

Abstract

The photochemical reactions of 5-methylcytosine (m⁵C), a minor component of mammalian DNA, have been studied at a concentration of 2 mM in frozen 10 mM aqueous NaCl solution at dry ice temperature (194.5 K). For these studies, low-pressure lamps emitting mainly UVB radiation were used. We have isolated and characterized three cyclobutane dimers, namely the cis–anti (c,a) the cis–syn (c,s) and the trans–syn (t,s) forms. While the c,a and the t,s cyclobutane dimers are relatively stable towards deamination upon standing in solution at 277 K, the c,s isomer is gradually converted into the corresponding c,s m⁵C-thymine (Thy) mixed dimer; this latter reaction occurs considerably faster at 310 K. The t,s cyclobutane dimer is converted into the corresponding m⁵C-Thy mixed dimer upon incubation at 373 K, while the c,a dimer is converted into a mixture of m⁵C and c,a mixed dimer when incubated at 310 K. Irradiation of equimolar mixtures of Thy (1 mM) and m⁵C (1 mM) under similar conditions yields each of the three m⁵C cyclobutane dimers, as well as significant amounts of c,a, c,s and t,s m⁵C–Thy mixed cyclobutane dimers. These m⁵C–Thy dimers undergo decompositions similar in nature to the processes undergone by m⁵C cyclobutane dimers. Pseudo-first order rate constants for deamination of the c,s m⁵C homodimer and c,s m⁵C–Thy heterodimer at various temperatures and at pH 7.7 have been measured and the enthalpies and entropies of activation have been evaluated for the deamination processes for these two compounds. The two dimers have half-lives of about 14 and 22 h, respectively, at 310 K; however, at 273 K, the corresponding half-lives can be evaluated as being around 30 and 36 days, respectively.