Reaction kinetics and cytosine adducts of chloroethylene oxide and chloroacetaldehyde: direct observation of intermediates by FTNMR and GC-MS.

As it is not yet known which are the important miscoding adducts formed in the reaction of the relatively unstable compound chloroethylene oxide (CEO) with double-stranded DNA, proton FTNMR and GC-mass spectroscopy were used to directly detect and characterize reaction intermediates. Reaction of CEO with cytidine gave the (hydrated) 2-oxoethyl derivative at the N-3 position prior to ring closure to 3,N4-ethenocytidine; 5-methylcytosine gave an analogous reaction. However, reactions of CEO or chloroacetaldehyde (CAA) with 3-methylcytidine - i.e., with the N-3 blocked as in double-stranded DNA (ds DNA) - were shown by GC-MS of the silylated products to give, at a much slower rate, a pattern of at least 17 adducts all of which contained chlorine. Based on MS fragmentation and considerations of positional, optical and cis/trans isomerism, the reaction products of the 3-methylcytosine moiety were assigned as cis/trans N4-(2-chlorovinyl)-3-methylcytosine which may have arisen from the corresponding N4-(1-hydroxy-2-chloroethyl) adduct. It is postulated that formation of these cytosine-N4 adducts would be more rapid in double-stranded DNA than in the model compound, and that the N4-(2-chlorovinyl) group may be a miscoding adduct. The kinetics for CEO rearrangement, hydrolysis and nucleophilic attack have been studied by proton FTNMR and lead to the hypothesis that concerted nucleophilic attack by cytosine-N4 and CEO rearrangement produce the N4 adducts.