The dehalogenation of halocytosines by bisulfite buffers

• Published in: Bioorganic chemistry 1975-01, Vol.4 (1), p.72-83
• Main Authors: Jacobson, Dan G., Sedor, Frank A., Sander, Eugene G.
• Format: Article
• Language: English
• ISSN: 0045-2068; 1090-2120
• DOI: 10.1016/0045-2068(75)90027-9

Both 5-bromo- and 5-iodocytosine are rapidly dehalogenated in dilute bisulfite buffers to yield cytosine. With 5-bromocytosine, but not with 5-iodocytosine, extrapolation of semilogarithmic plots of extent reaction versus time indicates the bisulfite buffer concentration-dependent formation of an intermediate which subsequently reacts to control the rate of 5-bromocytosine dehalogenation. The disappearance of both halocytosines has a second-order dependence on bisulfite buffer concentration. Both imidazole and acetate buffers catalyze the reaction of 5-iodocytosine, but not that of 5-bromocytosine, with bisulfite. In the case of acetate buffer catalysis of the reaction of 5-iodocytosine with bisulfite, the dependence of the observed rate constants changes from first order to zero order as a function of increasing buffer concentration. The observed rate constants for 5-bromocytosine dehalogenation increase, reach a maximum at about 4.5, and then decrease as a function of pH. Iodometric titration of sulfite utilization coupled with spectrophotometric analysis of pyrimidine reactants and products indicates that 1 mole of sulfite is consumed per mole of halocytosine dehalogenated. The spectrophotometrically determined p K a values for the conjugate acids of 5-bromo- and 5-iodocytosine at 25°C and ionic strength 1.0 M are 3.25 and 3.56, respectively. These results are discussed in terms of a multistep reaction pathway which is analogous to the bisulfite-catalyzed dehalogenation of the 5-halouracils.