Reversible addition of bisulfite buffer to the cytidine ring system |
| |
Authors: | Mamta Gautam-Basak Dan G. Jacobson Eugene G. Sander |
| |
Affiliation: | Department of Biochemistry and Biophysics, Texas Agricultural Experiment Station, Texas A&M University System, College Station, Texas 77843 USA |
| |
Abstract: | The rate constants for the reversible addition of protons and sulfite to the 5,6 double bond of cytidine and 3-methylcytidine have been spectrophotometrically measured under conditions (25°C, μ = 1.0 ) where the deamination of 5,6-dihydrocytidine-6-sulfonate is minimal. Both the addition and the elimination of sulfite from the ring system are subject to general catalysis of proton transfer. For the reaction in either direction, plots of the pseudo-firstorder rate constants against increasing buffer concentration are biphasic and indicative of at least a two-step reaction pathway with both steps being subject to general acid-base catalysis. Kinetic hydrogen-deuterium isotope effects were measured for both buffer-catalyzed steps of sulfite elimination from 3-methyl-5,6-dihydrocytidine-6-sulfonate and sulfite addition to 3-methylcytidine. Both H2O and D2O were used as solvent. For both the addition and the elimination of SO32− values of k2H/k2D were 6.3–7.1 and 2.3–2.6 at low and high imidazole buffer concentration, respectively. The large isotope effects values in the range of 6–7 can be attributed to rate-determining proton transfer to carbon-5 of the cytidine ring system. The smaller values are more likely caused by proton transfer to a electronegative atom such as the oxygen on carbon-2 of the cytidine ring. The equilibrium constants for bisulfite buffer addition to 3-methylcytidine and cytidine at 25°C, μ = 1.0 , pH 7.2, are 10.2 and 1.3 −1, respectively. |
| |
Keywords: | To whom inquiries should be addressed. |
本文献已被 ScienceDirect 等数据库收录! |
|