Covalent Addition of Bisulfite Ion to N-Alkylated Uracils and Thiouracils

Abstract

Rate and equilibrium constants at 25º and I 1.0 M have been calculated for the covalent additon of bisulfite ion across the C6=C5 double bond of uracil, 1-methyluracil, 1,3-dimethyluracil, uridine, uridine-5'-diphosphate (dianion), adenylyl(3'-5')uridine, uridylyl(3'-5')adenosine, 2-thiouracil and 1,3-dimethyl-2-thiouracil. Substitution of either uncharged (e.g. ribose) or charged (e.g. ribose 5'-diphosphate dianion) groups for the N1 proton of uracil resulted in a small reduction in both the rate and equilibrium constants for the formation of the adduct but had very little effect on the rate of elimination of bisulfite ion from the adduct. On the other hand, substitution at N3 of uracil substantially reduced both the rates of formation and decomposition of the covalent adduct. The rates of formation of the covalent adducts of 2-thiouracils were faster than those of the uracil adducts but alkyl substitution produced similar decreases in the magnitudes of rate and equilibrium constants.