Characterization of α-isosaccharinic acid:: Lactone and carboxylic conformations

The equilibrium concentrations of alpha-D-isosaccharino-1,4-lactone (ISA(L)) and the carboxylic alpha-D-isosaccharinic acid (HISA) conformations as a function of pH, as well as the rate of the structural rearrangements between the species, have been established in a 0.1 M NaClO4 matrix at 23degreesC using ion chromatographic analytical techniques. The equilibrium constant for the reaction HISA <---->(H+) ISA(L) + H2O was determined to be K-1 = 6.88 +/- 0.64 by curve fitting the experimental equilibrium data. The transformation rate from HISA to ISA(L) exhibits first-order behavior with a half-life of t(1/2) = 122.2 +/- 0.6 min. On the basis of kinetic data, the equilibrium constant was determined to be 6.60 +/- 0.07 with the forward and backward rate constants being k(+) = (4.85 +/- 0.03) x 10(-3) min(-1) and k(-) = (7.35 +/- 0.04) x 10(-4) min(-1), respectively, in agreement with the curve-fitted equilibrium constant. The transformation from ISA(L) to ISA(-) is more rapid and is complete in less than 8 min.