Infrared Frequency and Dissociation Constant of the Carboxylate Group
THE hexuronic acids of polyuronides such as the alginates and chondroitin sulphates vary in their acid dissociation constants (pKa) and in their abilities to react with periodate1. Both phenomena could be explained by the formation of a hydrogen bond between the carboxylate group and a hydroxyl group at C2 or C3 of the uronic acid1. Hanrahan2 inferred that the increase in frequency (ν) of the antisymmetric stretching vibration of the ionized carboxylate group in the mono-ions of dicarboxylic acids is due to the formation of CO−2 … HO hydrogen bonds. A similar increase in ν was found by Nakamoto3 when the carboxylate group underwent monodentate coordination by metal ions.