The Mutarotation of Some Aldoses

The kinetics of the mutarotation of a series of nine 5-substituted aldoses of the D-glucose configuration and five 2-substituted glucoses, catalysed by a wide range of catalysts has been studied. It was found that substituents in the 5-position of the pyranose ring had a much greater effect on the kinetics of mutarotation than substituents in the 2-position of the ring. Electron withdrawing substituents (in the 5-position) were found to enhance the rate of the base catalysed mutarotation but to decrease the rate of acid and water catalysed mutarotation. Reasons for this are discussed with reference to the possible mechanisms of mutarotation. Steric hindrance of general base catalysis of mutarotation has been observed with the bases 2,6-lutidine, diethanolamine and tris-hydroxymethyl methylamine as catalysts. Attempts have been made to establish conclusively the reasons for this steric hindrance and to use it as a tool in distinguishing between kinetically indistinguishable mechanisms of mutarotation. The kinetic data of the acid and base catalysed mutarotation of the 5-substituted aldoses have been fitted to linear free-energy relationships (Taft and Hammett Equations) and found to give, at best, only a moderately good correlation. Reasons for this have been suggested. The spontaneous mutarotation of the anions of 6-deoxy-alpha-D-glucohepturonic acid and 6-O-(o-hydroxyphenyl)-alpha-D-glucose have been found to be faster than anticipated on the basis of the inductive effects of the substituent on the 5-position of the ring. It is proposed that these rapid mutarotations result from intramolecular catalysis by the carboxylate and phenolate groups respectively. The possible mechanisms are discussed and a preference expressed for that mechanism which involves intramolecular general acid catalysis of the sugar anion in the rate determining step, rather than the kinetically equivalent true intramolecular general base catalysis. A brief comment is made on the possibility of testing the hypothesis of "tautomeric catalysis". Finally, the appendix describes the attempted preparation of suitable substrates for a kinetic investigation of the enzyme ribonuclease A. Although a substrate for ribonuclease was prepared in situ, the difficulty in preparation and isolation of such substrates proved too great