Search

The tautomerism of D -galactose in water has been studied by using g.l.c. to separate per- O -trimethylsilyl derivatives of D -galactose tautomers formed during the mutarotation reaction. The initial, rapid stage of the biphasic mutarotation of α- D -galactopyranose is due to the formation of α- D -galactofuranose and β- D -galactofuranose. The slow phase is due to the formation of β- D -galactopyranose. The kinetic data of D -galactose tautomerism have been tabulated. The free-energy difference between pyranose tautomers (anomers) is primarily entropic. The free-energy difference between pyranose and furanose tautomers is distributed between relatively large and opposing enthalpic and entropic energies. It is concluded that the “transition state” for these tautomerization reactions is quite unlike the structure of any of the tautomers, and that, for the activation of a compound for tautomerization reactions, the same entropy and enthalpy exists for the formation both of furanoses and pyranoses.

THE molecular feature common to the many different sweet tasting compounds has been sought for many years1. For the sugars, it was proposed2–5 that the sweet unit is the glycol group, and that intensity of sweetness varied inversely with the degree to which glycol OH groups appear to be intramolecularly hydrogen bonded. It is now apparent that vicinal OH groups in the glycol unit need to be approximately gauche, or in a staggered conformation. Vicinal OH groups which are in the anti conformation apparently are too far apart to cause sweet taste. Glycol OH groups which are eclipsed probably participate in an intramolecular hydrogen bond which competitively inhibits interaction of glycol with the receptor site. These steric features of sweet and non-sweet sugar glycol units are shown in perspective in Fig. 1. Glycol conformational parameters, and the gross conformation of pyranose and furanose rings, have been used to explain the varying sweetness of the sugars2–5.

IN an attempt to identify the feature common to various and seemingly unrelated compounds which taste sweet, we suggested1 that the saporous unit of all sweet tasting compounds was a bifunctional group consisting of an acidic (AH) and a basic (B) moiety with an A—H proton to B distance of about 3 A. The common unit described seems to be very similar to that proposed by Warfield2, in which the common molecular feature was visualized as a “taste couple” composed of a proton and a neighbouring, unshared electron pair.