The Thermodynamics of Mutarotation of Some Sugars: Ii. Theoretical Considerations

Abstract

iln il~terpretatio~~ of A H ancl A S for the nu tarot at ion of D-glucose, D-uylose, maltose, ldctosc, and ccllobiosc is given in terms of changes in the hydmtion of the reducing 11)1clro\yl group I n this part of the paper we shall attempt to give an iilterpretation of the experirneiltal results shown in Table I and Fig. 4 of Part I." We shall first consider the conformations of the sugars used in I'art I and note that in all of them the neighboring groups of the reducing carbon are identical.There is a large body of evidence for strong hydrogen bonding betweell the sugar hydroxyl groups and adjacent water nlolecules in aqueous solution. On the basis of the known coilformatioils of the sugars and of the structure of water it is possible to come to tentative conclusions about the packing of the water molecules about the sugar molecule, and to predict that an equatorial hydroxyl group will be more stroilgly hydrated than an axial group. We also fiild that the reducing hydroxyl of the sugars of Part I is bonded to the adjacent water molecule in the same manner, with only slight variations in bond length.The large value of A S for mutarotatioll of the various sugars call be associated with changes in the frequency of torsional oscillatioil of this reducing hydroxyl group about the C(I,-O axis. On the basis of a simple model of the hydrogen bond between the group and the water molecule, the order of magnitude of this frequency can be calculated and shown to be low enough to give a large contributiotl to the entropy.Using the hydrogen bond model, the order of magnitude of the variation of A S ancl A I I amongst the sugars is calculated, and the slope of A I I vs. A S in Fig. 4 of Part I can be estimated theoretically.Finally A F for the nlutarotatioil of the sugars call be viewed in the light of the free energies of interactioil between adjacent hydroxyl groups in the cyclitols which have beell the subject of thermodynamic study by Angyal and McHugh (1). Because the pyranose ring contains an oxygen atom, whereas the cyclitol ring does not, a difference exists between group interactions in the two cases. CONFORi\lI:lTIONS OF T H E SUGARSReeves (2) has shown that of the two chair and six boat coilformations possible lor the hexopyranoses the chair conformatiotls are ilormally preferred. More recent work in Reeves' laboratory (3, 4) has indicated the possibility of a boat conformation in some of the pyranoses, the tenclency of hydroxyl groups to take up equatorial positioils apparently overcoming the normal stereochemical preference for the chair conformation. 'JIanuscript received Novernber 12, 1957.

THE CONFORMATIONS IN SOLUTION OF trans-CYCLOHEXENE DIHALIDES AND cis- AND trans-1,2-CYCLOHEXANEDIOLS AND DERIVATIVES

“…The differences (so-called A-values) seem greater in protic solvents (3, p. 236) ; 0.8 (7370 aqueous acetic acid), 0.9 (water), 0.96 (isopropanol), 1.25 (deuterium oxide) (28). This change in A-value on change from aprotic to protic solvents has been assigned t o solvation phenomena (38,39). For the purpose of illustrating the present status of our appreciation of the conformation of the trans-diol, it is useful t o assume average A-values for the hydroxyl group of 0.6 kcal/mole (aprotic solvent) and 1.0 kcal/mole (protic solvent).…”

Pyranose–furanose and Anomeric Equilibria: Influence of Solvent and of Partial Methylation

“…In dimethyl sulfoxide, the hydrogenbonding pattern should be different; based on the chemical shifts of hydroxyl protons, both axial and equatorial groups are strongly bonded by this solvent (1,5,16,17). Hence, if the OH-2 group of mannose (or OH-4 of arabinose) were now to be reoriented away from the molecule by association with the solvent, the oxygen orbitals should become directed inwards (3). Molecular models suggest that this arrangement .vvould increase repulsive interactions between these orbitals and the erected para orbital of the ring oxygen (see ref.…”

“…I t was surmised initially that water might selectively stabilize pyranose forms of some of the sugars by hydrogen bonding, and thereby mask the relatively high stability of their furanose forms. hilore effective hydrogen bonding with \\later is also regarded as a factor that should stabilize /3-pyranoses (equatorial OH-1) more than the a-anomers (3,12). In attempting to relate the observed changes in equilibrium in the t\vo solvents with such hydrogen-bonding possibilities and with structural features that inight be common to, for example, arabinose and mannose, one or two possibilities suggest themselves.…”

Thermal Analyses of Polyol-Aqueous Solutions at Temperatures below 0.DEG.C..

“…4). Kabayama & Patterson ( 1958), Tait et al (1972) and Uedaira et a/ ( 1989) independently reported that there was a close correlation between hydration and the number of equatorial -OH groups or the number of dynamic hydration in reducing sugars. The slight difference in the curve of glucose In Fig.…”