1 CARBOHYDRATES: FIRST COUSINS OF WATER

摘要

1 INTRODUCTION Water has frequently been described as the most eccentric molecule in our ecosphere. Its many anomalous physical properties derive basically from the sp~3 hybridisation of the oxygen orbitals, which gives rise to an almost tetrahedral, quadrupolar bond orientation, in which the oxygen atom is placed at the centre of the water tetrahedron and four charges, two -OH groups (positive) and two lone electron pairs (negative) are directed towards the vertices, as shown in Figure 1. That, in itself, is not a unique molecular feature; for instance S1O_2 and GeO_2 have similar tetrahedral configurations. The unique features of H_2O are twofold: 1) its inability to form or participate in stable covalent bonds. Hydrogen bonding is thus the only method by which the molecule can interact with other molecules, including other water molecules, and 2) because of its quadrupolar nature with an equal number of proton donor and acceptor sites, H_2O is thus amphipathic, and as such, it can participate in a wide range of reactions: proton donor/acceptor, oxidation/reduction, and hydrolysis/aggregation. To some extent, carbohydrates are able to participate in the same types of reactions, although the rates in fused (vitreous) carbohydrates will differ vastly from those in liquid aqueous solutions.