Concentration and temperature dependences of the redox potentials of neutral and alkaline solutions of glucose, mannose, galactose, galacturonic acid, and xylose were measured. Axial orientation of the secondary hydroxy groups in epimeric hexopyranoses, oxidation of C 6 to carboxy group, and the lack of primary hydroxy group in going to aldopentopyranose favor increasing (in the order of listing) destabilization of their cyclic structure, formation of the oxo form, and thermally activated retro-aldol reaction of the latter in alkaline medium. This leads to enhancement of their reducing power and chemical reactivity toward 5,5′,7,7′-tetrabromoindigo.Diversity of natural low-molecular carbohydrates and polysaccharides is determined by specificities of stereoisomeric structure of monoses both in crystalline and solvated states. Numerous publications contain data on stereochemical behavior of pyranose ring, solvation, and specific thermodynamic and thermochemical properties of hexoses and pentoses with different configurational orientations of hydroxy groups in their cyclic forms [1][2][3][4][5][6]. This is responsible for deviations of physicochemical parameters of solutions of monosaccharides, their reactivity, and biochemical activity, which are related in turn to tautomeric transformation from cyclic structure to oxo form. Reductive properties of carbohydrate solutions have found various practical applications in the field of organic chemistry, including initiation of chemical transformations of heterocyclic compounds of the thiazine and thiazole series [7], stabilization of solutions of sulfite reducing agents [8], binding of dissolved oxygen in copper-ammonia solutions of cellulose while determining degree of its polymerization [7], biochemical methods for analysis of catalytic properties of enzymes that decompose polysaccharide compounds [9], etc. Antioxidant activity of monosaccharides is used in cosmetology and medicine [10,11].However, up to now there are no systematized published data on relations between the reducing power of sugars and steric configuration of cyclic forms of their molecules and the presence of substituents therein. Therefore, the goal of the present work was to reveal how stereoisomeric and chemical structure of polyhydroxy aldehydes affects their reducing power in solution.As subjects for study we selected chemically pure glucose (Glc), mannose (Man), galactose (Gal), xylose (Xyl), and galacturonic acid (GalA); the structures of their predominant anomers are shown below.
