Adsorption selectivity of sugars toward hydrous zirconium(IV) and hydrous iron(III) oxide surfaces

To substantiate stereochemical interactions between sugars and hydrous metal oxides, a selection of sugars including seventeen monosaccharides and sucrose were adsorbed on hydrous zirconium oxide (HZO) and hydrous iron oxide (HIO) suspended in aqueous solutions and their strengths of adsorption were compared according to the Langmuir monolayer adsorption isotherm. Among the eighteen sugars, the largest Langmuir constant a was observed for D- and L-ribose (a=37±5 and 35.0±0.7, resp.) and the smallest was for α-methyl-D-glucoside (a=1.0±0.2). Intermediate values were obtained for mannose, galactose and mannitol (a=17±2, 18±1 and 16±3, resp.). The configuration of a set of three neighboring hydroxy groups located either on the C(2), C(3) and C(4) carbon atoms of aldoses or on the C(3), C(4) and C(5) carbons of ketoses has a major effect on the Langmuir constant. The strength of adsorption of sugars decreased in the order ax–eq–ax>ax–eq–eq>eq–eq–eq>*–eq–eq (or *–eq–ax or eq–*-eq), where eq and ax indicate that the respective hydroxy groups are equatorial and axial toward the rings of the sugar molecules, and the asterisk indicates that the hydroxy group is replaced by a H– or CH3O– group. In addition, it is possible to correlate the order of a values more closely to the geometry of sugar molecules by considering the contribution from all hydroxy groups, together with hemiacetal oxygen atoms in the molecules. The numbers of NCONF and NTCSC were counted by manipulating a stereochemical molecular model of sugars and the ratio of R=NTCSC/NCONF was calculated; here, NCONF was the total number of conformations derived from a given sugar molecule and calculated by considering the existence of (i) α- and β-anomers, (ii) 1C- and C1-conformers, (iii) three rotamers arising from rotations around the C(5)–C(6) bond of aldohexoses and around the C(1)–C(2) bond of ketohexoses. NTCSC was the number of sets of three oxygen atoms included in the sugar molecule and expected to come close to each other to form a triangle, circumscribed by the smallest circle having an approximate radius of 1.6±0.2 Å. The Langmuir constant a was found to be linearly dependent on the ratio R=NTCSC/NCONF, the regression line being expressed by the equation a=28R+1.