Hydroxyl pair

Figure 3.8 STM images of the same area on Ti02(l 1 0) at 357 K (V, = 1.5 V, /, = 0.1 nA) as a function of time (At = 60s) (a) clean Ti02(l 1 0) with bridging oxygen (BBO) vacancies (b) Ti02 (1 1 0) with a geminate hydroxyl pair formed by adsorption and dissociation of a water molecule. Hv marks the...

However, when the silica has been calcined at 800°C or above only chlorochromate is formed from chromyl chloride. This mimics the behavior of other reagents such as chlorosilanes, TiCl4, or BC13, which have been used to determine the extent of hydroxyl pairing on silicas (20-23). The chlorochromate species does not polymerize ethylene. 

The furanose form of D-fructose with an available 5 yn-periplanar anomeric hydroxyl pair (C2-C3) is p-D-fructofuranose, Figure 26 (D). At equilibrium, this spedes accounts for an enormous 25% of the total spedation of D-fructose in deuterated water at 31°C. This value can be contrasted with that of the... 

From a simplistic viewpoint, we indicated that a very general statistical trend appears to exist between the natural speciation of saccharide forms containing the xy -periplanar anomeric hydroxyl pair arrangement (a premise of the form s stability) and the qualitative trend observed for their stability constants K) with monoboronic acids. Table 2. 

Table 2 Saccharide structures containing a syn-periplanar anomeric hydroxyl pair, equilibrated percentages in D2O and stability constants with phenylboronic acid are tabulated for a selection of saccharides...

These results indicate that the negligible observed stability constants (.Kobs) documented for certain disaccharides cannot be considered solely a function of the availability of the anomeric hydroxyl pair. In the case of the reducing sugars, maltose and leucrose it is the inability of the sugars to adopt their furanose forms, and thus a s yw-periplanar alignment of the anomeric hydroxyl pair, that inhibits complexation (Schemes 34 and 35). 

Computational data and observed experimental results indicate that strong binding between boronic acids and saccharides occurs preferentially with saccharides that have an available anomeric hydroxyl pair, which has the capacity to conform to a j yn-periplanar alignment. In the vast majority of cases, this requires formation of the furanose form of the saccharide. 

As discussed above (see Section 4.10, The Importance of Pyranose to Fura-nose Interconversion, page 75) current thinking requires that we consider saccharidic forms where a 5yn-periplanar arrangement of the anomeric hydroxyl pair can be attained. Generally, this requires formation of the furanose form of the saccharide. However, computational work has shown that in the case of D-galactose the a-D-furanose form of the saccharide is not the only species that can be considered with a syn-periplanar alignment of the anomeric hydroxyl pair (Figure 32). 

The observed stability constants (Afobs) for compounds 140( =3)-145( =g) and 146(pyrene) with saccharidcs, which have the ability to form a furanose ring with a jjn-periplanar anomeric hydroxyl pair (o-glucose, melibiose, o-fructose and lactulose) are large, consistent with our understanding of the boronic acid diol interaction. Equally, the low or zero values of the observed stability constants (Afobs) for saccharides unable to form a furanose ring with a. syw-periplanar anomeric hydroxyl pair (maltose and leucrose) are also consistent with current thinking. 

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