Carbohydrates stability constants

Stability Constants of Some Carbohydrate And Related Complexes by Potentiometric Titration... 

If it is true that the structural form of D-glucose which reacts with boric acid is the a-D-pyranose form, then that form probably exists in a boat or twist conformation in the complex. This implies that the study of the stability constants of sugar borate ester might give information about the ability of various carbohydrates to form such boat or twist conformations (10, 21). 

The measure of the strength of complex-formation between a cation (X" ") and a carbohydrate (Carb) is the stability (or formation) constant, defined as K = [Carb X ]/[Carb][X ]. The determination of the stability constants of cation-sugar complexes has not been wholly satisfactory. The constants being comparatively small, their accurate determination is difficult. The... 

Where / is the RTP intensity for a particular concentration of carbohydrate /o is the initial intensity without carbohydrate while /nm is the limiting intensity K is the stability constant of the receptor with guest [C] is the concentration of carbohydrate. The stability constants of BrQBA-carbohydrate complex were obtained as 2.6x103 mol/L for fructose, 1.8xl03 mol/L for galactose, 1.6xl03 mol/L for glucose and 1.3x 103 mol/L for mannose, respectively. 

Reports of the kinetics of ceric oxidation of a variety of different alcohols have been made. The substrate molecules include mono-, di- and trihydroxyalkanes, cyclo-alkanols (cychc alkane alcohols) and phenols (table 3). Hydroxyacids have also been investigated and will be discussed in the section on carboxylic acids. Ceric oxidation of carbohydrates is discussed along with aldehydes and ketones. In those studies with excess substrate, the dominant products (where discussed) are the corresponding aldehydes and ketones. The most remarkable aspect of these investigations is that the resolved values for the stability constants of many of the precursor complexes exceed those observed in analogous Ce(lV)-carboxylic acid oxidations. 

The majority of easily detected compounds at solid anodes under constant applied potentials are self-stabUized via tt-resonance. Therefore, a desirable characteristic of electrodes in dc amperometry is inert. The electrode serves as a sink to provide and remove electrons with no direct involvement in the reaction mechanism. Since TT-resonance does not exist in polar ahphatic compounds (e.g., carbohydrates), stabilization of reaction intermediates is actively achieved via adsorption at clean noble metal electrodes. Faradaic processes that benefit from electrode surface interactions are described as electrocatalytic. Unfortunately, an undesirable consequence of this apiproach is the accumulation of adsorbed carbonaceous materials, which eventually foul the electrode surface. 

Discussion of the effect of ligand structure on protein-carbohydrate affinity requires an evaluation of complex stability constants. A munber of biophysical techniques are appropriate for the study of protein-carbohydrate interaction many of the more enlightening strategies are the topics of separate chapters elsewhere in this volume. We describe below three techniques used extensively in glycobiology— inhibition of hemagglutination, enzyme-linked lectin assay (ELLA), and isothermal titration microcalorimetry—and we consider the types of information provided by each technique in order to facilitate appropriate interpretation of the data. 

Ascorbic acid (0.8% w/v) in aqueous solution degraded according to apparent first order kinetics, with a rate constant of 2.34 x 10 2/hour, when irradiated by artificial sunlight [40]. The presence of 5% aspartame in the solution decreased the rate constant to 1.48 x 10 2/hour, thus stabilizing ascorbic acid to photochemical degradation by about 37%. Similar effects were also seen with some carbohydrate sweeteners. 

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