Carbohydrates equilibrium solution

Chemistry of Small Carbohydrates - Equilibrium Solution Properties, Pure Appl. Chem. 59, 1189 (1987) and references dted therein. [160] L. A. Fedorov, D. N. Kravtsov, and A. S. Peregudov Metallotropic Tautomeric Transformations of the a, a-Type in Organometallic and Complex Compounds, Usp. Khim. 50, 1304 (1981) Russ. Chem. Rev. 50, 682 (1981). 

Franks, F. (1987). Physical chemistry of small carbohydrates-equilibrium solution properties, PureAppl Chem., 59 1189. 

A coirplete understanding of the role of carbohydrates in biological systems requires knowledge of the distribution at equilibrium of the various conformers in aqueous solution. The conformational behavior of carbohydrates in solution can be examined from different vantage points (1,), but the most relevant approach is, no doubt, study of dilute solutions themselves. At present, high resolution NMR spectroscopy is the primary tool for determination of three-dimensional structure of oligosaccharides in solution. Optical rotation is also very sensitive to conformation (2) and there is a new, semi-enqpirical theory of optical rotation of oligosaccharides ( ). 

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

Further structural studies of this material have been reported.78 In strain 49 and X6C61, this hexose is replaced by the C-5 epimer, a-o-Galp. L-Altrose was purified by preparative paper chromatography after hydrolysis of the polymer, and the L-configuration was confirmed by its optical rotation. L-Altrose is transformed into 1,6-anhydroaltrose on treatment with aqueous acid the acid catalyzed equilibrium is reached when the latter compound constitutes 60-65% of the total carbohydrate in solution. Apparently, the enantiomeric D-altrose has not been found in Nature. 

Studies of carbohydrates in solution have included an investigation of the intramolecular hydrogen bonding of methyl 4,6-O-benzylidene-a-D-hexopyrano-side derivatives. Diols, monomethyl ethers, and monodeoxy-compounds with various configurations were studied in the 3600 cm" region. A similar investigation was carried out on methyl ethers and benzylidene derivatives of D-aldo-pyranoses. A laser Raman study of D-fructose in aqueous solution indicated that furanose forms could be distinguished from pyranoses and that at equilibrium the ratio of the two forms was 41 59. ... 

Purify 2-deoxy-D-allose by two reciystallisations from absolute EtOH. The p-nitrophenylhydrazone has m 61-62°, [a]i 5 -55° (MeOH). An equilibrium solution at 31° in D2O contains 15% a-pyranose, 58% p-pyranose, 12% a-furanose and 15% P-furanose forms as estimated by HNMR spectroscopy, [see Angyal Adv Carbohydrate Chem Biochem 42 15 1984 for ratio of anomers in solution, Zoibach Ollapally J Org Chem 129 1790 1964] [Beilstein 1IV4283.]... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

Although carbohydrates exist almost entirely as cyclic hemiacetals m aqueous solution they are m rapid equilibrium with their open chain forms and most of the reagents that react with simple aldehydes and ketones react m an analogous way with the carbonyl functional groups of carbohydrates... 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

The reaction of Pt(C03)(dppp)] with a modest excess of vicinal diols in CH2C12 solution affords the corresponding [Pt(a,/3-diolato)(dppp)] species under equilibrium conditions, a reaction that is readily reversed by the addition of dry ice to the product. The reaction with triols such as glycerol and alditol carbohydrates also affords the corresponding diolato species, with the reaction exhibiting excellent equilibrium regioselectivities for a number of isomers, of which the 7, 6-threo diols are the most favored. 

A further informative example is the organic matter—CO2 couple, which is the principal reductant in natural systems. Consider a solution in equilibrium with atmospheric CO2 at neutral pH and containing 10p,M CH2O , where CH2O represents average organic C in natural systems, whose composition is similar stoichiometrically to that of carbohydrates. The half reaction is... 

Molecular dynamics (MD) simulation have been used for several years to get information on both equilibrium and dynamical conditions of various systems, including solutions of complex molecules. However, only a few carbohydrates have been studied (1-3). 

The reaction of carbohydrates in alkaline or acidic aqueous solutions results in a myriad of products, many of which have been recognized for well over a century. The number of identified products has greatly increased in recent years, owing to the development of sophisticated techniques for separation and identification. With the exception of anhydro sugars and oligosaccharides, found as concentration-dependent, equilibrium constituents (reversion products) in acidic solutions, all of the products result from reactions of intermediates present in the Lobry de Bruyn-Alberda van Ekenstein transformation. 

Knowledge of the composition of sugars in solution is fundamental to carbohydrate chemistry. The physical and chemical properties of the sugars in solution depend on the proportions of their various forms and their biological properties may also show such dependence. Enzymes that utilize these sugars as substrates may not be able to use each of the forms. Where only a single form is utilized, the other forms may either be converted into the reactive form or may function as inhibitors. The latter is especially important if the reactive form is present in very low proportion at equilibrium. It is possible that the substrate form is utilized faster than it is generated from the other forms the observed rate of the reaction is then that of the tautomeric interconversion.4... 

The equilibrium compositions of aqueous solutions of some aldoheptoses are listed in Table III. Because the additional carbon atom in the side chain does not introduce additional steric interactions, the composition of solutions of heptoses is similar to that of the homomorphous hexoses, with only one exception, namely, n-glycero-n-ido-heptose, 92 a-D-Idopyranose in solution is a mixture of the 4Ci and 1C4 conformant) S. J. Angyal and R. J. Beveridge, Carbohydr. Res., 65 (1978) 229-234. 

Some carbohydrates actively inhibit the crystallization of lactose, whereas others do not. Carbohydrates that are active possess either the /3-galactosyl or the 4-substituted-glucose group in common with lactose, so that adsorption can occur specifically at certain crystal faces (Van Krevald 1969). (3-Lactose, which is present in all lactose solutions [see Equilibrium in Solution (Mutarotation )], has been postulated to be principally responsible for the much slower crystallization of lactose compared with that of sucrose, which does not have an isomeric form to interfere with the crystallization process (Van Krevald 1969). Lactose solubility can be decreased substantially by the pres-... 

Examination of Figure 9.11 demonstrates an excellent separation of a-glucose from (3-glucose which points out the power of GLC for the separation of these materials as well as a significant complication in the study of carbohydrates. It is well known that solutions of some carbohydrates undergo anomerization and that an initially pure form of a sugar may result in an equilibrium mixture (mutarotation) as in Figure 9.12. 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a. and p structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1 ratio of p-D-glucose to a-D-glucose. Although their chemical constituents are identical, the biochemical properties between the a and the P forms can be quite different. Monosaccharides linked together to form disaccharides and polysaccharides cannot continue to interconvert and are therefore frozen in the a or p forms. Changing one monosaccharide in a complex carbohydrate to its opposite... 

Calorimetric measurements yield enthalpy changes directly, and they also yield information on heat capacities, as indicated by equation 10.4-1. Heat capacity calorimeters can be used to determine Cj , directly. It is almost impossible to determine ArCp° from measurements of apparent equilibrium constants of biochemical reactions because the second derivative of In K is required. Data on heat capacities of species in dilute aqueous solutions is quite limited, although the NBS Tables give this information for most of their entries. Goldberg and Tewari (1989) have summarized some of the literature on molar heat capacities of species of biochemical interest in their survey on carbohydrates and their monophosphates. Table 10.1 give some standard molar heat capacities at 298.15 K and their uncertainties. The changes in heat capacities in some chemical reactions are given in Table 10.2. 

---