Horace S. Isbell

Mechanisms for hydroperoxide degradation of disaccharides and related compounds

Abstract The reactions of disaccharides with alkaline hydrogen peroxide were studied under diverse conditions. Treatment of cellobiose, lactose, and maltose with aqueous sodium peroxide afforded, in each instance, the corresponding aldobionic acid, the next lower aldobionic acid 2- O - d -glucopyranosyl- d -erythronic acid, and formic acid. On the other hand, melibiose and gentiobiose afforded the corresponding aldobionic acid, the next lower aldobionic acid, and a 2- O - d -glycopyranosylglycolic acid. The yields of products varied widely with the experimental conditions, especially with the proportions of alkali peroxide and hydrogen peroxide. The reactions with alkali peroxide were slow, but rapid in the presence of hydrogen peroxide with the gradual addition of alkali. The results indicate that degradation of carbohydrates by alkaline hydrogen peroxide takes place by five reaction paths. These are designated the alpha-hydroxy hydroperoxide cleavage-mechanism, the Baeyer-Villiger mechanism, the ester mechanism, the dihydroxy-epoxide mechanism, and a newly proposed peroxy-radical mechanism. The last-named mechanism is more rapid than the others. With an excess of hydrogen peroxide and slow addition of alkali, it results in rapid, stepwise conversion of both reducing and nonreducing saccharides into formic acid. The process begins with formation of a hydroperoxide adduct of the carbohydrate. Reaction of the adduct with hydrogen peroxide affords a peroxy radical and a hydroxyl radical. The peroxy radical decomposes, affording formic acid, the next lower aldose and hydroxyl radical. Hydroxyl radical produced in a chain reaction oxidizes alditols and aldonic acids by reactions analogous to those of the Fenton reagent.

Contribution of the reaction pathways involved in the isomerization of monosaccharides by alkali

Abstract The isomerization of eight hexoses and four pentoses, in aqueous KOH at pH 11.5 under nitrogen, was monitored by l.c. on cation (Pb 2+ )-exchange and reverse-phase columns. The primary epimerization reaction of pentoses and hexoses accounts for ∼90% of the saccharides found in solution after one week. The remaining saccharides were formed in part by fragmentation and recombination. This is indicated by the formation of hexoses (glucose and sorbose) during the isomerization of pentoses, and by the simultaneous formation of 3- and 4-epimeric hexuloses from hexoses. The aldol reaction responsible for the formation of hexoses (glucose, fructose, and sorbose) from glyceraldehyde at pH 11.5 was found to be fast ( t 1/2 = 2 h), which accounts for the rapid fragmentation and recombination observed.

Evidence of stable hydrogen-bonded ions during isomerization of hexoses in alkali

Abstract Epimeric pairs of aldohexoses and the related ketohexose were isomerized in aqueous KOH at various temperatures and pH values, and the mixtures then analyzed by h.p.l.c. on either a cation-exchange resin or a reversed-phase column. It was found that the proportions of starting aldohexoses remaining after several days often exceeded those of the same that were formed from the epimeric aldoses and the corresponding ketoses. The difference with allose, gulose, and mannose was much larger than with the other aldohexoses. These differences are rationalized by assuming that anomers having the OH groups attached to C-1, C-2, and C-3 in an axial—equatorial—axial or an equatorial—axial—equatorial arrangement form especially stable, hydrogen-bonded ions or molecular complexes that disturb the equilibrium state and affect the isomerization and mutarotation reactions.

Oxidation of sodium salts of alduronic and glyculosonic acids by sodium peroxide

Abstract Because, in the presence of a large excess of alkaline hydroperoxides, aldoses are oxidized stepwise to formic acid, it was expected that alduronic acids would be degraded to formic acid and carbon dioxide by the mechanism previously proposed. However, in addition to the products expected, substantial yields of oxalic acid were found, indicating a second mechanism, proposed here. With alkali-metal hexuronates, one mechanism yields five moles of formic acid and one mole of carbon dioxide per mole of substrate, whereas the second mechanism yields four moles of formic acid and one mole of oxalic acid. The results show that, under highly alkaline conditions, the two mechanisms are of approximately equal importance. Oxidation of a D - xylo -5-hexulosonate begins with the cleavage of glycolic acid, and this is followed by degradation of the resulting tetruronic acid by the two mechanisms described for the degradation of hexuronic acids. With 2-hexulosonates, also, two reaction-mechanisms appear to be necessary to account for the products formed under highly alkaline conditions; the main reaction (80%) yields one mole each of carbon dioxide and pentonic acid per mole, whereas the other yields one mole of oxalic acid and four moles of formic acid. Under moderately alkaline conditions, both the alduronic acids and the 2-keto acids react almost entirely by the mechanism that yields carbon dioxide; no detectable amount of oxalic acid was found. In all cases, small amounts of unknown products, not further investigated, are formed. The following compounds were studied: sodium D -galacturonate, sodium D -glucuronate, potassium D -mannuronate, sodium D - lyxo -2-hexulosonate, sodium D - arabino -2-hexulosonate, calcium D - xylo -5-hexulosonate, and glyoxylic acid.

Transformations of sugars in alkaline solutions : Part II. Primary rates of enolization

Abstract A method has been developed for studying the enolization of sugars in D2O, and determining by infrared absorption the DOH formed. The method does not require isotopically labeled substrates and does not involve a primary isotope-effect. It can be applied to reducing sugars in general and to diverse reaction-systems. By use of radioactivity measurements in conjunction with measurements of infrared absorption, the isotope effect kT/kH, for the release of hydrogen atoms from deuterated d -glucose-2-t, was measured and found to be 0.13.

Degradation of reducing sugars and related compounds by alkaline hydrogen peroxide in the presence and absence of iron and magnesium salts

Abstract Traces of either ferrous or ferric salts greatly increase the rate of the stepwise degradation of reducing sugars by alkaline hydrogen peroxide, as measured by formation of formic acid; addition of larger proportions of iron salts causes relatively smaller effects. The results showed that, unless unusually strict precautions are taken to exclude traces of iron, the free-radical cleavage of the hydroperoxide adducts of reducing sugars is far more rapid than the ionic cleavage. The catalytic effect of iron salts is counteracted by addition of magnesium salts. With d -glucose, inhibition of the catalytic effect of iron by magnesium depends on both the magnesium-iron ratio and the concentration at a given ratio. Measurements with various molar proportions of the salts indicated that a magnesium-iron complex, containing six atoms of magnesium to one of iron, is formed. Presumably, removal of iron by formation of this complex inhibits the free-radical degradation of hydroperoxide adducts. In marked contrast to the results obtained with reducing sugars, the degradation of potassium glyoxylate and of glyoxal by alkaline hydrogen peroxide is extremely rapid, and not catalyzed by iron or inhibited by magnesium. The results are in accord with an ionic, rather than a free-radical, cleavage of the hydroperoxide adducts of these compounds. The rapidity of the ionic reaction may be attributed to the ready availability of an electron pair from the adjoining carbon atom.

Degradation of 2-deoxyaldoses by alkaline hydrogen peroxide

Abstract Reaction of 2-deoxy- D - arabino -hexose, 2-deoxy- D - lyxo -hexose, and 2-deoxy- D - erythro -pentose with alkaline hydrogen peroxide in the presence of magnesium hydroxide afforded the corresponding 2-deoxyaldonic acid, the 1,4-lactone, and the 1- O -formyl derivative of the next lower alditol. The 2-deoxyaldonic acids were separated in 60–80% yields, as new, crystalline lithium salts. The 1,4-lactones were obtained under conditions that precluded intermidiate formation of the free acids: presumably, the reaction proceeded by way of an intermediate, furanosyl hydroperoxide, which was converted into the lactone by elimination of water. With an excess of alkaline hydrogen peroxide, in the absence of magnesium hydroxide, the substrates were degraded to formic acid, with concurrent decomposition of hydrogen peroxide. It is shown that decomposition of hydrogen peroxide is catalyzed by hydroperoxide anion, and that it takes place by both a chain, and a non-chain, process. The decomposition reactions afford an abundant source of hydroxyl radical capable of oxidizing a wide variety of compounds.

Reactions of methyl aldohexopyranosides with alkaline hydrogen peroxide

Abstract Reaction of methyl α- d -glucopyranoside and methyl α- d -mannopyranoside with alkaline hydrogen peroxide and a ferrous salt, at room temperature and below, afforded the corresponding d -glycosiduronic acids. On dehydration, the acids gave the corresponding gamma lactones, with a shift of the pyranoid ring to a furanoid ring. Surprisingly, the glycosidic methyl group was retained during the oxidation reactions and pyranose-furanose interconversions. This retention is rationalized by a mechanism involving formation of a pseudo-acyclic intermediate. Another unexpected reaction was the conversion of slightly moist methyl d -glucopyranosiduronolactone syrup, on standing for 5–6 days at room temperature, into crystalline d -glucofuranurono-6,3-lactone, and of methyl α- d -mannopyranosidurono-6,3-lactone into crystalline d -mannofuranurono-6,3-lactone.

Preparation of D-mannitol-C-14 and its conversion to D-fructose-1-(and 6)-C-14 by acetobacter suboxydans

The Bureau recently undertook a program for the development of methods for the production of a variety of position-labeled sugars [l]. The preparation of D-glucose-1-C and D-mannose-1-C in high yield was described in a previous report [2]. This paper deals with the production of D-mannitol-1-C and its conversion to D-fructose-l-(and 6)-C by oxidation with Acetobacter suboxydans. Prior to the present investigation, randomly labeled fructose had been prepared by photosynthesis [3], but position-labeled fructose had not been reported. I t was known that D-mannitol could be obtained by reduction of D-mannose, and that it could be converted to D-fructose by oxidation with Acetobacter suboxydans [4]. D-Mannitol differs from D-mannose in that the ends of the molecule are alike. For this reason a 1-labeled mannitol (I) can be considered as either 1or 6-labeled, and any unsymmetrical derivative prepared from this substance would be 1,6-labeled, provided the carbon chain remains intact. Thus, by oxidation of D-mannitol-1-C one would obtain D-fructose-l-(and 6)-C (II).

Oxidation of Monosaccharides with Oxygen in Alkaline Solution. Separation, Identification and Estimation of the Aldonic Acids Produced by Liquid Chromatography1,2

Abstract A reliable liquid chromatographic method has been developed for the separation, identification and estimation of the products formed when saccharides are oxidized in alkaline solution with oxygen. The technique was then used to quantitatite the acids produced when glucose is subjected to such an oxidation. The Bio-Rad cationg exchange column HPX-87H+, when eluted with 0.01 N H2SO4 and attached to a UV detector, was found to be satisfactory. Quanitative estimation of the products formed during the oxidation of glucose was achieved by withdrawing aliquots from the reaction mixture and injecting them directly into the chromatographic system. It was found that about 90% of the oxidation products formed were produced via a 1,2-enediol and less than 10% via a 2,3-enediol. The results confirmed the mechanism proposed by Isbell to account for the acids produced.