Periodate oxidation structure determination

Fortunately for carbohydrate chemistry, and with Prof. Adams insistence, Dr. Jeanes decided to accept a position at the National Institutes of Health (NIH) in Washington D.C., in the laboratory of the celebrated carbohydrate pioneer Claude S. Hudson. Her support came from one of the first Fellowships of the Corn Industries Research Foundation. With Hudson she developed methods using periodate oxidation to determine the structure of starches, marking the start of her long fascination with carbohydrate polymers. Dr. Jeanes then continued her studies of carbohydrates with Dr. Horace Isbell at the National Bureau of Standards (now the National Institute of Standards and Technology). 

The formation of a y-lactone in the periodate oxidation of demethylene-oxodeltamine (LII) and the fact that deltamine (XLVIII) is stable to periodate oxidation unequivocally determines the position of the hydroxyl at C-10. Since this hydroxyl was eliminated in the conversion of deltaline to delpheline, it seemed possible to represent the structure of deltaline as XLIII with some assurance. 

The use of periodic acid oxidation in structure determination can be illustrated by a case in which a previously unknown methyl glycoside was obtained by the reaction of D-arabinose with methanol and hydrogen chloride. The size of the ring was identified as five-membered because only one mole of periodic acid was consumed per mole of glycoside and no formic acid was produced. Were the ring six-membered, two moles of periodic acid would be required per mole of glycoside and one mole of formic acid would be produced. 

Periodic acid oxidation (Section 25.23) Vicinal diol and a-hydroxy carbonyl functions in carbohydrates are cleaved by periodic acid. Used analytically as a tool for structure determination. 

Pacsu4 5 has suggested a structure for starch involving a small number of non-cyclic hemiacetal linkages, the number being presumably sufficient to account for the number of endgroups determined by the methylation method. Halsall, Hirst and Jones6 have commented on this structure, however, and have shown it to be incompatible with the results of periodate-oxidation studies. In addition, these authors pointed out that it would be difficult to explain enzymic hydrolysis and dextrin formation on the basis of such a structure. 

The oxidation may be performed at pH 5.0 by addition of an aqueous solution of sodium metaperiodate. The reaction is allowed to proceed in the dark at 4 °C and after 24 hours the excess periodate is destroyed by the addition of ethylene glycol. The aims in periodate oxidation are to elucidate the number of neighbouring hydroxyl groups by estimating the number of moles of periodate consumed and to determine the structure of the moiety remaining after the reaction. The amount of periodate used in the reaction may be determined in several ways, including titrimetric and spectrophotometric methods. 

If the products of periodate oxidation are reduced with sodium borohy-dride, polyalcohols are produced which may be readily hydrolysed under mildly acidic conditions and the reaction products can be determined. The identification of these products gives considerable information about the linkages between the polysaccharide components and also their sequence in the overall structure. 

Exact analysis of sialic acid is required in biologieal experiments where the biological role of sialic acid is frequently studied with the aid of sialidases, and the amount of sialic acids released is determined. This is also important for periodate oxidation studies on biological systems, where modification of sialic acids by periodate is only assumed, but chemical analysis of this effect by isolation and analysis of the modified sialic acids is seldom performed. These uncertainties in determinations of sialic acid can be overcome by the purification procedures already described. Furthermore, it must be stressed that unequivocal determination of the structure of a sialic acid, especially... 

Periodate oxidation of sialic acids had earlier been used for structural determination of O-substituted sialic acids.89 Whereas, for example, one mole of a 4-O-acetylated sialic acid consumes 2 moles of periodate within 10-20 min at 0°, the same amount of the 7-O-acetyl isomer is oxidized by only one mole. As already discussed in connection with the periodic acid-thiobarbituric acid assay, 9-O-substituted sialic acids exhibit a very low rate of oxidation as compared with the unsubstituted sialic acids13 this observation originally led to the erroneous assignment89 of the 9-O-acetyl group to 0-8. All other sialic acids having O-acetyl groups on the side chain are expected to be unaffected by periodate. 

Ellis and Honeyman80 have raised strong doubts as to the validity of the structural determinations that have been applied to glycosylamines these doubts are based on the fact that such compounds are readily isomerized, as evidenced by their mutarotation. The formation of a trityl ether cannot be considered valid evidence for the presence of a primary hydroxyl group in the compound tritylated, and periodate oxidation, which is often excessive, may lead to faulty conclusions. 

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