Methoxyl groups, estimation

Cumming s Modification.—(J. S. 0.1., 41, 20.)—A convenient apparatus for the estimation of methoxyl groups by the Zeisel method consists of a long-necked round-bottomed flask attached by a ground-glass joint to a bulbed U-tube. The methyl iodide generated by the interaction of hydriodic acid and the methoxyl group is absorbed in alcoholic silver nitrate. Pyridine may also be used as absorbent. (J. C. S., 117, 193.) For purification of hydriodic acid, see p. 506. 

For methoxyl and ethoxyl group estimations (p. 479) hydriodic acid (B.P. 127°) is distilled over phosphorus in a current of carbon dioxide. 

Estimating the reactivity of various aromatic nuclei in lignin involves studies to determine the protodedeuteration rates of certain selectively deuterated model compounds. The combined effects of hydroxyl, methoxyl, and side-chain substituents are approximately additive in guaiacyl models while syringyl models show significant deviation from the predicted behavior. The reactivity of the position meta to methoxyl groups is higher while those of the ortho and para positions are lower than anticipated on the basis of combined substituent effects. 

Many methylated sugars are known only in an amorphous state, but even those that are crystalline are frequently difficult to crystallize unless material is available for seeding. From the Rr value of an ether, a tentative estimate of the number of methoxyl groups can be made and, in some... 

At 400°C methanol replaced about 30% of the hydroxyl groups to form methoxyl groups. Failure to displace all of the hydroxyl groups may have been due to small traces of residual water in the reagents. From quantitative analysis of the methoxyl groups by oxidation to COa, McDonald estimated absorption coefficients as 3 x 104 cm2/mole for CH at 2857 and 2959 cm-1, 2 x 10 cm2/mole for CH at 2995 cm-1, and about 106 cm2/mole for OH at 3750 cm-1. However, these absorption coefficients may be subject to large error. 

Since lignin is not a uniform entity, chemical criteria for its characterization have centred around analytical detenninations of its functional groups, e.g. total hydroxyl content, phenolic hydroxyls (56), methoxyl and other ether groups, benzyl alcohol groups (7a), carbonyl groups (6), etc., and estimations of its content of special structural features, e.g. phenylcoumaran units (5), biphenylyl linkages (123), etc. 

Circulating catecholamines have an estimated half-life of about one circulation time (20 seconds). They are rapidly taken up by various tissues (notably the liver), where they may exert their effects before they are inactivated. DA may be converted to NE after uptake by nerve endings, kidney, heart, and other tissues. Two enzymes responsible for inactivation of catecholamines are present in most tissues but are particularly abundant in the liver. Catechol-O-methyltransferase (COMT) is a cytosolic, Mg +-dependent enzyme that catalyzes methoxylation of catecholamines at the hydroxyl group at position 3. COMT... 

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