Methylation methods

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. 

Methylation of a carbohydrate is most practically achieved by using the Hakomori procedure,9 which has largely replaced the Purdie, Haworth, and Kuhn methylation methods. 

The procedure given above is an adaptation of the methylation method first used by Sommelet and Ferrand3 and developed more fully by Clarke, Gillespie, and Weisshaus.2 /3-Phenylethyldi-methylamine has been prepared from /9-phenylethylamine by alkylation with dimethyl sulfate 4 by the reaction of fi-phenyl-ethylamine and of N-methyl- S-phenylethylamine with formaldehyde 6 by catalytic reduction of phenylacetonitrile in the presence of dimethylamine 3 by the reaction of dimethylamine with /3-phenylethyl chloride 7 8 9 and with /3-phenylethyl bromide 9 and by the reaction of phenylacetaldehyde with dimethylamine.10... 

Shantha, N.C., Decker, E.A., and Hennig, B. 1993. Comparison of methylation methods for the quantitation of conjugated linoleic acid isomers. J. AOAC Inti. 76 644-649. 

Three methods are usually adopted for confirmation of ochratoxin A (a) methylation (methods A and B), (b) ammonia derivative formation, and (c) LC-MS confirmation. 

As in the case of other polysaccharides, the methylation method was applied to agar to clarify its structure. Percival and Somerville8 succeeded in obtaining methylated agar by the acetylation of commercial... 

Complete structural analysis of galactinol by the methylation method was reported66 by Kabat and associates in 1953. On hydrolysis, completely methylated galactinol gives 2,3,4,6-tetra-O-methyl-D-galactose (identified as the anilide) and 2,3,4,5,6-penta-O-methyl-D-myo-inositol (identified by oxidation to the penta-O-methylinosose, reduction, and demethylation, to give mpo-inositol and L-inositol). 

Ingle, T. R., and Whistler, R. L. (1964). End-group analysis by methylation. Methods In Carbohydrate Chemistry, Vol. IV, pp. 83-86. Academic Press, New York. 

Another methylation method makes use of a reaction with iodomethane and a catalyst [478], as follows. About 30 mg of an oil dispersion of sodium hydride was washed three times with anhydrous diethyl ether, dried under a mild stream of nitrogen and suspended in 1 ml of freshly distilled dimethyl sulphoxide. A 200-jul portion of this suspension was added to ca. 1 /amol of phenylurea dissolved in 100 pi of pure dimethyl sulphoxide followed immediately by 50 pi of iodomethane. Reaction was allowed to proceed for 5—10 min with continuous stirring. Then 1 ml of water was carefully added and the mixture was extracted with three 1-ml portions of diethyl ether or light petroleum. The extracts were dried with anhydrous sodium sulphate, filtered and evaporated to dryness under a stream of nitrogen. Silicone stationary phases [OV-225 at 108°C or a mixture of DC-200 and QF-1 (1 3) at 170°C] were used. 

Methylation. Methylation methods should be applied to the purified fractions, particularly as an end-group assay, but also for whatever information may be gained relative to the degree of branching. 

Such degradations may be achieved in several ways, namely by heating the base or its quaternary salt with acetic anhydride, hydrochloric acid, or sodium ethoxide and ethyl alcohol at temperatures above 130-160° C. or by Hofmann s exhaustive methylation method. 

Reductive alkylation. A previously reported A-methylation method for secondary amines with aqueous HCHO, Zn and HOAc is applicable to amino acids. Mono- or dimethy-lation can be controlled by adjustment of pH, reagent stoichiometry, and reaction time. a-Branched amines can be prepared from amines, RCHO and alkyl halides by the action... 

Reductive amination. An A-methylation method consists of ZnCh-catalyzed condensation of the amine with paraformaldehyde and treatment with Zn(BH4)2. 

Part 1 contains mass spectromctric El data for confirmation of results. Part 2 deals with the updated cleanup methods, including SPE with various types of sorbents. Part 3 describes some single residue methods (SRMs), The pertinent methods for OP.s and CMs arc method 261-378-370 for methomyl, carbendazim, and thiophenate-methyl method 378 for car-bendazim method 658-344 for cabosulfan and carbofuran method 522 for fonofus method 405 for glyphosate and method 441 for oxamyl. Part 4 deals with MRMs, and a pertinent method for OPs and CMs is method S8, an updated version of method S19. Method S25 is for methyl CMs. Thin-layer diromaiographic methods using the automated multiple development technique are described in part 6. 

The simplest methylation method uses KOH (2 M in methanol). It is recommended, for instance, in the analysis of vegetable oUs or virgin ohve oil with reduced acidity (small amounts of free acids) by several European standards. The reaction is rapid and occurs at room temperature. It must be pointed out that sterol esters and waxes, as well as free FAs, do not react under these conditions, as mentioned earher. 

The methylation method has been the principal means for determining the type of ring of the monosaccharide constituents and their points of attachment to one another in the oligosaccharide molecule. The usual procedure involves (1) complete methylation of the disaccharide (2) hydrolysis of the methylated disaccharide to partially methylated monosaccharide units, and their identification by comparison of their properties with those of known methylated compounds (3) determination of the unmethylated positions in each unit, these being the positions which were available for ring formation and for linkage between units and (4) determination of the ring type (pyranose or furanose) by supplementary evidence, such as determination of the rate of hydrolysis of the partially methylated lactone (31). 

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