Methylation of carboxyl groups

The carboxyl group of a fatty acid must be depolarized by methylation in order to facilitate gas chromatographic separation or separation by fractional distillation. Reaction with diazomethane is preferred for analytical purposes. Diazomethane is formed by alkaline hydrolysis of N-nitroso-N-methyl-p-toluene sulfonamide. 

The gaseous CH2N2 released by hydrolysis is swept by a stream of nitrogen into a receiver containing the fatty acid solution in ether-methanol (9 1 v/v). The reaction  

All unsaturated fatty acids which contain an isolated cis double bond absorb UV fight at a wavelength close to 190 nm. Thus, the acids can not be distinguished spectrophotometrically. 

---

B. Methylation of carboxyl groups Bacterial glutamyl Aspartyl Protein phosphatase 2A Ras Section D,5 Box 12-A Section C,3 Section D,3... 

Methylation of nucleic acids is considered further in Chapers 27 and 28. Methylation of carboxyl groups... 

Therefore these groups have to be blocked by derivatization. This can be done in one step by trimethylsilylation, but since trimethylsilylesters are easily hydrolyzed a two step derivatization -first by methylation of carboxylic groups followed by... 

After calcium enters secretory cells, how does it trigger secretion This question cannot be answered with certainty, but a few pertinent theories have been formulated. First, calcium may activate certain cytoplasmic enzymes such as protein carboxyl methylase, thereby causing methylation of carboxyl groups on granule membrane surfaces (25). This change in granule membrane surfaces may promote the interaction between granule membrane and plasma membrane and initiate the secretory process. 

The reaction of S,6-0-isopropylidene-L-ascorbic add with superoxide followed by methylation of carboxyl groups has afforded oxidation products 46 and 47. In contrast, 5,6-0-isopropylidene-3-0-methyl-L-ascorbic add under the same conditions gave only 47 in 82% yield. ... 

The blocking and deblocking of carboxyl groups occurs by reactions similar to those described for hydroxyl and amino groups. The most important protected derivatives are /-butyl, benzyl, and methyl esters. These may be cleaved in this order by trifluoroacetic acid, hydrogenolysis, and strong acid or base (J.F.W. McOmie, 1973). 2,2,2-Trihaloethyl esters are cleaved electro-lytically (M.F. Semmelhack, 1972) or by zinc in acetic acid like the Tbeoc- and Tceoc-protected hydroxyl and amino groups. 

Pectins are generally classed according to their ester content as high methoxyl pectins (>50% of the carboxyl groups esterified) or low methoxyl pectins (<50% of carboxyl groups esterified) (pectic acid, methyl ester [9049-34-1]). Low methoxyl pectins, like algins, require calcium for gelation. 

To aid in determining the number of carboxyl groups, prepare a derivative using trideuterated Methyl-8 (Pierce cat. no. 49200), using the same procedure previously given. Inject 1-2 pd of the trideuterated methyl ester separately or mix equal portions of the nondeuterated methyl ester with the trideuterated methyl ester, and inject 2 pd immediately into the GC/MS system. From the mass difference, it is easy to determine the number of carboxyl groups present. 

B. Method for determining the number of —COOH groups in a molecule The TMS derivative of an acid can be converted to the methyl ester using anhydrous methanolic HQ. 1 2 3 Obtain a mass spectrum of the TMS derivative of the acid, and then evaporate the TMS reaction mixture with clean, dry nitrogen. Add 250 pd of anhydrous methanolic HC1 (Pierce cat. no. 33050) and heat at 60° for 20 min. Many TMS derivatives of acids are converted to methyl esters at room temperature after 20 min. If the sample is rerun as the methyl ester, the number of carboxyl groups can be determined by the mass differences before and after making the methyl ester from the TMS derivative. 

Hayama et al.132 discussed the catalytic effects of silver ion-polyacrylic add systems toward the hydrolyses of 2,4-dinitrophenylvinylacetate 84 (DNPVA) by using the weak nudeophilicity of carboxylic groups and the change-transfer interactions between olefinie esters and silver ions133Metal complexes of basic polyelectrolytes are also stimulating as esterase models. Hatano etal. 34, 13S) reported that some copper(II)-poly-L-lysine complexes were active for the hydrolyses of amino acid esters, such as D- and L-phenylalanine methyl ester 85 (PAM). They... 

FIGURE 15.1 Scheme showing PLC group fractionation of soluble organic matter into fractions of aliphatic hydrocarbons, aromatic compounds with application of urea clathra-tion, and methylation of carboxylic acids in polar fractions based on experimental data given in Reference 36 to Reference 52, Reference 77 to Reference 81, and Reference 88 to Reference 89. 

Characteristic markers of Venice turpentine are larixol and larixyl acetate, but in many cases they may remain undetected, as larixyl acetate hydrolyses and the methylation of hydroxyl groups is not as effective as for carboxylic acids. On-line derivatization with tetramethylammonium hydroxide (TMAH) is, for instance, scarcely effective on hydroxyl groups and involves a number of secondary reactions (isomerization, dehydration and cleavage of hydrolysable bonds) due to the strong alkalinity of the TMAH solution. When the experimental conditions are such that THM of labdanes occurs larixol and larixyl acetate are detected as a single peak since in the derivatization process larixyl acetate is hydrolysed to larixol [29]. 

The distribution of carbon in the form of aromatic, hydroaromatic, and methyl carbon (as assessed in the present work) is presented in Table VIII. The sum of the three forms of carbon accounts for about 97.5-98.09f of the total carbon in lower rank bituminous coal (C < 86% carbon, dmf basis) and about 99.4% of the total carbon in case of the highest rank bituminous coal (C = 90.4%) studied. Small proportions of carboxyl groups are present in lower rank bituminous coal, and if this carbon is taken into account, it may bring the total up to 98% at the maximum. 

Figure 19-5 Schematic representation of an important chemotactic system of E. coli, S. typhimurium, and other bacteria. The transmembrane receptor activates the autokinase CheA, which transfers its phospho group to proteins CheY and CheB to form CheY-P and CheB-P. CheY-P regulates the direction of rotation of the flagella, which are distributed over the bacterial surface. CheR is a methyltransferase which methylates glutamate carboxyl groups in the receptor and modulates the CheA activity. CheZ is a phosphatase and CheB-P a methylesterase.

Figure 5-13 Translation of a projection formula to a ball-and-stick model and then to a Newman formula. The first step is to assemble the model over the projection formula in the same way as shown in Figure 5-12, except that configurations at each of two atoms are involved. Next, the model is turned and rotated around the center C-C bond to put the methyl and carboxyl groups anti to one another. The Newman formula then is drawn to correspond to the ball-and-stick model.

Sequential Methylation Technique. A sequential methylation-extraction technique has been devised primarily to separate phenolics from aromatic and aliphatic acids. The procedure, outlined below, allows quantitative conversion of phenolic hydroxyls to methyl ethers in one step and esterification of carboxyl groups in another. 

Incorporation of carboxyl groups in vinyl polymers (J) and polyolefins (1, 7) improves the adhesion of these polymers to various materials. However, many of these carboxylated polymers, particularly the carboxylated polyolefins, have limited solubility in volatile, lacquer-type solvents such as butyl acetate or methyl ethyl ketone and thus are limited in their ability to improve the adhesion of coatings applied from solvents. Carboxylated polyesters that are soluble in these solvents can be prepared. We were therefore interested in determining the effects of structure and carboxyl content on the adhesion of coatings of various classes of polymers blended with carboxylated polyesters. 

---