Derivatization permethylation

Permethyl-derivatized cyclodextrin, 6 98 Permethylhydroxypropyl-derivatized cyclodextrin, 6 98... 

Additional experiments were performed to examine the role of the molecular size of the host enantioselectivity. Since the methyl groups in methylated jS-CD orient themselves toward the center of the cavity, it is expected that decreasing the extent of methyl derivatization in jS-CD from 21 (the permethylated jS-CD) to 14 methyl groups increases the effective size of the cavity." " As a matter of fact, the enantioselectivity of valine (kolkL = 0.32 with 21-methyl j8-CD) increases to W l = 0.18 with 14-methyl jS-CD. No significant effect of the cavity size is observed with the smaller alanine. 

El and Cl are normally unsuitable for most flavonoid glycosides because of their polarity and thermolability. Preparation of permethylated or trimethylsilylated derivatives may overcome this problem. However, derivatization often produces mixtures of partially derivatized compounds and may involve rearrangements,and usually only weak molecular ion signals is observed when permethylated compounds are studied under El conditions. 

The ESI-MS characteristics of ohgosaccharides are strongly determined by the identity. The ESI-MS of oligosaccharides is complicated by their hydrophihcity, which limits their surface activity in the ESI droplets. Derivatization strategies to reduce their hydrophihcity, e.g., permethylation or introducing chromophores or fluorophores (Ch. 20.3.5), generally result in an improved response. Permethylated oligosaccharides can be analysed as protonated molecules or as sodium adducts in positive-ion ESI-MS. 

Cellulose pyrolysis has been studied in detail from a variety of points of view mainly related to chemical utilization of wood pyrolysis products or to fire related problems. Analytical pyrolysis of cellulose is not often used as a tool for cellulose detection, but it is a common procedure for studying the pyrolysis products. A variety of analytical procedures have been applied for this study, pyrolysis/gas chromatography/mass spectrometry (Py-GC/MS) being the most common [11-16]. Besides Py-GC/MS, other analytical procedures also have been utilized, such as Py-MS [17,18], Py-IR [19], and off-line Py followed by HPLC [20]. The Py-MS spectrum of cellulose was shown in Figure 5.4.1 (B). Some procedures applied GC/MS on derivatized pyrolysis products (off-line), the derivatization being done by silylation [21], permethylation, perbenzoylation [22], etc. Information about cellulose also has been obtained from the analysis of pyrolysis products of several cellulose derivatives, such as O-substituted cellulose [23]. Also the study of cellulose crystalline structure with X-ray during pyrolysis has been used [23a] to generate information about the pyrolysis mechanism. 

Several anhydrodisaccharides, and anhydrocello-n-oses (n = 3, 4, 5, 6) have been detected in the pyrolysate, but they were not seen in regular GC/MS separations due to their low volatility. They cannot be detected even after derivatization as trimethylsilyl derivatives (TMS). These substances can be detected, however, in the pyrolysate using an LC procedure or using otf-iine permethylation of the pyrolysate [22], which increases the volatility more than trimethyisilylation. 

The selection of derivatization methods for amides and imides is not as great as for other classes of amino compounds. The active hydrogen atom in the structural fragments —CO — NH— or SO2 — NH— is highly acidic and, hence, sometimes recommended TMS, acetyl, or TEA derivatives of these compounds are unstable during hydrolysis. The best derivatization method is their exhaustive alkylation (preferably methylation), because permethylated amides and imides are volatile enough for their GC analysis. This general statement can be illustrated by retention data... 

Methylation by diazomethane is a simple method for derivatization of relatively acidic compounds [e.g., phenols (pA = 9-10) or carboxylic acids (pi = 4.4 0.2)]. The application of this reagent for methylation of aliphatic alcohols requires additional acid catalysis. Methyl iodide is the most convenient reagent for synthesis of permethylated derivatives of polyols (including carbohydrates) and phenols. Dimethyl sulfate [(01130)2802] can be used in basic aqueous media for methylation of phenols, but the yields of methyl ethers, in this case, are not enough for quantitative determinations of initial compounds by GC. 

Permethylation. The process of derivatizing all hydroxyl groups (—OH) in an organic molecule to methoxyl groups (—OCH3). 

Chemical derivatization methods as an aid to MS analysis continue to be important. Derivatization can be divided into tagging of reducing ends and protection of most or all of the functional groups. Permethylation is still an important type of full derivatization. 

In another study [243] an alternative to -acylation was presented initial permethylation of the amino group using formaldehyde and the imino group-specific reducing agent, sodium cyanoborohydride (NaBH3CN) (cf. Ref. 239). Subsequent acetylation proved necessary, nonetheless, to derivatize any hydroxyl and sulfhydryl groups present in the amino acid. 

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