Polysaccharides derivatised

A similar procedure was adopted for synthesis of nanoparticles of cellulose (CelNPs). The polysaccharide nanoparticles were derivatised under ambient conditions to obtain nanosized hydrophobic derivatives. The challenge here is to maintain the nanosize even after derivatisation due to which less vigorous conditions are preferred. A schematic synthesis of acetyl and isocyanate modified derivatives of starch nanoparticles (SNPs) is shown in scheme 3. The organic modification was confirmed from X-ray diffraction (XRD) pattern which revealed that A- style crystallinity of starch nanoparticles (SNPs) was destroyed and new peaks emerged on derivatisation. FT-IR spectra of acetylated derivatives however showed the presence of peak at 3400 cm- due to -OH stretching indicating that the substitution is not complete. 

Figure 3.1 Chemical structures of the current most successfully employed derivatised polysaccharide CSPs. (a) CHIRALPAK AD Amylose tris (3,5-dimethylphenylcarbamate) coated onto a silica support, (b) CHIRALPAK AS Amylose tris [(S)-a-methylbenzylcarbamate] coated onto a silica support, (c) CHIRALCEL OD Cellulose tris (3,5-dimethylphenylcarbamate) coated onto a silica support, (d) CHIRALCEL OJ Cellulose tris (4-methylbenzoate) coated onto a silica support.

Silylation reactions on polysaccharides with chlorosilanes and silazanes were attempted more than 50 years ago resulting in hydrophobic silyl ethers with both increased thermal stability and solubility in organic solvents [376]. The silylation reaction for the protection of hydroxyl groups in mono- and polysaccharides exhibits many advantages, e.g. fast silylation, solubility of silylether in organic solvents suitable for subsequent derivatisation, stability of the resulting silylether under basic conditions but easy deprotection of the silyl moieties by acid hydrolysis or nucleophilic agents like fluoride and cyanide ions [377]. The partial and complete silylation of dextran was studied in detail by Ydens and Nouvel [215-217]. 

Several intermediates are available commercially for instance, CNBr-Sepharose, Sepharose and Bio-Gel P (polyamide) intermediates with reactive spacer groups attached, and Sepharose already coupled to a ligand specific for polysaccharides and glycoproteins. An outline of the reactions developed to couple ligands to Sepharose is illustrated in Figure 4.15 dextran and cellulose matrices can be derivatised in a similar manner. 

The current success and dominance of the commercial CSP market by derivatised polysaccharide, macrocyclic antibiotic and, to a lesser extent, synthetic multiple-interaction Pirkle-type materials is such that it could be considered questionable whether or not there is a need to maintain an awareness of the capabihties of some of the earlier developed CSP. Indeed in a rather shrewd marketing move some Diacel products have been dubbed historical CSP. This suggests, at the same time, that the newer CSP represent a major advance in technology but that the older ones are well worth preserving. The latter point is especially true. No matter how successful multi-column screening approaches... 

As in analytical chiral LC, Daicel derivatised polysaccharide CSPs are the most frequently used materials in preparative scale chiral separations. Recently CSPs have been prepared in which derivatised polysaccharides have been covalently bonded to the solid support rather than coated on as in the Diacel materials. The rationale for this is that it is advisable to reduce the chance of the chiral selector leeching off the column in trace amounts to contaminate samples of chiral dmgs isolated by production scale LC. However, the extent to which the Daicel coated CSPs are now used in production scale chiral LC would tend to suggest that such a problem, if it exists, is not a very significant risk. 

A range of precursors for saccharides or polysaccharides are readily accessible through the described two-step process. Each fragment can be derivatised prior to assembly of the hexose, which significantly reduces the constraints of the respective late-stage derivatisation approach. The method is extremely versatile and enables the formation of a variety of hexoses, depending on the applied conditions for the second step. 

Natural thickeners can be defined as products obtained from natural sources such as plants, seeds, seaweeds and microorganisms. These products are high molecular weight polymers composed of polysaccharides and are often referred to as hydrocolloids. Production processes vary from simple collection of tree exudates and milling in the case of gum arabic to more complex production by fermentation as in the case of xanthan gum. A number of these natural thickeners are also derivatised in order to modify their properties. Table 2.1 provides a simple classification of these products by source. Tables 2.2-2.4 provide an overview of the main natural thickening agents and their applications. A brief description of each class of hydro colloids is given below but for more detailed information on each of the hydrocolloids there are a number of publications available [ 1—3]. 

The eluent compatibility of a polymeric adsorbent will be dependent upon the chemical structure of the polymer backbone, chemical type of the cross-linking agent, degree of cross-linking, and any subsequent covalent or dynamic modifications carried out. The natural polysaccharide polymers in their native state are hydrophilic and are therefore compatible with aqueous eluents whereas the synthetic polymers can be hydrophobic, as in the case of polystyrene, and hence compatible with organic eluents, or hydrophilic, as in the case of polyacrylamide, and so be compatible with aqueous mobile phases. It is of course possible to modify the eluent compatibility of a polymeric matrix by surface coating or derivatisation. For example, the very hydrophobic maeroporous polystyrene matrices may be coated with a hydrophilic polymer to make ion exchange adsorbents or materials suitable for aqueous size separations [25]. 

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