Steric interactions, cellulosics

Inspired by Easson and Stedman s work, Dalgliesh established the three-point attachment model in 1952 to elucidate the chromatographic separation of D-/L-amino acids in cellulose paper chromatography [22]. The Dalgliesh s model was later improved by Lochmuller and Souter [23]. According to this three-point attachment model, it is necessary to have at least three attractive interactions, or two attractive and one repulsive (steric) interaction between the receptor and one of the... 

Numerous investigators have proposed theories for the temperature dependence of pitch. " However, none completely explain the experimental results obtained for the various cellulosic systems. Of those proposed, Osipov s approach, which is based on a molecular statistical theory,takes into account steric and chiral interactions in solution to predict the influence of temperature and solvent on the pitch and twist sense of cellulose deiivatives. Assuming the cellulosic chain adopts a twisted belt as opposed to a helix, and the persistence length, /, of the cellulose chains is much smaller than that of a rigid chain, the twisting power could be expressed by... 

The theory predicts that the handedness of cellulosic liquid crystalline solutions, designated by the sign of the pitch, depends not only on temperature (T) and on steric repulsion of the chain X), but also on an attractive interaction parameter, %, which depends on the nature of the solvent. The chiral forces are balanced when (x XkT) = 0. In this compensated condition, the pitch of the mesophase should become infinite, and the mesophase resembles a normal nematic phase. 

Fig. 6. Cellobiohydrolase-cellulose interaction. In the cellulose chain, the (31—4 glycosidic bond alternate sides of the ribbon-like structure. Steric hindrance prevents the cellulose chain from bending or turning within the catalytic domain of cellobiohydrolases. This forces the product into dimer units as the enzyme progresses along the chain...

Water-soluble cellulose derivatives themselves adsorb onto solid particles and may for instance affect the suspension properties of these insolubles. The mechanisms involved are quite complex and depend on the polymer concentration. At low concentrations macromolecules influence the electrophoretic mobility and the flocculation of the particles. At higher concentrations, surface coverage by the adsorbed polymer is sufficient to prevent particle-particle interaction and thus to stabilize the suspension sterically. As an example, the effect of NaCMC (among other polymers) on the zeta potential, flocculation and sedimentation properties of sulfadimidine has been investigated by Kellaway and Najib [115,116],... 

Another example of modeling the structure of this type of CSP is presented by Francotte and Wolf [47]. They prepared benzoylcellulose beads, in a pure polymeric form as a sorbent, for the chromatographic resolution of racemic compounds like benzylic alcohols and acetates of aliphatic alcohols and diols. Their experimental results implicated multiple interaction sites to be involved in the complexation. Rationalizing the interaction mechanism required a more systematic investigation of the factors influencing separations and, to address the structural features of the cellulose tribenzoate, they carried out molecular modeling with molecular mechanics. The key question being addressed is to what extent is the polysaccharide backbone exposed to small molecules when sterically encumbered benzoates are attached ... 

Other packings have used esterified celluloses, chiral peptides and /3-cyclo-dextrins as enantioselective media [12]. The cyclodextrins are chiral carbohydrates formed from up to 12 glucose units. The monomers are configured such that the cyclodextrin has the shape of a hollow truncated cone or barrel-like cavity within which stereospecific guest-host interactions can occur, though other features such as steric repulsion, solvent, pH, ionic strength and temperature all affect retention. 

The derivatised glucose can act as a chiral site and result in diastereomeric interactions with enantiomers which together with the steric fit requirements within the cavity and the different interactions with the cellulose strands provides the basis for enantioselective interaction and subsequent resolution. The acetate, benzoate and phenylcarbamate glucose ester give superior resolution and selectivity compared with the parent material. Hydrophobic mobile phases are most commonly encountered though aqueous based eluants can be used with many versions of these materials. These stationary phase packings have been used to separate a wide range of pharmaceutical compounds [80]. 

Ward and Upchurch (340) found an inverse relationship between the solubilities of 18 substituted anilines and their adsorption by nylon (R2 = 0.77) and cellulose triacetate (R = 0.80). It was suggested that steric and electronic effects of the molecules were also responsible for the adsorption differences of the compounds. Because of the low solubilities of the substituted anilines, adsorption by organic matter probably occurs at lipophilic sites through dipole-dipole interactions—e.g., hydrogen bonding or charge-transfer complexes. Low adsorption at clay surfaces probably occurs because the hydrophobic molecules do not readily associate with hydrated surfaces. Adsorption of the substituted anilines by dry clays might occur on the soil surface for instance, but in the... 

There has been another proposal that plane-structures consisting of cellulose molecules in the lOl plane of native cellulose are held together by hydrophobic interactions even in the presence of alkali, and that hydrophilic surfaces of the 101 plane-structures are solvated with alkali and water (24). However, if such planar structures were solvated with aqueous alkali, they would be expected to result in the formation of a dispersion of micelles. It seems to us more likely that some strong or sterically protected inter-molecular hydrogen bonds of native cellulose survive even in alkali cellulose. On the other hand, since some hydrogen bonds are cleaved by NaOH and water which penetrate into the crystalline lattice of cellulose, new lattice planes can be formed as, for example, in Na-Cellulose I or other soda celluloses. 

Non-covalent interactions between carbohydrate chains can be very powerful, as in the hydrogen-bonded structures of cellulose and chitin, and this is especially true if the bonds have a polar character and are in a hydro-phobic environment. Likewise, saccharide chains may interact through coordination of metal ions for example, the hydroxyl group—calcium ion interaction can be strong if steric factors favour it. Carbohydrates may also interfere dramatically with the structure of water, but all of these fascinating possibilities do not carry clear biosynthetic implications and so must lie largely outside the scope of this chapter. 

In the latter the free rotation is impeded to a much greater extent by steric hindrance and by the interaction of hydroxyl groups. For similar reasons the number v in cellulose nitrate becomes larger as more NOg groups are substituted in the glucose residue (see below). 

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