Optical activity, polysaccharides

Colloid catalysts with chiral stabilizing agents can be used as asymmetric catalysts in the reactions of prochiral compounds. One such catalyst is the well-known "Skita-catalyst"(colloidal Pt or Pd with gum-arabicum as an optical active polysaccharide as a protective colloid). These catalysts have been used very often in a number of hydrogenations of unsaturated compounds, including prochiral molecules, but never were their asymmetrizing action noted. Nevertheless, including chiral components as protective colloids in such catalysts allowed for the discovery of asymmetric effects in their action. Indeed, Balandin, Klabimovskii et al. found small... 

Polarimetric determination of the sucrose concentration of a solution is vaUd when sucrose is the only optically active constituent of the sample. In practice, sugar solutions are almost never pure, but contain other optically active substances, most notably the products of sucrose inversion, fmctose and glucose, and sometimes also the microbial polysaccharide dextran, which is dextrorotatory. Corrections can be made for the presence of impurities, such as invert, moisture, and ash. The advantage of polarization is that it is rapid, easy, and very reproducible, having a precision of 0.001°. 

The optical activity of this polysaccharide in water and cupram-monium solution so closely resembles that of methyl 2-methyl-/3-D-glucoside that Reeves68 has suggested that the D-glucopyranose units of the polysaccharide are linked chiefly through the 2-position (see page 231). 

In living systems, optically active macromolecules, such as proteins, nucleic acids, and polysaccharides, are extensively involved in life processes. These macromolecules often possess specific conformational and higher... 

Both synthetic and naturally occurring polymers have been used as CSPs. Figure 3.2 shows typical CSPs prepared from optically active polymers (1-18) 1-15 are totally synthetic polymers, including vinyl polymers (1-7), polyamides (8-12), polyurethanes (13), polyacetylene (14), and polysaccharide analogue (15). The CSPs 16-18 are based on natural polymers, proteins (16), and polysaccharides (17, 18). 

Among optically active polymers, polysaccharide derivatives are particularly valuable. Polysaccharides such as cellulose and amylose are the most readily available optically active polymers and have stereoregular sequences. Although the chiral recognition abilities of native polysaccharides are not remarkable, they can be readily converted to the esters and carbamates with high chiral recognition abilities. The chiral recognition mechanism of these derivatives has been clarified to some extent. 

Complexes between chiral polymers having ionizable groups, and achiral small molecules become, under certain conditions, optically active for the absorption regions of the achiral small molecules. Dyes such as acridine orange and methyl orange have been used as achiral species, since they are in rapport with biopolymers through ionic coupling. This phenomenon has been applied to the detection of the helix chirality in poly-a-amino acids, polynucleotides, or polysaccharides when instrumental limitations prevent direct detection of the helices. 

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

Roussel [3] prepared chiral polysaccharide esters by grafting a-phenylpro-pionic acid, (R)-, (IV), and (S)-ibuprofen, or (R)- and (S)-naproxen onto cellulose. These agents were then used for isolating optically active acids. 

Liang, J. N., Stevens, E. S., Morris, E. R., and Rees, D. A. 1979. Spectroscopic origin of conformation-sensitive contributions to polysaccharide optical activity vacuum-ultraviolet circular dichroism. Biopolymers 18 yil-yyi. 

Apart from protein systems, the optical activity of various other substances and systems of biological interest has also been investigated recently, such as nucleic acids, polysaccharides, ribosomes or membrane fragments [see, for example, (11—13, 15—17)]. Although these are not included in the present review, many analogies with regard to the generation of optical activity and the interpretation of data can be found between these and protein systems. 

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