Circular dichroism synthetic polymers

To understand how measurement of optical activity can provide us an idea about the conformation of a maciomolecule, we will consider an example of synthetic polypeptide pofy-L-lysine. This synthetic polymer is readily soluble in water at pH 5.7. At this pH the e-amino groups are protonated. Consequently the pol)mier can exist in a variety of conformations. The potymer may then be thought as existing in a remdom coil state in solution. Observe the CD (Figure 9.4(B)) and ORD (Figure 9.4(A)) of this random eoU (only CD is discussed observe, however, that wherever extinction is maximum for CD, ORD is zero). There is a weak positive circular dichroism at 217 nm and a strong negative eircular dichrolsm at 197 nm. 

Circular dichroism is extensively employed in the stmctural study of biological polymers, but not in the study of synthetic polymers. This is because most biological polymers are optically active, whereas most synthetic polymers are not. However, if the optically active vinyl or vinylidene monomers are incorporated in a synthetic polymer, the synthetic polymr could become optically active. 

Optically active polymers show another properties namely thermosensitivity, e.g., main chains helical poly(iV-isopropylacrylamide) and thermosensitive part as side chain of poly(A -isopropylacrylamide) (PNlPAm). Such type of polymers synthetic method described elsewhere [137]. The polymer with optically active cores (helical polyacetylenes) and thermosensitive shells (PNlPAm) brashes self-assembled core/shell structured nanoparticles in aqueous solution. Another example of optically active polymer is poly[/V-(L)-(l-hydroxymethyl)-pro-pylmethacrylamide] (P(l-HMPMA)) of lower critical solution temperature and thermosensitivity. Circular dichroism and microcalorimetric measurements of the polymer showed the polymer chains in a state of relatively low hydration compared to that of by racemate synthesized monomers by free-radical reaction formed P(d,l-HMPMA). Thermosensitivity and structural effects were obtained by microscopic observation of aqueous solution of polymers and its hydrogels [138]. 

These reactions are very important in biochemical processes in vivo and are responsible for their exceptional specificity and selectivity. They can however also occur in unbiological conditions in vitro or in synthetic polymers. Since the chirality of the components is changed in all these cases, such reactions are usually studied by optical rotatory dispersion (ORD) and circular dichroism (CD), which permit conclusions on reaction mechanisms, conformation of the macromolecules, helix-coil transition and equilibrium constants. 

In the previous decades, the optical activity of these synthetic polymers were studied by polarimetry. Presently, advances in Optical Rotatory Dispersion (ORD) and in Circular Dichroism (CD) allow more detailed investigations in the far UV essential to interpret chiroptical properties. Explaining observed ORD and CD changes and establishing relationships between spectra and structures of backbones and side-chains are two of the problems which arise when synthetic optically active polymers are studied. 

Abstract The optical achvities of poly-(R)-lactide, poly-(S)-lactide, poly(beta -hydroxybutyrate) and two beta -hydroxyvalerate copolymers were measured in soluhon, as solid powders in suspension, and where possible, as films. Poly-(plus)-3-methyl-I-pentene was also reinvestigated. In some cases the specific rotahon values of powder samples showed significant differences from the values of the solution measurements. The discrepancies of the data observed seem to reflect the local environment of the polymer chains in supermolecular assemblies and consequently the sohd state structure (morphology) of the polymers. The circular dichroism (CD) spectra of the polymers were also measured in solution and in the form of their films. For comparison, the CD spectra of the namrally occurring protein casein and of the synthetic polypeptide poly-(i-)-proline were also measured. (Author abstract) 2 I Refs. 

In the present paper we hope on the one hand to present an original conception of interpreting the optical activity of synthetic polymers, and on the other one to show that optical activity is not only a means to study conformational phenomena but can also prove an excellent method of detection, and even of study, of the ionisation of polyelectrolytes and of the complexation of macromolecules with ions or small molecules. In order to show both the advantages as well as the disadvantages of such a technique, we shall first of all give a brief reminder of the origins of optical rotation and circular dichroism and of their sensibility to secondary structures and chemical modifications. 

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