Optical birefringence dispersion

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Rotation of the polarization plane (or the axes of the dichroic ellipse) by a small angle a occurs when the phases for the two circular components become different, which requires a difference in the refractive index n (Pearlman and Nguyen 1991). This effect is called circular birefringence. The change of optical rotation with wavelength is called optical rotary dispersion (ORD). 

The most commonly encountered manifestations of chiroptical phenomena are circular birefringence (also known as optical rotation), optical rotatory dispersion (ORD), and circular dichroism (CD). An explanation as to the nature of circularly and linearly polarized light is provided, and the origins of the various chiroptical effects are discussed. In each instance, a concise summary of the calculations used by workers in the field to report the results of their investigations is provided. 

As it is easily derived from Eqs. 6.4-18, the relations are valid also for birefringence and dichroism. As far as optical activity in the UV/VIS range is concerned transformations on this basis have already been widely used for decades to correlate circular-dichroism bands and Cotton effects, i.e. anomalies of the optical rotatory dispersion (Moffit and Moscowitz, 1959 Blout et al., 1967). Another relation is mentioned in the following section. 

Fig. 4.8. Basic geometry for the observation of the dispersed Faraday effect the polariser P is crossed with an analyser A, and propagation along an axis Oz is parallel to the field lines B in the region of the atomic absorption cell. Rotation of the plane of polarisation through an angle (p occurs as a result of magneto-optical birefringence (after J.-P. Connerade [161]).

An extensive but unfortunately, as yet, unpublished study by Yamaoka (28) was concerned with the mode of orientation of several polypeptides in varied solvents under the influence of a rectangular voltage pulse. While measurements could be made in most organic solvents, he was unable to obtain steady-state values for the birefringence of PBLG dissolved in benzene and dioxane, except at low concentrations in dioxane. Extremely long rise times were observed in these solvents, and the 1.4-millisecond limit on his pulse width prevented establishment of equilibrium. Yamaoka showed by means of optical rotatory dispersion that PBLG assumes a helical conformation in benzene. 

When NaOH is increased to 0.22M, the linewidths of the C NMR signals are abruptly narrowed, resulting in the conformational transition to the random coil. The transition behavior is consistent with that of optical rotatary dispersion, viscosity and flow birefringence ( ). (In particular, a viscosity minimum was achieved at 0.2M NaOH, in contrast to that of amyloser33,33a]). 

Uchiyama, A, Ono, Y, Ikeda, Y, Shuto, H., and Yahata, K. (2012). Copolycarbonate optical films developed using birefringence dispersion control, Pol m., 44,995-1008. 

Higgins, D. A., X. Liao, J. E. Hall and E. Mei (2001). "Simultaneous near-field optical birefringence and fluorescence contrast sqjplied to the study of dye-doped polymer-dispersed liquid crystals." Journal of Physical Chemistry B 105(25) 5874-5882. 

The net result is a rotation of the plane of polarization. An example of a circular birefringence is optical activity, which is observed when a medium composed of chiral molecules (with an excess of one enantiomer) is subject to linearly polarized electromagnetic radiation. The dependence of the corresponding anisotropy on the wavelength is described by the optical rotatory dispersion (ORD). The Faraday effect, discussed below, is another well-known example of a circular birefringence. 

Constantine S, Zhou Y, Morals J and Ziegler L D 1997 Dispersed optical heterodyne birefringence and dichroism of transparent liquids J. Phys. Chem. A 101 5456-62... 

---