The Origin of Circular Dichroism

The difference between the dipole strengths for left- and right-circularly polarized light is characterized experimentally by the rotational strength 91 of an absorption band  

Early CD spectrometers actually measured ellipticity, but most modem instmments measure Ae more directly and sensitively by switching a light beam rapidly between right- and left-circular polarization (Chap. 1). In studies of proteins or polynucleotides, the molar ellipticity usually is divided by the number of amino acid residues or nucleotide bases in the macromolecule to obtain the mean residue ellipticity. 

Circular dichroism (CD) is a very small effect, typically amounting to a difference of only about 1 part in 10 or 10 between the extinction coefficients for light with left- or right-circular polarization. But in spite of its small magnitude, CD proves to be a very sensitive probe of molecular structure. 

The resultant field will oscillate in amplitude from E iax = Er + / when the two fields are aligned in parallel (the times labeled 0 and 4 in Fig. 9.5B) to Emin = Er — Ei whcn they are antiparallel (times 2 and 6). The minor and major half-axes of the elipse swept out by the resultant field (Fig. 9.5B) are l l and Emax, which have the ratio 

Rosenfeld [11] showed that the rotational strength of a transition depends on the dot product of the magnetic transtion dipole and the electric transition 

---

Circular dichroism (CD) spectroscopy is a sensitive analytical tool for assessing protein structure. It can detect changes in both the secondary and tertiary structure of proteins, as well as provide information regarding prosthetic groups, bound ligands and co-factors. The origin of circular dichroism in proteins is described and various applications of CD spectroscopy to the study of protein structure, function, and folding is discussed. 

Essentially, the origin of spontaneous chiral resolution is the same as the previous example. When molecules with the same chiral conformation form small chiral domains due to packing entropy effects, the same chiral conformation of molecules is stabilized when they approach the chiral domain. Thus both chiral domains with different chiral conformations grow, resulting in spontaneous chiral resolution [6-8]. Chirality enhancement occurs even in such chiral domains. For instance, chirality in both segregated chiral domains is enhanced by doping nonchiral bent-shaped molecules (BSMs) with nonchiral rod-shaped molecules (RSMs), as observed by circular dichroism (CD) or optical rotatory power (ORP) [9],... 

S. F. Mason, From Pasteur to parity nonconservation theories of the origin of molecular chirality , in Circular Dichroism, ed. K. Nakanishi, N. Berova and R. W. Woody, VCH, New York, 1993, pp. 39-57. 

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. 

The interaction of polarized light with chiral compounds is of great interest since chiroptical techniques are extremely useful as methods of characterization. It is equally true that although most scientists are aware that enantiomerically rich solutions will rotate the plane of linearly polarized light, the origins of this effect are not as simple as might be imagined. In this first article, the phenomena of polarimetry and optical rotatory dispersion will be discussed. A subsequent note will concern the related phenomenon of circular dichroism. 

In the years to follow the key to the measurement of vibrational circular dichroism was the development of photoelastic modulators suitable for work in the infrared spectral region. The first successful measurements of circular dichroism originating from vibrational transitions in the infrared were done by Hsu and Holzwarth (1973) on thin slices of monocrystalline a-NiS04 6 H2O and a-ZnSe04 6 H2O. For this measurements the authors used a normal dispersive IR spectrometer supplemented by a linear polarizer and a photoelastic modulator made from Germanium. 

Modern CD instrumentation measures the difference in absorbance of right-add left-handed circularly polarised light directly, so CD spectra are often reported as As, with units the same as for s itself. Sometimes, however, for historical reasons, the circular dichroism of a sample is recorded as ellipticity. The origin of the term lies in the fact that if one of the circularly polarised beams emerging from the sample is attenuated relative to the other, on recombination the two circularly polarised beams form elliptically polarised light, not plane polarised light. The ellipticity 0, is tan (Z / ) where b is the minor and a the major axis of the ellipse. The conventional molar ellipticity [9]... 

Some experiments attempting to simulate the origin of life by chemical synthesis from inorganic starting materials have yielded mixtures of amino acids (and other molecules) similar to those found in biology. However, the solutions do not show any circular dichroism. Why is this ... 

Mizutani, 7., Ema, T. Yoshida, T. Renn, T. Ogoshi, H. Mechanism of induced circular dichroism of amino acid ester-porphyrin supramolecular systems—Implications to the origin of the circular dichroism of hemoprotein. Inorg. Chern. 1994, 33, 3558. 

---