The variation of optical rotation as a function of wavelength is called optical rotary dispersion (ORD). Circularly-polarized light rays will travel through an optically active medium with different velocities due to the different indices of refraction for right- and left-circularly polarized light called optical rotation or circular birefringence. Inherently asymmetric chromophores (uncommon) or symmetric chromophores in asymmetric environments will interact differently with right- and left-circularly polarized light resulting in two related phenomena. Circular dichroism spectra of "pure" secondary structures. green:strand, and yellow:other.įigure 22. Secondary structures are color coded red:helix. Modern secondary structure determination by CD are reported to achieve accuracies of 0.97 for helices, 0.75 for beta sheet, 0.50 for turns, and 0.89 for other structure types (Manavalan & Johnson, 1987).įigure 21. In fact, optical rotary dispersion (ORD, see below) data suggested a right-handed helical conformation as a major protein structural element before the Pauling and Corey model (Pauling & Corey, 1951) and Kendrew's structure of myoglobin. A number of excellent review articles are available describing the technique and its application (Woody, 1985 Johnson, 1990). It has been shown that CD spectra between 260 and approximately 180 nm can be analyzed for the different secondary structural types: alpha helix, parallel and antiparallel beta sheet, turn, and other. Circular dichroism (CD) spectroscopy is a form of light absorption spectroscopy that measures the difference in absorbance of right- and left-circularly polarized light (rather than the commonly used absorbance of isotropic light) by a substance. The phenomenon of circular dichroism is very sensitive to the secondary structure of polypeptides and proteins ( Figure 21 and Figure 22 ).