Update based on the original article by Alison Rodger, Encyclopedia of Analytical Chemistry, ©2000, John Wiley & Sons, Ltd.
Circular Dichroism and Linear Dichroism
Electronic Absorption and Luminescence
Published Online: 13 JAN 2014
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Rodger, A. 2014. Circular Dichroism and Linear Dichroism. Encyclopedia of Analytical Chemistry. 1–34.
- Published Online: 13 JAN 2014
Circular dichroism (CD) is the difference in absorption, A, of left and right circularly polarized light, Equation (1):
For randomly oriented systems such as solutions of molecules, only chiral molecules will show any CD intensity corresponding to their absorption bands. Chiral molecules are those molecules that cannot be superposed on their mirror images. Chiral is derived from the Greek word χϵιρ meaning hand, hence the alternate term for chirality, ‘handedness’. Two molecules that are mirror images of each other are often referred to as enantiomers, and equimolar mixtures of two enatiomers form a racemic mixture, which has no net CD intensity in solution. CD can be used to analyze chiral structures such as protein secondary structure in molecules and to probe interactions between chiral molecules and other molecules.
Linear dichroism (LD) is the difference in absorption of light linearly polarized parallel and perpendicular to an orientation axis, Equation (2):
LD can be used to provide orientation information about subunits of a molecular system such as small molecules absorbed onto stretched films, flow-oriented DNAs and fibrous proteins, and lipid bilayer systems.