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Circular Dichroism and Linear Dichroism

Electronic Absorption and Luminescence

  1. Alison Rodger

Published Online: 13 JAN 2014

DOI: 10.1002/9780470027318.a5402.pub2

Encyclopedia of Analytical Chemistry

Encyclopedia of Analytical Chemistry

How to Cite

Rodger, A. 2014. Circular Dichroism and Linear Dichroism. Encyclopedia of Analytical Chemistry. 1–34.

Author Information

  1. University of Warwick, Department of Chemistry, Coventry, UK

  1. Update based on the original article by Alison Rodger, Encyclopedia of Analytical Chemistry, ©2000, John Wiley & Sons, Ltd.

Publication History

  1. Published Online: 13 JAN 2014

Abstract

Circular dichroism (CD) is the difference in absorption, A, of left and right circularly polarized light, Equation (1):

  • mathml alt image(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):

  • mathml alt image(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.