Circular Dichroism and Linear Dichroism

Abstract Circular dichroism ( CD ) is the difference in absorption, , of left and right circularly polarized light: For randomly oriented systems such as solutions, only chiral molecules will show any CD intensity corresponding to their absorption bands. Chiral is derived from the Greek word χϵιρ meaning hand and describes something that cannot be superposed on its mirror image by rotation. Two molecules that are mirror images of each other are often referred to as enantiomers and equimolar mixtures of two enantiomers form a racemic mixture which has no net CD intensity in solution. CD can be used to analyze chiral structures, such as a protein secondary structure, 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: 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. Both circular and LD are absorbance techniques and most CD instruments will also measure LD. Both CD and LD can also be enhanced by fluorescence detection with either incident circularly or linearly polarized light (fluorescence detected circular and linear dichroism) and/or measurement of the circular or linear nature of the emitted light.