Circular dichroism (CD) is a spectroscopic technique used to study the structural characteristics of chiral (asymmetric) molecules, particularly biomolecules like proteins, nucleic acids, and certain small organic molecules. Circular dichroism measures the differential absorption of left-handed circularly polarized light compared to right-handed circularly polarized light as it passes through a sample.
Key points about circular dichroism:
Chirality: Chirality refers to the asymmetry in the spatial arrangement of atoms in a molecule, resulting in a non-superimposable mirror image. Many biological molecules, including proteins and nucleic acids, exhibit chirality.
Polarized light: Circular dichroism involves shining circularly polarized light through a sample. Circular polarization means that the electric field vector of the light rotates in a circular manner as it propagates.
Differential absorption: Chiral molecules exhibit differential absorption of left- and right-circularly polarized light due to their structural asymmetry. This difference in absorption is measured as circular dichroism.
Spectroscopy: The CD spectrometer records the changes in absorbance as a function of wavelength. The resulting CD spectrum provides information about the secondary structure, conformational changes, and overall folding of the molecules in the sample.
Units: The CD spectrum is usually expressed in terms of molar ellipticity (θ), which is the degree of rotation of polarized light per unit path length and concentration. The units are often reported in degrees times square centimeters per decimole (deg·cm^2·dmol^−1).
Applications of circular dichroism:
Protein structure: CD spectroscopy is widely used to study the secondary structure of proteins, providing information about the presence of alpha helices, beta sheets, and random coils.
Nucleic acid conformation: It is used to analyze the conformation of nucleic acids, revealing information about DNA and RNA secondary structures.
Chirality in small molecules: CD can be applied to study chiral small molecules in various fields, including chemistry, pharmaceuticals, and materials science.