Circular dichroism

Circular dichroism is the difference in absorption of left- and right-circularly polarized light by a material. Its wavelength-dependent signal reveals optical activity and can probe molecular structure.

Circularly polarized light: Light whose electric field rotates in a circle as the wave propagates. Circular dichroism compares absorption for its left- and right-handed forms.
Protein secondary structure: The local arrangements of a protein backbone, including alpha helices, beta sheets, and turns. Far-ultraviolet spectra help estimate the proportions of these structural motifs.
Linear polarization: A state of light in which the electric field oscillates along one fixed direction. It differs from the rotating electric field used to define circular polarization.
Linear dichroism: The difference in absorption of light polarized along two perpendicular linear directions. It compares linear orientations rather than left- and right-circular polarizations.
Optical activity: The interaction of chiral matter with light that produces different optical effects for opposite handedness. It is the broader phenomenon underlying differential absorption in circular dichroism.
Protein folding: The process by which a protein adopts its functional three-dimensional structure. Circular dichroism tracks structural changes during folding and unfolding.
Electromagnetic radiation: Energy propagated through space by oscillating electric and magnetic fields. Circular dichroism measures a polarization-dependent interaction with electromagnetic radiation.
Optical rotatory dispersion: The wavelength dependence of the rotation of linearly polarized light by an optically active material. It measures polarization rotation, a related optical-activity effect rather than differential absorption.
Chirality: The property of an object that makes it distinguishable from its mirror image. Chiral structures generally provide the handedness needed for a circular-dichroism signal.
Circular dichroism spectroscopy of proteins: The use of circular-dichroism spectra to assess protein structure and conformational change. This is the principal biomolecular application of the method.

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