Circular Dichroism in Protein Analysis

Abstract

Proteins are chiral objects. Elements of local, regular structure, called secondary structure, within proteins have a distinct handedness. For example, α-helices are mostly right-handed. Proteins absorb ultraviolet light, exciting electrons into high-energy states. Plane-polarized light (such as that created by Polaroid sunglasses) is light that propagates in only a single plane. It is possible to generate circularly polarized light. Left- and right-handed circularly polarized light may be considered as two components of plane-polarized light. When plane-polarized light passes through a chiral medium, its plane of polarization is rotated. If the light excites an electronic transition, a chiral molecule will absorb the left- and right-handed circularly polarized components differently. This differential absorption is termed circular dichroism. Circular dichroism spectroscopy may be used to estimate quantitatively the secondary structure content of proteins, giving the fraction of residues in α-helices, β-sheets, turns, and in coil conformations. It is widely used to characterize proteins under equilibrium conditions and to measure the kinetics of protein folding and unfolding.