Cotton effect

The Cotton effect is a phenomenon of optical activity. It is the characteristic change of the optical rotatory dispersion (ORD ) and of circular dichroism ( CD) in the vicinity of an absorption band of the substance. In a wavelength region in which the light is absorbed, the absolute value of the first optical rotation varies rapidly with wavelength, the zero crossing at the absorption maxima and then again varies rapidly with wavelength, but in the reverse direction. This phenomenon was discovered in 1895 by French physicist Aimé Cotton.

The Cotton effect is called positive if the optical rotation first increases as the wavelength decreases ( as Cotton as first observed ) and negative if it first reduces the rotation.

Enantiomers have opposite Cotton effects.

Protein structures as " β -sheet " and " α -helix " show a negative Cotton effect with maxima at 199 and 205 nm Disordered structures show a positive Cotton effect.