Circular Dichroism (CD)
Definition and meaning of Circular Dichroism (CD) in chemistry.
Circular dichroism (CD) is the differential absorption of left- and right-circularly polarized light by a chiral (optically active) sample, measured as a function of wavelength.
In more detail
A CD signal arises only when a molecule lacks a mirror plane or center of symmetry, so that it responds unequally to the two circular polarization states near an absorption band. The result is plotted as ΔA = A_L − A_R (or as molar ellipticity) versus wavelength, giving a spectrum whose sign, shape, and intensity are sensitive to a molecule's absolute configuration and three-dimensional conformation. Because different secondary structures scatter and absorb circularly polarized light distinctively, CD spectroscopy is a key analytical tool in biochemistry for monitoring protein folding, secondary structure content, and nucleic acid conformation without crystallizing the sample.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Measured quantity | ΔA = A_L − A_R, often reported as molar ellipticity [θ] (deg·cm²·dmol⁻¹) |
| Requirement | Molecular chirality (no internal mirror symmetry) |
| Typical range for proteins | Far-UV, 190–250 nm |
The far-UV CD spectrum of a largely α-helical protein shows two characteristic negative bands near 208 nm and 222 nm along with a strong positive band near 190 nm, a signature used to estimate helix content.
Frequently asked questions
How does CD differ from optical rotatory dispersion (ORD)?
CD measures the differential absorption of left- versus right-circularly polarized light, while ORD measures the differential refractive index (net rotation of plane-polarized light) versus wavelength; both effects stem from the same molecular chirality and are mathematically related through the Kramers-Kronig relations.
Why do achiral molecules show no CD signal?
An achiral molecule interacts identically with left- and right-circularly polarized light, so the absorbance difference ΔA is zero at every wavelength.