Chirality and Circular Dichroism Spectroscopy

Chirality and Circular Dichroism Spectroscopy

Many amino acids exist as chiral molecules

Chiral molecules are mirror images or enantiomers. There is no symmetry operation in 3D-space that can be performed on one enantiomer to make it overlay the other. Enantiomerism is a special form of isomerism. The physical properties of enantiomers are identical in every way except two: how the molecules interact with polarized light and how they interact with other chiral molecules.

Circular dichroism (CD), measured as a function of wavelength, is the difference in absorbance of left-handed circularly polarized light (L-CPL) and right-handed circularly polarized light (R-CPL). This difference can be detected when a molecule contains one or more light-absorbing groups - so-called chiral chromophores.

Circular dichroism (CD) = ΔA(λ) = A(λ)L-CPL - A(λ)R-CPL, where λ is the wavelength

Generating a CD spectrum

When chiral chromophores are present, one state of circularly polarized light will be absorbed to a greater or lesser extent than the other. Over corresponding wavelengths, a CD signal can therefore be positive or negative, depending on whether L-CPL is absorbed to a greater extent than R-CPL (CD signal positive) or to a lesser extent (CD signal negative).

Chirascan CD spectrometers measure alternately the absorbance of L- and R-CPL and then calculate the CD signal.

CD spectra vary according to differences in absorbance of
L- and R-CPL.

CD spectrum of vitamin B12 (cobalamin). Multiple peaks show how CD varies as a function of wavelength and that a CD spectrum may exhibit positive and negative peaks.

Typical protein CD spectra

The chiral center is the α-carbon atom in the protein backbone.

Disulphide bonds, often found in proteins, may influence both far- and near-UV CD spectra.

The principle chiral chromophore that dominates in the far-UV spectrum of proteins is the peptide carbonyl bond (C=O): π → π* at ~ 190 nm; n → π* at ~ 210-230 nm.

In the near-UV, aromatic amino acids are the predominant chromophore.