Self-Assembly of Synthetic and Biological Polymeric Systems of ...

ackbone of proteins. Since proteins, then, are built by units that are chiral, they will interact
with left and right circularly polarized light to different extents giving rise to circular dichroism
(40)(42). Hence, circular dichroism can be defined as the difference between the absorption of
left and right circularly polarized light by a chiral molecule:
where AL and AR are the absorption of the left and right circularly polarized light, respectively,
by the chiral molecule. For achiral molecules , and no CD signal is present. For chiral
molecules, at some wavelength, and the resulting will be non-zero; it can have
either a positive or negative value depending on the relative intensities of the left and right-
handed light absorbance. Usually, in the analysis of protein structures CD instruments scale the
chirality as ellipticity, , expressed in millidegrees. Therefore, a CD signal can be viewed either
as an absorption difference or as the produced ellipticity, although both are related each
other. However, ellipticity is not a useful quantity to compare different samples as its
magnitude differs depending on the amount of protein present in the sample. For CD spectral
data, the most useful units are delta epsilon the molar circular dichroism, Δ, and the mean
residue ellipticity []MRE (MRE).
Figure 2.17. Schematic representation of a typical circular dichroism spectrometer.
To obtain a circular dichroism spectrum (Figure 2.17), a periodic variation in the polarization of
the light beam is induced by the polarization modulator through all ellipticities. This polarized
light passes through the sample to a photomultiplier detector. If the sample is not optically
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