56
Circular Dichroism in Protein Analysis
Bibliography
71
Books and Reviews
71
Primary Literature
72
Keywords
Chiral
Possessing a nonsuperposable mirror image (like left and right hands).
Chromophore
Chemical moiety that absorbs light through excitation of one of its electrons.
Circular Dichroism
The differential absorption of left- and right-circularly polarized light.
Circularly Polarized Light
The resultant from combining two beams of linearly polarized light (
vide infra
)ofequal
intensity, but orthogonal polarizations, with a phase difference of a quarter of
a wavelength.
Linearly Polarized Light
A beam of light in which the electric Felds of all the photons oscillate in the same plane
(also called
plane-polarized light
).
Optical Rotation
The rotation of plane-polarized light by a chiral compound (equivalent to circular
birefringence).
¥
Proteins are chiral objects. Elements of local, regular structure, called
secondary
structure
, within proteins have a distinct handedness. ±or 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
po
lar
izedl
ight
.Left
-andr
ight
-handedc
ircularly 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.
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