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03-232 Biochemistry
Lecture 5 Supplemental Reading
Optical Activity:
Light is composed of oscillating electromagnetic waves. The change in the electric field as a function of time
can be represented by a vector that oscillates in one direction.
Unpolarized light: The direction of the electric field oscillation is random.
Linearly polarized light: The electric field
oscillates in one direction, this is also referred to
as plane-polarized light.
The figure illustrates converting unpolarized
light into linearly polarized light. Light wave on
the left is unpolarized, but after passing through
the filter in the middle, it becomes polarized in
one direction, vertically in this example (Image
from Anton-Parr.com).
Optical activity: If a molecule has a
chiral center then it can interact with
linearly polarized light in such a way as to
change the angle of polarization when
the light exits the sample. The diagram
on the right shows a chiral compound
rotating light to the right by an angle α
(Image from chemwiki.ucdavis.edu).
 If the light is rotated clockwise to the right the compound is often referred to as D for dextrorotary
(latin for right). To avoid confusion with stereochemical nomenclature it is best to refer to compounds
that rotate light to the right as “+”.
 If the rotation is to the left, the compound is often referred to as L, for levorotary. Again, to avoid
confusion, it is best to refer to these compounds as “-“.
Stereochemistry and Optical Rotation: The optical rotation of
two enantiomers (compounds that are mirror images of each
other) will be the same magnitude but in opposite directions.
The direction of optical rotation is not necessarily related to the
stereochemical definition of chirality. Glyceraldehyde is an
example of a molecule where the chirality is the same as the
direction of optical rotation, the R enantiomer rotates light to
the right (D,+) and the S enantiomer rotates light to the left(L,-).
The absolute stereochemistry is determined by labeling
the groups W, X, Y, Z with Z being the lowest atomic number. If
two atoms attached to the chiral carbon have the same atomic
number, differentiate them based on the next atom, moving
out until a difference is found. Now point Z away from you and
if the order of WXY is clockwise it is R (left), if the order of WXY
is counterclockwise it is S. R-glyceraldehyde is shown as an
example on the right.
Applications of Optical rotation:
 Quantification of purity of solutions.
 Determining inversion of chiral centers during reactions.
01/01/2016
L-glyceraldehyde
(S)-glyceraldehyde
(-)-glyceraldehyde
D-glyceraldehyde
(R)-glyceraldehyde
(+)-glyceraldehyde
clockwise
=R
X
W
Z
X
W
Z
rotate Z
to back
Y
Y