CHM 114: Bioorganic Molecules
Stereochemistry and Polarimetry
One of the great unsolved mysteries of life concerns the origin of optically active compounds. Living systems are
composed of dissymmetric (chiral) molecules, and molecular dissymmetry is necessary for life as we know it, but
how did such dissymmetry come about? Was there one single dissymmetric molecule at one point in history that
gave rise to all molecular dissymmetry on Earth? Or is molecular dissymmetry a phenomenon that appeared
independently at many different locations as a consequence of some kind of fundamental dissymmetry in the
universe? The discovery, in 1997, that some amino acids in an Australian meteorite contain an excess the Lenantiomer appears to support the hypothesis that some natural process favors one enantiomer over another.
But convincing proof of such a hypothesis is hard to come by and the answer may never fully be known.
The basic molecules of life (proteins, carbohydrates, nucleic acids) are chiral and are built up of smaller units that
are also chiral. A strand of DNA, for example, consists of two long chains, each having a backbone of linked
chiral sugar (D-2-deoxyribose) molecules twisted into a right-handed double helix. DNA and RNA regulate the
synthesis of proteins from L-amino acids.
Enantiomers have identical physical properties except the direction in which they rotate plane-polarized light. If
one enantiomer rotates plane-polarized light in a clockwise (+) direction, the other enantiomer will rotate planepolarized light by the same angle in a counterclo0ckwise (-) direction. Enantiomers may also differ in some
chemical properties, such as the way in which they interact with other chiral molecules. These differences are
particularily important in biochemical systems. For example, some kinds of olfactory receptors are apparently
chiral, so the (+) and (-) enantiomers of a compound that interacts with these chiral receptors may smell different.
Carvone, a terpenoid found in the essential oils of both caraway seeds and the spearmint plant, is an example of
a chiral compound whose enantiomers have decidedly different odors.
CH3
CH3
O
H
(R)-Carvone
O
H
(S)-Carvone
In the first part of this experiment, you will determine which direction (R) and (S)-carvone rotate plane polarized
light and determine their specific rotation. Your calculated specific rotation can then be compared to the accepted
literature value for (+) and (-)-carvone.
In the second part of this experiment, you will be given a sample of limonene (another chiral terpene) and using
the literature value for its specific rotation, you will determine the concentration of your limonene sample.
Polarimetry
Light can be considered a wave phenomenon (remember your physics) with vibrations occurring in an infinite
number of planes perpendicular to the direction of propagation. When a beam of ordinary light passes through a
polarizer, such as a Nicol prism, only the light wave components that are parallel to the plane of the polarizer can
pass through. The resulting beam of plane-polarized light has light waves whose vibrations are restricted to a
single plane. When a beam of plane-polarized light passes through an optically active substance, molecules of
the substance interact with the light so as to rotate its plane of polarization. The angle by which a sample of an
optically active substance rotates the plane of polarization is called its observed rotation, α. The observed
rotation of a sample depends on the length of the light path through the sample (l) and the concentration of the
sample (c). These two variables are not intrinsic properties of the sample itself, so the observed rotation can be
divided by these factors to yield the sample’s specific rotation, [α].The cell length and sample concentration must
be given in standard units of dm for cell length and g/mL for the concentration.
Where:
[α] = αobs/(l)(c)
♦
[α] = specific rotation
♦
αobs = observed rotation
♦
l = sample cell length, in decimeters (dm)
♦
c = concentration of the solution in grams of solute per milliliter of solution (g/mL). For a pure compound,
substitute the density in g/mL.
Procedure:
Measure the optical rotation of both (R) and (S)-carvone from the samples provided. Be sure to record the name
and concentration of the sample. The polarimeter cells used today have a 5 cm cell length. Calculate the specific
rotation using the above equation. Be sure to have the proper units for concentration and cell length. Compare
your calculated specific rotation to the literature value. You will need to look-up the literature value in the CRC or
some other handbook.
Measure the optical rotation of the two limonene unknowns. First determine whether each sample has a (+) or (-)
rotation. Then look-up the specific rotation for limonene. Using this information, calculate the concentration of
limonene in each of the unknowns. (+)-Limonene has the R absolute configuration at its chiral center and (-)limonene has the S absolute configuration. Using the structure of limonene given below, draw the correct 3dimensional structure for both (R) and (S)-limonene.
Limonene