Structure Elucidation - Natural Products Chemistry: Sources, Separations, and Structures - page 26

O O

H 3 C CH 3

O -

- O

NH 3 + NH 3 +

FIGURE 3.4 Example of a chiral molecule, alanine showing both forms: (S)-alanine and
(R)-alanine.

3.4 CIRCULAR DICHROISM
Circular dichroism (CD) refers to the differential absorption of left and right circu-
larly polarized light. It is exhibited in the absorption bands of optically active chi-
ral molecules. CD spectroscopy has a wide range of applications in many different
fields. Most notably, UV CD is used to investigate the secondary structure of proteins
and can allow a measure of the differences in the absorption of left-handed polarized
light versus right-handed polarized light, which arise due to structural asymmetry.
The absence of a regular structure results in zero CD intensity, whereas an ordered
structure results in a spectrum, which can contain both positive and negative signals.
A chiral molecule has a non-superimposable mirror image, due to the fact that
the carbon atom is asymmetric when fully substituted. An example is the amino acid
alanine shown in Figure 3.4.
The classical example of chirality is our hand. The left hand is non-superimposable
on the right hand and is thus the mirror image. In chemical terms, the two mirror
images of a chiral molecule are called either enantiomers or are known as optical
isomers. The exciton chirality method is a nonempirical method developed from the
“dibenzoate chirality method.” The theoretical base of the exciton chirality method
is the coupled oscillator theory or group polarizability theory. When two (or more)
strongly absorbing chromophores are spatially near to each other and constitute a
chiral system, the interactions between their transition dipoles is responsible for large
rotational strengths, often surpassing those associated with the perturbations on each
chromophore exerted by the chiral nonchromophoric skeleton. One important inter-
action is the coupling between two (or more) electric transition moments (exciton
coupling), which results in the splitting of the excited energy level. The sign of exciton
chirality determines the stereochemistry. An example is shown in Figure 3.5.

3.5 X- RAY CRYSTALLOGRAPHY
X-ray crystallography, illustrated in Figures 3.6 and 3.7, is used for determining the
arrangement of atoms within a crystal, in which a beam of X-rays strikes the crystal
and causes the beam of light to spread into many specific directions. From the angles
and intensities of these diffracted beams, a crystallographer can produce a three-
dimensional picture of the density of electrons within the crystal. From this electron