Structure Elucidation - Natural Products Chemistry: Sources, Separations, and Structures - page 27

FIGURE 3.5 Example of a circular dichroism (CD) spectrum.

density, the mean positions of the atoms in the crystal can be determined, as well as
their chemical bonds, their disorder, and various other information.
In an X-ray diffraction measurement, a crystal is mounted on a goniometer and
gradually rotated while being bombarded with X-rays, producing a diffraction pat-
tern of regularly spaced spots known as reflections. . The two-dimensional images
taken at different rotations are converted into a three-dimensional model of the
density of electrons within the crystal using the mathematical method of Fourier
transforms, combined with chemical data known for the sample. Poor resolution
(fuzziness) or even errors may result if the crystals are too small or not uniform
enough in their internal makeup. Producing good quality crystals is still recognized
as an art!

HISTORICAL NOTE

Wilhelm Conrad Röntgen discovered X-rays in 1895, just as the studies of
crystal symmetry were being concluded. The use of X-rays created the X-ray
diffraction pattern of crystals. The study of X-ray crystallography of bio-
logical molecules accelerated with the contributions from Dorothy Crowfoot
Hodgkin, who solved the structures of cholesterol (1937), vitamin B 12 (1945),
and penicillin (1954), for which she was awarded the Nobel Prize in Chemistry
in 1964. In 1969, she succeeded in solving the structure of insulin on which she
worked for over 30 years.