Circular dichroism

Right and left circularly polarized light is influenced differently in a layer that contains an enantiomer of an optically active chiral substance.

Circular dichroism , also known as circular dichroism , or CD for short , is a special property of optically active ( chiral ) molecules . It is a special form of dichroism in which the two enantiomers of a chiral substance absorb circularly polarized light to different degrees . Alternatively, one can also measure how strongly an enantiomer of a chiral substance absorbs two light rays that only differ in the direction of rotation of their circular polarization (see figure).

Circular dichroism enables the structure of optically active chiral molecules to be elucidated by means of CD spectroscopy : the absorption difference is measured for light of different wavelengths , and the desired CD spectrum is obtained, which should be a mirror image for the two enantiomers of a molecule or for the two directions of rotation of the circular polarization.

Applications

A CD spectrum provides information about the stereochemistry of the examined chiral substances. In combination with optical rotational dispersion and other chiroptical methods, CD spectra enable the definition of the absolute configuration of molecules.

Often the method is used to study simple biological substances such as amino acids and sugars . It is also used to determine the secondary structure of more complex substances, such as the helix and sheet structures of peptides and nucleic acids . In this case, however, X-ray structure analysis and NMR spectroscopy are often much better suited for structure elucidation. The X-ray structure analysis assumes that single crystals of the sample to be examined are present.

Chiroptical spectroscopic methods

• for chiral molecules:
  • ECD spectroscopy (electronic circular dichroism, ECD): CD spectroscopy in the UV / vis range
  • ORD ( optical rotary dispersion )
  • VCD spectroscopy (vibrational circular dichroism, VCD): CD spectroscopy in the IR range
• for achiral molecules (a magnetic field also induces a change in polarization and intensity of polarized light transmitted through the Cotton-Mouton effect ):
  • M CD spectroscopy ( magnetocircular dichroism )
  • M ORD spectroscopy ( magneto-optical rotation dispersion )

literature

• Alfred Pingoud , Claus Urbanke: Working methods of biochemistry . Walter de Gruyter, 1999, ISBN 978-3-11-016513-5 , p. 245 ( limited preview in Google Book Search).
• Walter Hoppe : Biophysics . Springer-Verlag, 1982, ISBN 978-0-387-11335-7 , pp. 152 ( limited preview in Google Book search).
• Martin Holtzhauer, J. Behlke: Methods in protein analysis . Springer, 1996, ISBN 978-3-540-60210-1 , pp. 103 ( limited preview in Google Book search).
• Gerhard Richter: Practical Biochemistry . Georg Thieme Verlag, 2003, ISBN 978-3-13-132381-1 , p. 122 ( limited preview in Google Book search).
• Kelly SM, Jess TJ, Price NC: How to study proteins by circular dichroism . Biochimica et Biophysica Acta 1752, Elsevier Verlag, 2005 ( available from ScienceDirect ).

Web links

• Circular dichroism at chemgapedia.de

Individual evidence

1. ↑ Science Online Lexica : Entry on chiroptic methods in the Lexicon of Chemistry. Retrieved August 30, 2009.