Lec2_Spectroscopy_ML - Circular Dichroism For linear...

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Biophysical Methods Slide 1 Spectroscopy-2 Circular Dichroism For linear polarization the electric field vector of a plane wave is arbitrarily divided into two perpendicular components. Selecting a particular polarization means that one of the components is blocked such that the amplitude of that components is zero For circular polarization one of the components is phase shifted relative to the other, by ! /2 (90 degrees). Because of the phase shift, at the maximum of one component the other component is zero. Therefore, the direction of polarization rotates. This is called circular polarization. The x component may either be 90 degrees ahead of the y component or 90 degrees behind, leading to right-hand circular polarization or left-hand circular polarization, respectively. Biophysical Methods Slide 2 Spectroscopy-2 Circular Dichroism Circular Dichroism (CD) provides information about the unequal absorption of left- and right-handed circularly polarized light by chiral molecules. The difference in absorption of left- and right-circularly polarized light is Which gives applying Beer’s law:
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Biophysical Methods Slide 3 Spectroscopy-2 Circular Dichroism The different absorption of the left- and right-circularly polarized light gives rise to an ellipticity ( ! ) of the polarization defined as: E R and E L are the magnitudes of the electric field vectors of the right-circularly and left-circularly polarized light, respectively. ! is usually small so we approximate The light intensity I is proportional to E 2 and we get: Using Lambert Beer’s law Gives Biophysical Methods Slide 4 Spectroscopy-2 Circular Dichroism The ellipticity ! of the polarization, converted to units of degrees instead of radians, is thus related to the difference in molar absorption: Accordingly a molar ellipticity is defined as [ ! ]=100 ! /cL. Which yields
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Biophysical Methods Slide 5 Circular Dichroism Circular Dichroism (CD) provides information about the unequal absorption of left- and right-handed circularly polarized light by chiral molecules. Proteins are chiral molecules , and CD has become a standard technique for assessing conformational attributes of proteins as well as peptides. The CD bands of proteins and peptides are found in the same spectral regions as their absorption bands. CD bands in the far-UV region (170 - 250 nm) contain information about the peptide bonds of proteins. These bands can be used to monitor changes in secondary structure during a structural transition such as folding or ligand binding. For example, an " -helix displays a strong, characteristic CD spectrum in the far-UV region. CD bands in the near-UV region (250 - 300 nm) originate from aromatic amino acid residues and disulfide bonds. The near-UV spectrum provides a fingerprint of the folded conformation of a protein that is surpassed only by NMR spectroscopy.
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This note was uploaded on 03/29/2009 for the course A&EP 470 taught by Professor Lindau during the Fall '08 term at Cornell University (Engineering School).

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Lec2_Spectroscopy_ML - Circular Dichroism For linear...

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