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W6 - Ch24a Handout ZeNan Chang Changyi Li I Optical...

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Unformatted text preview: Ch24a Handout; lectures 11, 13 ZeNan Chang, Changyi Li I. Optical Activity i. a. Optical Rotation Molecules with chiral centers are optically active , meaning that when plane ‐ polarized light passes by, the plane of polarization is rotated. One can think of plane polarized light as the sum of left ‐ and right ‐ handed circularly polarized components of equal magnitude. A chiral molecule refracts the two components differently, thus altering the plane of polarization. where l = path length of cell in decimeters, c = concentration of solute in grams per 100ml, and [ α ] λ t is the “specific rotation” at a particular wavelength of light ( λ ) and temperature (t). The specific optical rotation for a polymer of optically active monomers is: Optical rotation at a single wavelength is generally used for determining concentration of chiral molecules and purity of chiral molecules (in comparison to a known standard). b. Optical Rotatory Dispersion (ORD) Optical rotatory dispersion is the dependence of [ α ] λ t or [m’] upon the wavelength of light. Enantiomorphs have mirror image ORD spectra (vertically reflected). ii. Circular Dichroism (CD) Dichroism refers to a molecule absorbing light of different polarization differently. Circular dichroism is the difference in absorption between left ‐ and right ‐ handed circularly polarized light. Note that when the absorptions of left ‐ and right ‐ handed circularly polarized light are unequal, the transmitted light will be elliptically polarized, where the tip of the electric field vector traces out an ellipse (rather than a circle). See the appendix to this handout, "Origin of Circular Dichroism," for an outline of the physical basis of CD, where it is shown why rotational strength is the imaginary component of the dot product of the electric and magnetic transition dipoles. Circular dichroism is often used to analyze the secondary structure of macromolecules, particularly proteins ( α‐ helices, β‐ sheets), and how they change with environmental conditions or upon interaction with other molecules. In addition to the figures in the slides, here is a sample image of optical rotatory dispersion (green) and circular dichroism (red), with the absorbance (blue) as well. (Taken from www.photophysics.com) iii. Circular Dichroism and Optical Rotatory Dispersion provide theoretically equivalent information In actuality, the ORD spectra ( φ ( λ )) and the CD spectra ( θ ( λ )) are the real and imaginary parts of the...
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