Comparison of ORD and CD

In what follows, we compare ORD and CD by sketching the typical curves for proteins and polypeptides in a-hehcal, p-hehcal, and random coil forms. We first give a few theoretical remarks to provide background. It is now well known that the spectral region of the amide involves and nj-n transitions. According to 

Moffit, the electronic dipole transition moments and the optical activity of helical molecules are coupled. The electronic transition 7i° — n occurs at around 190 nm, while that of rii — n occurs at around 225 nm. The actual values, of course, depend on the stmcture of the molecules at specific conditions. They are not exactly at 190 and 225 nm, respectively. Still, it is worthwhile to focus on the curve near these two wavelengths. 

FIGURE 12.15 CD of a polypeptide —, helical structure —, disordered state. 

Djerassi, C., Optical rotatory Dispersion. New York McGraw-HiU, 1960. 

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For these authors an OA sulfoxide would be an interesting co-monomer to induce asymmetry into the backbone of a polymeric chain but this is not very clear again. Comparison of ORD and CD curves of the model Xc and of the copolymer Xa indicates that copolymerization occurs with retention of configuration of the sulfur atom and that the sulfoxide group does not lose its configuration when adjacent to a carbanion. 

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