Glutamic acid Cotton effects

As shown in Fig. 16, artifactual deflections are also observed at wavelengths at which the lamp fails to emit light, in this case the region of resonance absorption by the mercury itself between 255 and 260 m/t, a situation identical with absorption by a chromophore in the optical path. A second deflection is obtained with poly-L-glutamic acid in the random coil as the peptide absorption band below 250 mu is entered. Since this deceptive effect, which will be more marked in the steeper dispersions characteristic of helices, can at lower wavelengths obscure the authentic Cotton effect feature observed at 233 m/t by Simmons et al. (1961) in helical polypeptides and proteins (Fig. 4), and since these authors propose the amplitude of this... 

Fig. 15. Rotatory artifacts that simulate Cotton effects at an absorption band. The dependence of the rotatory artifact on absorbance of p-cresol solutions placed in series with the same poly-L-glutamic acid solution is shown. The concentration of p-cresol was adjusted to give the total absorbance of chromophore plus polypeptide background that appears with each curve. The rotator, poly-L-glutamic acid, was at concentration of 0.5% at pH 7.0 in a 10-cm cell. The rotations are those actually observed, a, in degrees. The rotatory dispersion at Am 2 coincides almost exactly with that for the polypeptide alone, so that it has been omitted from the figure. At Am 4, an interference filter, /, with maximum transmission between 280 and 285 m/i, was placed in the optical path. The absorption spectrum, in arbitrary units, is typical of p-cresol plus poly-L-glutamic acid background. The emission spectrum is represented in arbitrary units, uncorrected for detector response. (Urnes et al., 1961a.)...

The presence of additional amino or carboxyl groups, as long as they are not close to the a-carbon atom (e.g., lysine or glutamic acid), has very little influence on the CD Cotton effect of the carboxylic n-7c transition. However, the presence of sulfide or other dissociable groups close to the a-carbon can strongly influence the CD spectra. 

---