CIRCULAR DICHROISM AND OPTICAL ROTATORY DISPERSION OF ALPHA-GLIADIN

The circular dichroism spectrum of α-gliadin, a wheat protein, was measured in 10−3 m HCl (pH 3), 10−5 m HCl (pH 5), and 10−5 m HCl plus 0.005 m KCl (pH 5). Bands at 222, 208, and ca. 191 mμ indicate the presence of some helical structure. The ellipticity 11,300 (deg cm2)/dmole−1 at 222 mμ for pH 5 solutions suggests about one-third helical structure. Although the protein aggregates when a small amount of salt is added at pH 5, no corresponding change was found in the circular dichroism associated with the peptide bonds. We infer that no major conformational change occurs when the protein aggregates. Bands associated with side-chain optical activity at 277 and 294 mμ became more intense when the protein aggregated. This change suggests that the environment of aromatic side chains changes in a specific way during interaction between protein subunits. Optical rotatory dispersion and circular dichroism measured at various temperatures between 4 and 90° showed the molecule to be conformationally stable below about 30°. Above this temperature conformation changed progressively, the helical content decreasing, but even at 90° about 65% of the helical structure present at 25° remained intact. © 1968, American Chemical Society. All rights reserved.