A new basis for interpreting the circular dichroic spectra of proteins.

Experimental circular dichroic (CD) spectra of three proteins have been combined with estimates of the content of peptide-chain structural modes obtained from x-ray diffraction studies of the same proteins. Solution of the simultaneous equations at a series of wavelengths permits the construction of a CD spectrum for each of three structural modes: alpha-helix, beta-structure, and the so-called "random". The CD spectra thus obtained are compared with those obtained from polypeptide models. The alpha-helical spectra from the two approaches are nearly congruent, the beta-structure spectra are in fair agreement, and the third forms agree qualitatively, but are substantially different quantitatively. Comparisons are made between the present approach and earlier approaches to interpreting protein CD spectra. Certain advantages of the present approach are indicated.