Side-chain optical activity in cystine-containing proteins: circular dichroism studies.

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Abstract
Ellipticity bands in the circular dichroism spectra of insulin, ribonuclease, and lysozyme occur at wavelengths longer than 240 m[mu], and these are due to optically active transitions in the side chains (as distinct from peptide bond transitions at shorter wavelengths). In insulin, variation of pH from 8.4 to 11.5 leads to changes in the circular dichroism spectra which are incompatible with assignment of the bands to tyrosine transitions. The most probable source of optical activity in insulin at these wavelengths is the disulfide chromophore. Circular dichroism spectra of cystine, oxidized glutathione, and cystine disulfoxide exhibit el-lipticity bands of substantial magnitude at wavelengths corresponding to the (weakly allowed) adsorption bands in these compounds. The effects of long exposure to alkali, and of addition of mercaptoethanol and urea on these ellipticity bands in insulin are presented and discussed in terms of possible alteration of configuration about the disulfide bond in cystine residues. The origin of the bands in lysozyme and ribonuclease is discussed.