Studies on secondary structure in chicken egg-white lysozyme after reductive cleavage of disulfide bonds

Native lysozyme and three reduced derivatives (carboxymethyl, carboxamidomethyl, and triphenylethyl-phosphonium) were examined by circular dichroism, and fractions of the protein chain present as alpha-helix, beta structure, and unordered structure were estimated by a computeradapted curve-fitting program. The alpha helix ranged from 2 to 23% in the three products, while native lysozyme exhibited 26 to 31%. However, beta structure in the reduced samples occupied 23-62% of the chain length (the latter in 50% methanol), consistently in excess of the native content(11-16%). Secondary structure in the reduced samples increased with pH, while that of the native protein remained nearly constant. In 8 M urea alpha helix was mostly eliminated from the reduced protein, while beta structure was nearly unaffected. Qualitatively, a partial loss of beta structure appeared to result from peptic digestion of the reduced samples, with further loss on exposure of the digest to urea. Stability of the observed beta structure indicates its existence prior to the oxidation of SH groups, which is concomitant with development of the native conformation. This structure could, therefore, constitute at least part of a precursor conformation in the formation of native structure.