Physical characterisation of the wheat protein purothionin

The α‐ and β‐purothionins, which differ slightly in amino acid composition and have slightly different mobilities in gel electrophoresis, were not different in other physical properties. Molecular weights determined by sedimentation equilibrium were 5100 and 5300, respectively, by osmotic pressure, 6100, and by gel permeation chromatography, about 7500. Circular dichroism (c.d.) spectra were compatible with about 40% α‐helical structure for the purothionins. An unusually strong negative band near 268 nm was found in the near‐u.v. region of the c.d. spectrum. The optical activity in this region may be attributed primarily to the high disulphide content of the molecule, since the purothionins contain no tryptophan and only a single tyrosine residue which appears to contribute some fine structure in the 270 to 290 nm region of the c.d. spectrum. The u.v.‐fluorescence spectrum was characteristic of tyrosine, but the low quantum yield was indicative of quenching by disulphide or other groups in the protein. An intrinsic viscosity of 2.7 cc/g suggests a compactly folded, globular structure for the purothionins. Mobilities of 11 × 10−5 cm2 V−1 s−1 at pH 3.1 and 7.2 × 10−5 cm2 V−1 s−1 at pH 8.6 were obtained from free‐boundary electrophoresis.