37—THE ULTRAVIOLET ABSORPTION SPECTRUM OF SOLID KERATIN

Abstract

A technique has been developed for determining the ultraviolet absorption spectrum of native keratin in the form of thin longitudinal sections of horsehair, using a standard spectrophotometer with only minor modifications. The technique, which includes a procedure of correcting for scattering by the specimen, is adequate to determine such spectral features as peak positions; it cannot be used at present for reliable measurements of the true absorption due to chromophoric groups, on account of uncertainties in the scattering correction. It is found that, in native keratin, the tyrosine absorption peak occurs at 278—279 mμ, compared with 274·5 mμ for the free aminoacid in aqueous solution. The magnitude of this bathochromic shift is similar to that observed in other proteins, and is generally interpreted¹ as being caused by a hydrogen-bond interaction involving the phenolic −OH group of the tyrosyl residue. The position of the peak is unchanged when the specimen is treated, at 20°C, with acid at pH 1·5 or with unbuffered 8M urea, but a slight shift is sometimes observed when a specimen is stretched to 37% extension. Immersion in 8M LiBr at 20°C shifts the peak to about 275 mμ. These results may be linked with supercontraction data; it is found that, under the stated conditions, a specimen supercontracts in lithium bromide, but not in urea. The polarization spectrum indicates a small amount of parallel dichroism. but this may be due to dichroic scatter rather than to dichroic absorption. Difference spectra, obtained by subtracting optical densities of a specimen taken in 8M LiBr at 20°C from those taken in water at the same temperature, exhibit the characteristic tyrosine difference peaks at about 279 and 287 mμ. The spectrum of specimens treated with hot sodium hydroxide or exposed to ultraviolet light show increased absorption, particularly at wavelengths above 300 mμ.