Infra-Red Spectra of Films of Native and Denatured Pepsin

Abstract

WE report here some observations on the infrared absorption of films prepared by drying native, denatured, and regenerated pepsin solutions. On thermodynamic grounds¹ the alkali denaturation of pepsin molecules unrestricted in space (dilute aqueous solution) would be expected to involve the breaking of a large number of intra-molecular hydrogen bonds. As yet, direct evidence as to the formation of new intermolecular hydrogen bonds between uncoiled molecules is lacking. Differences in the infra-red absorption spectra of native and denatured pepsin in the range of hydrogen-bonding frequencies can, with reasonable assurance, be assumed to represent evidence for overall changes in hydrogen bonding. The infra-red spectra of films of pepsin and other enzymes measured in the wave-length region 2–15 microns^2,3 show, in addition to the strong bands around 3, 6 and 6.5 microns always obtained with proteins and polypeptides, several less intense absorption bands in both the 3-micron region and at wave-lengths longer than 6.6 microns. In this communication we are concerned only with the region 3,400–2,800 cm.⁻¹ (2.9–3.6 microns), in which the spectral measurements were made using calcium fluoride and lithium fluoride prisms, since this is a region of absorption by hydrogen-bonding frequencies.