The middle ultraviolet spectra of proteins. I. Studies on model compounds

Abstract

In order to facilitate a study of the middle ultraviolet (220–250 nm) spectra of proteins, an examination of the spectral properties of model compounds (histidine, phenylalanine, tryptophan, tyrosine, and related compounds) containing protein chromophores was first undertaken. The object was to evaluate the effect of changes in the solvent composition or pH on the spectra. Considerable numerical data were collected, which permit the following conclusions, (a) The addition of ethylene glycol, urea, or salts to an aqueous solution of these chromophores causes shifts in their spectra, with resultant difference spectra. Values at the extrema of these difference spectra are usually linear functions of the solvent composition. (b) The extrema of the middle ultraviolet spectra arising from changes in pH can be used to follow the protonation of titratable groups, (c) The solvent effects observed support the conclusions that the major absorption band of the aromatic compounds is due to a π-π* transition, and that a minor transition is involved in the spectrum of tyrosine.