Changes in ultraviolet absorption produced by alteration of protein conformation.

Transfer of an aromatic chromophore from the interior of a protein into the water solvent upon denaturation of the protein produces an absorption change throughout the ultraviolet region, 215 to 320 rnp, approximately 6 times the magnitude of that produced by transfer of the same chromophore from 20% ethylene glycol into water. In the 230 rnE.1 wave length region, the transfer of an indole chromophore produces an absorption change approximately 7 times that produced by the transfer of a phenolic chromophore; the chromophores of the amino acids cystine, histidine, and phenylalanine produce only slight changes in absorption. Difference spectra for protein denaturation were calculated by summation of the 20% ethylene glycol perturbation difference spectra of the amino acids, and multiplication by the factor of 6. Between 215 and 320 mp, difference spectra produced by acid, autolysis, or urea denaturation closely resemble calculated ones, but are red shifted 2 to 3 rnp near 280 rnp and 4 to 10 rnp near 230 mp. Thus, the absorption changes observed near 230 rnp for globular proteins result primarily from changes in the environment of the aromatic chromophores indole and phenol. The helix to coil transition and the denaturation-produced solvent perturbation of the amide group must contribute less than 10% of the absorption changes observed near 230 rnp.