PARTIALLY FOLDED STATES OF PROTEINS - CHARACTERIZATION BY X-RAY-SCATTERING

Partially folded states of proteins are found to occur with a wide var iety of degrees of unfolding, ranging from the compact molten globule to the fully unfolded forms, depending on solvent conditions and the s pecific protein involved. Small to intermediate angle X-ray scattering from partially folded states of proteins yields low resolution scatte ring profiles that may be used to explore the degree of folding of a p rotein under given solution conditions. By Monte Carlo simulation of a highly simplified homopolymer model, we show that such partially fold ed states will yield a characteristic scattering profile that may be w ritten as a Linear superposition of scattering from a compact core and of scattering from random coil loops that emerge from this core. We a lso find a term resulting from interference of X-rays scattering from the core with those scattering from the loops. This interference term oscillates in sign and tends to enhance the core portion of the scatte ring profile. We compare the model calculations of the scattering prof ile with measurements of the scattering profile as a function of salt concentration for cytochrome c at pH 2. Because of our characterizatio n of the scattering profiles, we suggest that these results may be re- interpreted in terms of the presence of a range of partially folded st ates as a function of pH and salt concentration, and that the observed scattering profiles are consistent with the characterization of the p artially folded states in terms of random coil loops emerging from a c ompact core with the loop fraction increasing as the salt concentratio n is decreased. This characterization is consistent with data on amide protection against H-H-2 exchange of compact regions within partially folded states observed for a number of proteins, including cytochrome c. (C) 1995 Academic Press Limited