Formation of a CH-π Contact in the Core of Native Barstar during Folding

An important part of the protein folding process is the consolidation of the protein core through the formation of specific, directional contacts after the initial hydrophobic collapse. Here, we simultaneously monitor formation of core contacts and assembly of secondary structure through salt-induced folding by using resonance Raman spectroscopy. Unfolded barstar at pH 12 was refolded by gradual addition of sodium sulfate salt. Altered spectral characteristics of the Trp53 residue suggest that the core of the protein attains a CH-pi interaction at a low concentration of the salt, with an increase in the packing density. Further increase in salt concentration produces a reduction in the solvent accessibility of the core. These data provide evidence that the core of the protein becomes rigid upon the addition of 0.6 M sodium sulfate. This is the first time that the formation of a CH-pi interaction has been directly monitored during the folding of a protein.