Studies of Early Events of Folding of a Predominately β‑Sheet Protein Using Fluorescence Correlation Spectroscopy and Other Biophysical Methods

The interplay between the early collapse of the unfolded state and the formation of the secondary structure has been the subject of extensive research in protein chemistry. In this study, we used the intestinal fatty acid binding protein (IFABP), a small model protein with predominately β-sheet structure, to study the early events, including the early chain collapse and the formation of the secondary structure. We used a combination of fluorescence correlation spectroscopy and far-UV circular dichroism (CD) to understand how these early processes influence the late folding events like the stabilization of the secondary structure and aggregation. Acid-induced unfolded IFABP was found to collapse in the presence of low concentrations of added salt and aggregate at higher concentrations. Both the formation of the collapsed state and aggregation were conveniently probed by fluorescence correlation spectroscopy, a sensitive fluorescence technique with single-molecule resolution. In contrast, the formation of the secondary structure was monitored by far-UV CD. The results suggested that backbone hydrogen bond formation, not only the overall hydrophobicity of IFABP, may play crucial roles in the early collapse. Two mutant proteins positioned at a crucial nucleating site, namely, G80V and L64G, although being opposite in their overall hydrophobicity, collapsed relatively rapidly compared to the wild-type protein. The interconnection among the early collapse, the formation of the secondary structure, and aggregation was similar for these two mutants. Another mutant, G44V, which was identical in its overall hydrophobicity to G80V but situated in a region distant from the hydrophobic core, was found to be very different from G80V and L64G