Preferential Solvation by Trifluoroethanol Drives α-Helical Folding in the Disordered S2 Region of the Escargot Protein.

The N-terminal domain of the Drosophila melanogaster Escargot transcription factor (Esg) is an intrinsically disordered region (IDR) that complements the DNA-binding activity of its C-terminal zinc fingers. Within this IDR, the S2 segment (residues 120-152) is predicted to form an α-helical molecular recognition feature, a transient structural element implicated in protein-protein interactions. We examined the conformational equilibrium of the S2 peptide in water and in helix-promoting 2,2,2-trifluoroethanol (TFE)/water solutions using replica exchange with solute tempering 2 (REST2) simulations and circular dichroism measurements. We show that the peptide can display substantial ellipticity, with TFE nearly doubling the helix population at 40% v/v compared to pure water. Minimum-distance distribution functions and the Kirkwood-Buff theory of solvation show that TFE preferentially accumulates on the peptide domain. This effect primarily arises from nonspecific contacts between TFE and uncharged polar and nonpolar side chains of the peptide. These findings support the view that the S2 region's structural plasticity is critical for modulating the function of Esg and provide further insights into TFE-induced helix stabilization.