Repeating Aspartic Acid Residues Prefer Turn-like Conformations in the Unfolded State: Implications for Early Protein Folding.

Protein folding can be described as a motion of the polypeptide chain in a potential energy funnel, where the conformational manifold is narrowed as the chain traverses from a completely unfolded state until it reaches the folded (native) state. The initial folding stages set the tone for this process by substantially narrowing the manifold of accessible conformations. In an ideally unfolded state with no long-range stabilizing forces, local conformations (i.e., residual structures) are likely to drive the folding process. While most amino acid residues tend to predominantly adopt extended structures in unfolded proteins and peptides, aspartic acid exhibits a relatively high intrinsic preference for turn-forming conformations. Regions in an unfolded polypeptide or protein that are rich in aspartic acid residues may therefore be crucial sites for protein folding steps. By combining NMR and vibrational spectroscopies, we observed that the conformational sampling of multiple sequentially neighbored aspartic acid residues in the model peptides GDDG and GDDDG even show an on average higher propensity for turn-forming structures than the intrinsic reference system D in GDG, which suggests that nearest neighbor interactions between adjacent aspartic acid residues stabilize local turn-forming structures. In the presence of the unlike neighbor phenylalanine, nearest neighbor interactions are of a totally different nature in that it they decrease the turn-forming propensities and mutually increase the sampling of polyproline II (pPII) conformations. We hypothesize the structural role of aspartic residues in intrinsically disordered proteins in general, and particularly in small linear motifs, that are very much determined by their respective neighbors.