Nascent β-Hairpin Formation of a Natively Unfolded Peptide Reveals the Role of Hydrophobic Contacts

Despite the important role of the unfolded states in protein stability, folding, and aggregation, they remain poorly understood due to the lack of residue-specific experimental data. Here, we explore features of the unfolded state of the NTL9 protein by applying all-atom replica-exchange simulations to the two fragment peptides NTL9(1–22) and NTL9(6–17). We found that while NTL9(6–17) is unstructured, NTL9(1–22) transiently folds as various β-hairpins, a fraction of which contain a native β-sheet. Interestingly, despite a large number of charged residues, the formation of backbone hydrogen bonds is concomitant with hydrophobic but not electrostatic contacts. Although the fragment peptides lack a proposed specific contact between Asp⁸ and Lys¹², the individually weak, nonspecific interactions with lysines together stabilize the charged Asp⁸, leading to a pK_a shift of nearly 0.5 units, in agreement with the NMR data. Taken together, our data suggest that the unfolded state of NTL9 likely contains a β-hairpin in segment 1–22 with sequence-distant hydrophobic contacts, thus lending support to a long-standing hypothesis that the unfolded states of proteins exhibit native-like topology with hydrophobic clusters.