Triaspartate: a model system for conformationally flexible DDD motifs in proteins.

Understanding the interactions that govern turn formation in the unfolded state of proteins is necessary for a complete picture of the role that these turns play in both normal protein folding and functionally relevant yet disordered linear motifs. It is still unclear, however, whether short peptides can adopt stable turn structures in aqueous environments in the absence of any nonlocal interactions. To explore the effect that nearest-neighbor interactions and the local peptide environment have on the turn-forming capability of individual amino acid residues in short peptides, we combined vibrational (IR, Raman, and VCD), UV-CD, and (1)H NMR spectroscopies in order to probe the conformational ensemble of the central aspartic acid residue of the triaspartate peptide (DDD). The study was motivated by the recently discovered turn propensities of aspartic acid in GDG (Hagarman; et al. Chem.-Eur. J. 2011, 17, 6789). We investigated the DDD peptide under both acidic and neutral conditions in order to elucidate the effect that side-chain protonation has on the conformational propensity of the central aspartic acid residue. Amide I' profiles were analyzed in terms of two-dimensional Gaussian distributions representing conformational subdistributions in Ramachandran space. Interestingly, our results show that while the protonated form of the DDD peptide samples various turn-like conformations similar to GDG, deprotonation of the peptide eliminates this propensity for turns, causing the fully ionized peptide to exclusively sample pPII and ß-strand-like structures. To further explore the factors stabilizing these more extended conformations in fully ionized DDD, we analyzed the temperature dependence of both the UV-CD spectrum and the (3)J(H(N),H(α)) coupling constants of the two amide protons (N- and C-terminal) in terms of a simple two-state (pPII-ß) thermodynamic model. Thus, we were able to obtain the enthalpic and entropic differences between the pPII and ß-strand conformations of the central and C-terminal residue. For the central residue, we obtained ΔH(3) = -12.0 kJ/mol and ΔS(3) = -73.8 J/mol·K, resulting in a much larger room-temperature Gibbs free energy of 10.0 kJ/mol, which effectively locks the C-terminal in a ß-like conformation. A comparison of the temperature dependence of the chemical shifts reveals that there is indeed some type of protection of the amide protons from solvent in ionized DDD. This finding and several other lines of evidence suggest that both conformations of ionized DDD are stabilized by hydrogen bonding between the carboxylate groups of the central and C-terminal residue and the respective amide protons. These hydrogen bonds can be expected to be eliminated by side-chain protonation and substituted by hydrogen bonds between the N-terminal amide proton and the C-terminal carbonyl group as well as between the central aspartate side chain and the N-terminal amide proton. Hence, our results are indicative of a pH-induced switch in hydrogen-bonding patterns of aspartic acid motifs.