Long-range interactions stabilize the fold of a non-natural oligomer

Toward designing nonbiological polymers that fold into predictable tertiary structures, we report a "beta-oligomer" composed of beta-amino acids that adopts a cooperatively folded structure. We have computationally designed a C(2)-symmetrical pair of interacting 14-helical beta-oligomers stabilized via long-range interhelical interactions and stapled together by a disulfide bond. The reduced (BHBred) and oxidized (BHBox) forms of the synthetic beta-oligomer represent the individual isolated helices and the two-helix bundle, respectively. We also prepared a third monomeric synthetic beta-oligomer (BHBmon) to avoid inadvertent disulfide formation during characterization. Circular dichroism spectroscopy revealed that BHBox showed a 2-fold increase in secondary structure, relative to the monohelical controls, BHBred and BHBmon. Further, BHBox showed a sigmoidal thermal unfolding curve with a per-residue van't Hoff enthalpy of approximately 0.7 kcal/(mol.residue), analogous to folded proteins. In contrast, BHBmon shows a broad thermal transition, typical of multistate unfolding for monomeric helices. Also, analytical ultracentrifugation showed that BHBmon and BHBox were monomeric at concentrations < or =800 and 280 microM, respectively. Therefore, the enhanced helicity of BHBox could be attributed to intramolecular helix-helix interactions.