Characterising Halogen Bonding in a Cooperative Model System

Abstract: Non-covalent interactions are the main driving factor for secondary structure formation of biomacromolecules and are of enormous importance for the direction of molecular recognition events, for example those of importance for drug discovery. Profound understanding of these interactions is of vital importance to achieve high affinity and selectivity of a ligand to a chosen target. Halogen bonding adds another interaction to the toolbox of non-covalent interactions of biomolecules, currently encompassing hydrogen bonds, van der Waals interactions, π-π interactions and salt bridges. As the halogen bond has similar strength and directionality to the hydrogen bond, it is suitable for implementation into biomolecules and drug candidates. Regardless its potential, examples where intuitive introduction of halogen bonding in protein-like structures was exploited for structure stability, target selectivity or binding enhancement, are scarce. Motivated to make halogen bonds applicable in biomolecules, we designed a β-hairpin forming peptide inheriting a XB interaction site R-I···O-R. High resolution solution NMR spectroscopy provided insights in the dynamic conformational behaviour influenced by an array of interactions directing the structure of flexible molecules in solution. The combination of NOE-based interproton distance analysis and computational investigation demonstrates the capability of conformational stabilisation of a β-hairpin foldamer by a halogen bond. Residual dipolar coupling (RDC) analysis proved that the bond orientations are compatible with a geometry, in which a conformation stabilizing I···O halogen bond is formed. This study demonstrates the use of a cooperatively folding system to detect a weak secondary interaction, a halogen bond, in solution by detecting its influence on the overall folding stability.