Examining the Effect of an Anion-Binding Reagent on the Structure of Deprotonated Leucine Enkephalin Using Cryogenic-Ion Infrared Action Spectroscopy.

Biomolecular systems feature a complex interaction network comprising numerous intra- and intermolecular interactions. By isolating biomolecules under vacuum conditions, the intricate balance between specific interaction motifs can be characterized with precise control over conditions. In this study, we apply cryogenic-ion infrared action spectroscopy and electronic structure methods to examine the structural changes in the deprotonated form of the model peptide leucine enkephalin (YGGFL) upon complexation with diserinol isophthalamide (DIP), an anion-binding reagent. The low-energy conformer of the uncomplexed, deprotonated peptide ([YGGFL - H]) adopts a noncanonical turn structure stabilized by intramolecular ionic hydrogen bonding to the C-terminal carboxylate moiety. Despite the favorability of DIP to strongly coordinate with carboxylate residues, we find that the structure of the peptide is largely unaffected by the binding of DIP. Instead, DIP only partially coordinates with the carboxylate moiety and is positioned below the backbone turn of YGGFL to engage in additional hydrogen bonding interactions. These findings underscore the stability of the turn structure and the strong energetic penalty imposed by disruption of this motif even when strong intermolecular coordination is expected.