Functional, metal-based crosslinkers for α-helix induction in short peptides

Abstract

Many protein-protein interactions that play a central role in cellular processes involve α-helical domains. Consequently, there has been great interest in developing strategies for stabilizing short peptides in α-helical conformations toward the inhibition and interrogation of protein-protein interactions. Here, we show that tridentate Hybrid Coordination Motifs (HCMs), which consist of a natural (histidine, His) and an unnatural (8-hydroxyquinoline, Quin) metal binding functionality, can bind divalent metal ions with high affinity and thereby induce/stabilize an α-helical configuration in short peptide sequences. The Quin functionality is readily introduced onto peptide platforms both during or after solid-state peptide synthesis, demonstrating the preparative versatility of HCMs. A systematic study involving a series of HCM-bearing peptides has revealed the critical importance of the length of the linkage between the Quin moiety and the peptide backbone as well as the metal coordination geometry in determining the extent of α-helix induction. Through ZnII coordination or modification with ReI(Quin)(CO)3, the HCM-bearing peptides can be rendered luminescent in the visible region, thus showing that HCMs can be exploited to simultaneously introduce structure and functionality into short peptides.

Tunable helicity, stability and DNA-binding properties of short peptides with hybrid metal coordination motifs

“…The dissociation constants of the metal-HCM complexes ( K d,M ) range from 30 nM for Co II to 600 fM for Cu II (Table 1, Table S3), the affinities roughly following the Irving-Williams series: Co II < Ni II <Cu II ≫ Zn II , which was also observed in the case of His-Phen HCMs placed on the cyt cb 562 surface and His-Quin HCMs (also tridentate) incorporated into 10-residue long peptides. 37-39 The dissociation constants corresponding to the metal-mediated dimerization event ( K d,dimer ) are all in the micromolar regime (1-200 μM), again similar to the dimerization constants of His-Phen modified cyt cb 562 and His-Quin modified peptides. 37-39 Importantly, the K d,M values are ∼3-fold (for Co II ) to >3 orders of magnitude (for Cu II higher than those determined for the free Phen ligand, 40 and up to 6 orders of magnitude higher than those for i/i +4 bis-His motifs incorporated into short peptides.…”

“…37-39 The dissociation constants corresponding to the metal-mediated dimerization event ( K d,dimer ) are all in the micromolar regime (1-200 μM), again similar to the dimerization constants of His-Phen modified cyt cb 562 and His-Quin modified peptides. 37-39 Importantly, the K d,M values are ∼3-fold (for Co II ) to >3 orders of magnitude (for Cu II higher than those determined for the free Phen ligand, 40 and up to 6 orders of magnitude higher than those for i/i +4 bis-His motifs incorporated into short peptides. 32,41 Electrospray ionisation mass spectrometry (ESI-MS) measurements were carried out on HCM-bearing peptides after incubation with 1 or 2 equiv.…”

“…Interestingly, this trend is quite distinct from that observed for the peptides bearing His-Quin HCMs, where Cu II binding yielded the most helicity and Ni II frequently the least. 39 Although Phen and Quin are both planar bidentate ligands with similar bite angles, they differ in terms of the size of the aromatic moiety, composition of the donor atoms (N/N vs N/O), and, potentially in charge of the ligand upon metal coordination (0 vs -1). These differences could result in Phen and Quin ligands directing the formation of alternate crosslinking geometries which would change the effective length of the metal crosslink across i and i +7 positions, and thereby modulate the structure of the metal-bound peptide backbone ( vide infra ).…”

“…1a), but they would also provide stable coordination motifs whose metal binding properties can be modulated through the choice of metal, chelating ligand or the solution pH. This modularity can in turn be exploited for the incorporation of metal-based functions (e.g., luminescence and catalysis) 39 or metal-mediated oligomerization (as we exploit in this report).…”

“…39 In the current report, we build further upon this work by examining 1) whether His-Phen HCMs are also capable of helix induction in small peptides in order to expand the possible peptide-HCM ligand set, 2) the effects of secondary elements such as the identity of the i +4 residue on HCM-mediated helix induction, and 3) the protection of HCM-stabilized peptides from proteolysis. Furthermore, motivated by our observations on the HCM-induced formation of discrete V-shaped protein dimers, we investigate 4) the ability of an HCM-bearing peptide scaffold to form α-helical dimers for selective DNA recognition and binding.…”