Twists or turns: stabilising alpha vs. beta turns in tetrapeptides

Recent Advances in the Field of Amino Acid-Conjugated Aminoferrocenes-A Personal Perspective

The development of turn-based inhibitors of protein-protein interactions has attracted considerable attention in medicinal chemistry. Our group has synthesized a series of peptides derived from an amino-functionalized ferrocene to investigate their potential to mimic protein turn structures. Detailed DFT and spectroscopic studies (IR, NMR, CD) have shown that, for peptides, the backbone chirality and bulkiness of the amino acid side chains determine the hydrogen-bond pattern, allowing tuning of the size of the preferred hydrogen-bonded ring in turn-folded structures. However, their biological potential is more dependent on their lipophilicity. In addition, our pioneering work on the chiroptical properties of aminoferrocene-containing peptides enables the correlation of their geometry with the sign of the CD signal in the absorption region of the ferrocene chromophore. These studies have opened up the possibility of using aminoferrocene and its derivatives as chirooptical probes for the determination of various chirality elements, such as the central chirality of amino acids and the helicity of peptide sequences.

Promising Perspectives of the Antiproliferative GPER Inverse Agonist ERα17p in Breast Cancer

The estrogen receptor α (ERα) corresponds to a large platform in charge of the recruitment of a panel of molecules, including steroids and related heterocyclic derivatives, oligonucleotides, peptides and proteins. Its 295-311 region is particularly targeted by post-translational modifications, suggesting that it could be crucial for the control of transcription. In addition to anionic phospholipids, the ERα 295-311 fragment interacts with Ca²⁺-calmodulin, the heat shock protein 70 (Hsp70), ERα and possibly importins. More recently, we have demonstrated that it is prone to interacting with the G-protein-coupled estrogen receptor (GPER). In light of these observations, the pharmacological profile of the corresponding peptide, namely ERα17p, has been explored in breast cancer cells. Remarkably, it exerts apoptosis through GPER and induces a significant decrease (more than 50%) of the size of triple-negative breast tumor xenografts in mice. Herein, we highlight not only the promising therapeutic perspectives in the use of the first peptidic GPER modulator ERα17p, but also the opportunity to modulate GPER for clinical purposes.

A conserved sequence in the small intracellular loop of tetraspanins forms an M-shaped inter-helix turn

Tetraspanins are a family of small proteins with four transmembrane segments (TMSs) playing multiple roles in human physiology. Nevertheless, we know little about the factors determining their structure. In the study at hand, we focus on the small intracellular loop (SIL) between TMS2 and TMS3. There we have identified a conserved five amino acid core region with three charged residues forming an M-shaped backbone, which we call M-motif. The M´s plane runs parallel to the membrane surface and the central amino acid constitutes the inter-helix turning point. At the second position of the M-motif, in tetraspanin crystal structures we identified a glutamate oriented towards a lysine in the juxtamembrane region of TMS1. Using Tspan17 as example, we find that by mutating either the glutamate or juxtamembrane-lysine, but not upon glutamate/lysine swapping, expression level, maturation and ER-exit are reduced. We conclude that the SIL is more than a short linking segment but propose it is involved in shaping the tertiary structure of tetraspanins.

Conformational Preferences and Antiproliferative Activity of Peptidomimetics Containing Methyl 1'-Aminoferrocene-1-carboxylate and Turn-Forming Homo- and Heterochiral Pro-Ala Motifs

The concept of peptidomimetics is based on structural modifications of natural peptides that aim not only to mimic their 3D shape and biological function, but also to reduce their limitations. The peptidomimetic approach is used in medicinal chemistry to develop drug-like compounds that are more active and selective than natural peptides and have fewer side effects. One of the synthetic strategies for obtaining peptidomimetics involves mimicking peptide α-helices, β-sheets or turns. Turns are usually located on the protein surface where they interact with various receptors and are therefore involved in numerous biological events. Among the various synthetic tools for turn mimetic design reported so far, our group uses an approach based on the insertion of different ferrocene templates into the peptide backbone that both induce turn formation and reduce conformational flexibility. Here, we conjugated methyl 1'-aminoferrocene-carboxylate with homo- and heterochiral Pro-Ala dipeptides to investigate the turn formation potential and antiproliferative properties of the resulting peptidomimetics 2-5. Detailed spectroscopic (IR, NMR, CD), X-ray and DFT studies showed that the heterochiral conjugates 2 and 3 were more suitable for the formation of β-turns. Cell viability study, clonogenic assay and cell death analysis showed the highest biological potential of homochiral peptide 4.

Effect of Differential Geminal Substitution of γ Amino Acid Residues at the (i + 2) Position of αγ Turn Segments on the Conformation of Template β-Hairpin Peptides

The effect of insertion of three geminally dimethyl substituted γ amino acid residues [γ^2,2 (4-amino-2,2-dimethylbutanoic acid), γ^3,3 (4-amino-3,3-dimethylbutanoic acid), and γ^4,4 (4-amino-4,4-dimethylbutanoic acid)] at the (i + 2) position of a two-residue αγ C₁₂ turn segment in a model octapeptide sequence Leu-Phe-Val-Aib-Xxx-Leu-Phe-Val (where Xxx = γ amino acid residues) has been investigated in this study. Solution conformational studies (NMR, CD, and IR) and ab initio calculations indicated that γ^3,3 and γ^4,4 residues were well accommodated in the β-hairpin nucleating αγ C₁₂ turns, which gave rise to well-registered hairpins, in contrast to γ^2,2, which was unable to form a tight C₁₂ β-hairpin nucleating turn and promote a well-registered β-hairpin. Geminal disubstitution at the C^α carbon in γ^2,2 led to unfavorable steric contacts, disabling its accommodation in the αγ C₁₂ hairpin nucleating turn unlike the γ^3,3 and γ^4,4 residues. Geminal substitutions at different carbons along the backbone constrained backbone torsion angles for the three γ amino acid residues differently, generating diverse conformational preferences in them. Folded hairpins were energetically more stable (∼8 to 9 kcal/mol) than the unfolded peptides. Conformational preference of the peptides was independent of the N-terminal protecting group. Such fundamental understanding will instrumentalize the future directed design of foldamers.

Geometrically Diverse Lariat Peptide Scaffolds Reveal an Untapped Chemical Space of High Membrane Permeability

Constrained, membrane-permeable peptides offer the possibility of engaging challenging intracellular targets. Structure-permeability relationships have been extensively studied in cyclic peptides whose backbones are cyclized from head to tail, like the membrane permeable and orally bioavailable natural product cyclosporine A. In contrast, the physicochemical properties of lariat peptides, which are cyclized from one of the termini onto a side chain, have received little attention. Many lariat peptide natural products exhibit interesting biological activities, and some, such as griselimycin and didemnin B, are membrane permeable and have intracellular targets. To investigate the structure-permeability relationships in the chemical space exemplified by these natural products, we generated a library of scaffolds using stable isotopes to encode stereochemistry and determined the passive membrane permeability of over 1000 novel lariat peptide scaffolds with molecular weights around 1000. Many lariats were surprisingly permeable, comparable to many known orally bioavailable drugs. Passive permeability was strongly dependent on N-methylation, stereochemistry, and ring topology. A variety of structure-permeability trends were observed including a relationship between alternating stereochemistry and high permeability, as well as a set of highly permeable consensus sequences. For the first time, robust structure-permeability relationships are established in synthetic lariat peptides exceeding 1000 compounds.