Targeting of a Helix-Loop-Helix Transcriptional Regulator by a Short Helical Peptide.

The Id proteins (Id1-4) are cell-cycle regulators that play a key role during development, in cancer and vascular disorders. They contain a conserved helix-loop-helix (HLH) domain that folds into a parallel four-helix bundle upon self- or hetero-association with basic-HLH transcription factors. By using such protein-protein interactions, the Id proteins inhibit cell differentiation and promote cell-cycle progression. Accordingly, their supporting role in cancer has been convincingly demonstrated, which makes these proteins interesting therapeutic targets. Herein we present a short peptide containing an (i,i+4)-lactam bridge and a hydrophobic (Φ) three-residue motif Φ(i)-Φ(i+3)-Φ(i+6), which adopts a helical conformation in water, shows Id protein binding in the low-micromolar range, penetrates into breast (MCF-7 and T47D) and bladder (T24) cancer cells, accumulates in the nucleus, and decreases cell viability to ∼50 %. Thus, this cyclopeptide is a promising scaffold for the development of Id protein binders that impair cancer cell viability.

Visible-light photoredox-catalyzed desulfurization of thiol- and disulfide-containing amino acids and small peptides.

A scalable protocol for the desulfurization of cysteine by using visible light, the photocatalyst Ir(dF(CF3 )ppy)2 (dtb-bpy)PF6 and triethylphosphite under biphasic reaction conditions has been developed. The loading of the catalyst can be as low as 0.01 mol%, which can be efficiently removed during the workup (≤0.3 ppm), giving rise to the corresponding desulfurized product in high yields. This method has been applied also to cystine, penicillamine, and reduced and oxidized glutathione. The desulfurization has been found to be pH sensitive, with an optimal pH value of 6.5 and 7.0 for the cysteine derivatives and glutathione, respectively. In addition, during the desulfurization of a decapeptide containing cysteine and methionine, concurrent oxidation of the two sulfur-containing residues to disulfide and sulfoxide has been observed. Therefore, whereas the presented protocol allows a straightforward visible light-mediated desulfurization of simple thiols by using very low catalyst loading and a cost-effective trialkylphosphite as thiyl radical trapping agent, its application to complex substrates needs to be carefully validated. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Impact of the amino acid sequence on the conformation of side chain lactam-bridged octapeptides.

Synthetic helical peptides are valuable scaffolds for the development of modulators of protein-protein interactions involving helical motifs. Backbone-to-side chain or side chain-to-side chain constraints have been and still are intensively exploited to stabilize short α-helices. Very often, these constraints have been combined with backbone modifications induced by Cα-tetrasubstituted, ß-, or γ-amino acids, which facilitate the α-peptide or α/ß/γ-peptide adopting an α-helical conformation. In this work, we investigated the helical character of octapeptides that were cyclized by a Lys-Asp-(i,i + 4)-lactam bridge. We started with two sequences extracted from the helix-loop-helix region of the Id proteins, which are inhibitors of cell differentiation during development and in cancer. Nineteen analogs containing the lactam bridge at different positions and displaying different amino acid core triads (i + 1,2,3) as well as outer residues were prepared by solid-phase methodology. Their conformation in water and water/2,2,2-trifluoroethanol mixtures was investigated by circular dichroism (CD) spectroscopy. The cyclopeptides could be grouped in helix-prone and non-helix-prone structures. Both the amino acid core triad (i + 1,2,3) and the pendant residues positively or negatively affected the formation of a helical structure. Computational studies based on the NMR-derived helical structure of a cyclopeptide containing Aib at position (i + 2) of the triad were generally in agreement with the secondary structure propensity of the cyclopeptides observed by CD spectroscopy. In conclusion, the Lys-Asp-(i,i + 4)-lactam bridge may succeed or fail in the stabilization of short helices, depending on the primary structure. Moreover, computational methods may be valuable tools to discriminate helix-prone from non-helix-prone peptide-based macrolactams. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.