Fluorinated 2,6-Xylenesulfonyl Group: A Protective Group for Amines

We have developed a fluorinated 2,6-xylenesulfonyl group (fluorinated xysyl, ^fXs) as a protective group for amines. The sulfonyl group could be attached to amines by reactions with the corresponding sulfonyl chloride, and survived various conditions, including acidic, basic, and even reductive conditions. The ^fXs group could be cleaved by treatment with a thiolate under mild conditions.

Cyclic peptides bearing the d-Phe-2-Abz turn motif: Structural characterization and antimicrobial potential

The effect on secondary structure and antimicrobial activity of introducing different cyclic constraints in linear β-hairpin antimicrobial peptides has been investigated with the intention of generating cyclic β sheets as promising antimicrobials with improved therapeutic potential. The linear peptides were cyclized head to tail either directly or after the addition of either a second turn motif or a disulfide bridge. The propensity of these peptides to adopt a cyclic β-sheet structure has been correlated to their antibacterial activity. All cyclic peptides showed enhanced activity, compared with their linear counterparts against methicillin-resistant Staphylococcus aureus. Scanning electron microscopy and transmission electron microscopy studies showed that this family kills bacteria through membrane lysis. The peptide that showed the best efficacy against all strains (exhibiting nanomolar activity), while retaining low haemolysis, bears two symmetrical, homochiral d-phe-2-Abz-d-ala turns and adopted a flexible structure. Its twin peptide that bears heterochiral turns (one with d-ala and one with L-Ala) showed reduced antibacterial activity and higher percentage of haemolysis. Circular dichroism and nuclear magnetic resonance spectroscopy indicate that heterochirality in the two turns leads to oligomerization of the peptide at higher concentrations, stabilizing the β-sheet secondary structure. More rigid secondary structure is associated with lower activity against bacteria and loss of selectivity.

Turn Conformation of β-Amino Acid-Based Short Peptides Promoted by an Amidothiourea Moiety at C-Terminus

A C-terminal amidothiourea motif is shown to promote a β-turn-like folded conformation in a series of β-amino acid-based short peptides in both the solid state and solution phase by an intramolecular 11-membered ring hydrogen bond.

Crystal and NMR Structures of a Peptidomimetic β-Turn That Provides Facile Synthesis of 13-Membered Cyclic Tetrapeptides

Herein we report the unique conformations adopted by linear and cyclic tetrapeptides (CTPs) containing 2-aminobenzoic acid (2-Abz) in solution and as single crystals. The crystal structure of the linear tetrapeptide H₂ N-d-Leu-d-Phe-2-Abz-d-Ala-COOH (1) reveals a novel planar peptidomimetic β-turn stabilized by three hydrogen bonds and is in agreement with its NMR structure in solution. While CTPs are often synthetically inaccessible or cyclize in poor yield, both 1 and its N-Me-d-Phe analogue (2) adopt pseudo-cyclic frameworks enabling near quantitative conversion to the corresponding CTPs 3 and 4. The crystal structure of the N-methylated peptide (4) is the first reported for a CTP containing 2-Abz and reveals a distinctly planar 13-membered ring, which is also evident in solution. The N-methylation of d-Phe results in a peptide bond inversion compared to the conformation of 3 in solution.

Protective effects of β-sheet breaker α/β-hybrid peptide against amyloid β-induced neuronal apoptosis in vitro

Alzheimer's disease is most common neurodegenerative disorder and is characterized by increased production of soluble amyloid-β oligomers, the main toxic species predominantly formed from aggregation of monomeric amyloid-β (Aβ). Increased production of Aβ invokes a cascade of oxidative damages to neurons and eventually leads to neuronal death. This study was aimed to investigate the neuroprotective effects of a β-sheet breaker α/β-hybrid peptide (BSBHp) and the underlying mechanisms against Aβ₄₀ -induced neurotoxicity in human neuroblastoma SH-SY5Y cells. Cells were pretreated with the peptide Aβ₄₀ to induce neurotoxicity. Assays for cell viability, cell membrane damage, cellular apoptosis, generation of reactive oxygen species (ROS), intracellular free Ca²⁺ , and key apoptotic protein levels were performed in vitro. Our results showed that pretreatment with BSBHp significantly attenuates Aβ₄₀ -induced toxicity by retaining cell viability, suppressing generation of ROS, Ca²⁺ levels, and effectively protects neuronal apoptosis by suppressing pro-apoptotic protein Bax and up-regulating antiapoptotic protein Bcl-2. These results suggest that α/β-hybrid peptide has neuroprotective effects against Aβ₄₀ -induced oxidative stress, which might be a potential therapeutic agent for treating or preventing neurodegenerative diseases.

Residue dependent hydrogen-bonding preferences in orthanilic acid-based short peptide β-turn motifs

This communication describes the competition between native β-turn (C10) and 2-aminobenzenesulfonic acid (^SAnt)(orthanilic acid)-based pseudo β-turn (C11) in their hybrid peptides.

Synthesis of hybrid hydrazino peptides: protected vs unprotected chiral α-hydrazino acids

Peptidomimetics based on hydrazino derivatives of α-amino acids represent an important class of peptidic foldamers with promising biological activities, like protease inhibition and antimicrobial activity. However, the lack of straightforward method for the synthesis of optically pure hydrazino acids and efficient incorporation of hydrazino building blocks into peptide sequence hamper wider exploitation of hydrazino peptidomimetics. Here we described the utility of N^α-benzyl protected and unprotected hydrazino derivatives of natural α-amino acids in synthesis of peptidomimetics. While incorporation of N^α-benzyl-hydrazino acids into peptide chain and deprotection of benzyl moiety proceeded with difficulties, unprotected hydrazino acids allowed fast and simple construction of hybrid peptidomimetics.

Reversal of H-bonding direction by N-sulfonation in a synthetic reverse-turn peptide motif

This communication depicts an intriguing example of hydrogen-bonding reversal upon introduction of a sulfonamide linkage at the N-terminus of a synthetic reverse-turn peptide motif. The ready availability of two sulfonyl oxygen atoms, as hydrogen-bonding acceptors, combined with the inherent twisted conformation of sulfonamides are seen to act as switches that engage/disengage the hydrogen-bond at the sticky ends/termini.

Face-to-face stacking in sulfonamide based bis-ethylene bridged heteroaromatic dimers

Four sulfonamide based bis-ethylene bridged heteroaromatic dimers were crystallized in offset face-to-face stacked geometry. Further, density functional theory revealed that crystallized structures were the most stable conformers.

Conformational modulation of peptides using β-amino benzenesulfonic acid (SAnt)

This communication describes the utility of the conformationally restricted aromatic β-amino acid (2-aminobenzenesulfonic acid, ^SAnt) for inducing various folding interactions in short peptides.

Synthetic turn mimetics and hairpin nucleators: Quo Vadimus?

Structural mimicry of peptides has witnessed perceptible progress in the last three decades. Reverse turn and β-hairpin units are the smallest secondary structural motifs that are some of the most scrutinized functional cores of peptides and proteins. The practice of mimicking, without altering the function of the bioactive core, ranges from conformational locking of the basic skeleton to total replacement of structural architecture using synthetic analogues. Development of heterogeneous backbones--using unnatural residues in place of natural ones--has broadened further opportunities for efficient structural rigidification. This feature article endeavours to trail the path of progress achieved hitherto and envisage the possibilities that lie ahead in the development of synthetic turn mimetics and hairpin nucleators.

Conformational modulation of peptide secondary structures using β-aminobenzenesulfonic acid

This communication describes the influence of β-aminobenzenesulfonic acid ((S)Ant) on the conformational preferences of hetero foldamers. The designed (Aib-(S)Ant-Aib)n and (Aib-(S)Ant-Pro)n oligomers display a well-defined folded conformation featuring intramolecular mixed hydrogen bonding (7/11) and intra-residual (6/5) H-bonding interactions, respectively.

Formation of a pseudo-β-hairpin motif utilizing the Ant–Pro reverse turn: consequences of stereochemical reordering

Herein, we report a special case of pseudo-β-hairpin formation by tetrapeptide sequences featuring a two-residue Ant–Pro dipeptide motif (Ant = anthranilic acid and Pro = proline) at the loop region.