Integrating 3,4-Dihydro-2-1,4-oxazine into Peptides as a Modification: Silver Triflate-Catalyzed Cyclization of -Propargyl -Sulfonyl Amino Alcohols for SPPS Applications.

We present a methodology yielding 3,4-dihydro-2-1,4-oxazine by cyclization of -propargyl -sulfonyl amino alcohols using silver triflate as a catalyst at ambient temperature. Additionally, we showcase the applicability of this methodology in solid phase peptide synthesis (SPPS) to introduce the oxazine heterocyclic ring into short peptides containing serine and threonine. Notably, Rink amide resin supported the on-resin formation of 3,4-dihydro-2-1,4-oxazine, while 2-CTC resin facilitated the oxazine formation in a one-pot process involving peptide cleavage, deprotection, and subsequent C-O ring formation, thus offering a versatile method for the late-stage modification of peptides.

Metal-Free One-Pot Domino Synthesis of Oxazolidinethione Derivatives of Quaternary Amino Acids from α-Amino Esters and Aldehydes Using CS.

We present a streamlined, metal-free, one-pot domino approach to efficiently synthesize oxazolidinethione derivatives containing substituted quaternary amino acids. This method employs α-amino esters, aldehydes, and CS under mild conditions, constructing three new bonds (C-N, C-C, and C-O) to produce oxazolidinethione compounds featuring a quaternary center and a beta-hydroxy derivative in high yields. This scalable protocol enables the creation of libraries of biologically significant, intricate amino acid derivatives using amino esters and aldehydes.

Total synthesis, structure elucidation and expanded bioactivity of icosalide A: effect of lipophilicity and ester to amide substitution on its bioactivity.

The first total synthesis of icosalide A, an antibacterial depsipeptide that is unique in that it contains two lipophilic beta-hydroxy acids, has been achieved by following Fmoc solid-phase peptide synthesis in combination with solution-phase synthesis. The ambiguity in the absolute stereochemistry of icosalide A has been resolved by synthesizing the reported structures and other relevant diastereomers of icosalides and comparing their NMR data. NMR-based structure elucidation of icosalide A revealed a well-folded structure with cross-strand hydrogen bonds similar to the anti-parallel beta-sheet conformation in peptides and displayed a synergistic juxtaposition of the aliphatic sidechains.

Possible Fine-Tuning of Methane Activation toward C2 Oxygenates by 3d-Transition Metal-Ions Doped Nano-Ceria-Zirconia.

In this work, we demonstrate a simple sol-gel technique to prepare metal-ion(s)-doped ceria-zirconia solid solution for efficient catalytic methane activation. The cation-depicting formula units are CeZr (CZ), CeZrM (CZM), and CeZrMM (CZMM) (M and M = V, Mn, Fe, Co, and Cu), employed for undoped, mono-metal-ion-doped, and bi-metal-ion-doped solid solutions, respectively. Methane activation with Mn, Fe, Cu mono-metal-ion-doped CZ favors the C1 product, while CZCo assists C-C coupling with the formation of acetaldehyde.

Correction: Residue-based propensity of aggregation in the Tau amyloidogenic hexapeptides AcPHF6* and AcPHF6.

[This corrects the article DOI: 10.1039/D0RA03809A.].

[1,3]-Claisen Rearrangement via Removable Functional Group Mediated Radical Stabilization.

A thermal O-to-C [1,3]-rearrangement of α-hydroxy acid derived enol ethers was achieved under mild conditions. The 2-aminothiophenol protection of carboxylic acids facilitates formation of the [1,3] precursor and its thermal rearrangement via stabilization of a radical intermediate. Experimental and theoretical evidence for dissociative radical pair formation, its captodative stability via aminothiophenol, and a unique solvent effect are presented.

HDAC6 ZnF UBP as the Modifier of Tau Structure and Function.

Histone deacetylase 6 is a class II histone deacetylase primarily present in the cytoplasm and involved in the regulation of various cellular functions. It consists of two catalytic deacetylase domains and a unique zinc finger ubiquitin binding protein domain, which sets it apart from other HDACs. HDAC6 is known to regulate cellular activities by modifying the function of microtubules, HSP90, and cortactin through deacetylation.

EGCG impedes human Tau aggregation and interacts with Tau.

Tau aggregation and accumulation is a key event in the pathogenesis of Alzheimer's disease. Inhibition of Tau aggregation is therefore a potential therapeutic strategy to ameliorate the disease. Phytochemicals are being highlighted as potential aggregation inhibitors.

Residue-based propensity of aggregation in the Tau amyloidogenic hexapeptides AcPHF6* and AcPHF6.

In Alzheimer's disease and related tauopathies, the aggregation of microtubule-associated protein, Tau, into fibrils occurs the interaction of two hexapeptide motifs PHF* VQIINK and PHF VQIVYK as β-sheets. To understand the role of the constituent amino acids of PHF and PHF* in the aggregation, a set of 12 alanine mutant peptides was synthesized by replacing each amino acid in PHF and PHF* with alanine and they were characterized by nuclear magnetic resonance (NMR) spectroscopy, circular dichroism (CD), transmission electron microscopy (TEM) and ThS/ANS fluorescence assay. Our studies show that while the aggregation was suppressed in most of the alanine mutant peptides, replacement of glutamine by alanine in both PHF and PHF* enhanced the fibrillization.

N-Heterocyclic Carbene Catalysis Exploiting Oxidative Imine Umpolung for the Generation of Imidoyl Azoliums.

Although NHC-catalyzed umpolung of imines are known, the related reactions under oxidative conditions are limited. Described herein is the two-step process involving the initial formation of aldimines from the corresponding aldehydes and 2-amino benzyl alcohols followed by NHC-catalyzed cyclization proceeding via the imidoyl azoliums under oxidative conditions. The reaction allowed the synthesis of trifluoromethylated 3,1-benzoxazines in good yields and broad scope.

Novel cilengitide-based cyclic RGD peptides as αvβ integrin inhibitors.

In this letter, we report a series of five new RGD-containing cyclic peptides as potent inhibitors to αvβ integrin protein. We have incorporated various unnatural lipophilic amino acids into the cyclic RGD framework of cilengitide, which is selective for αvβ integrin. All the newly synthesized cyclic peptides were evaluated in vitrosolid phase binding assay and investigated for their bindingbehaviourtowards integrin subtypes.

Melatonin interacts with repeat domain of Tau to mediate disaggregation of paired helical filaments.

Tau is the major neuronal protein involved in the stabilization of microtubule assembly. In Alzheimer's disease, Tau self-assembles to form intracellular protein aggregates which are toxic to cells. Various methods have been tried and tested to restrain the aggregation of Tau.

Selective Targeting of Integrin αvβ8 by a Highly Active Cyclic Peptide.

Integrins play important roles in physiological and pathophysiological processes. Among the RGD-recognizing integrin subtypes, the αvβ8 receptor is emerging as an attractive target because of its involvement in various illnesses, such as autoimmune diseases, viral infections, and cancer. However, its functions have, so far, not been investigated in living subjects mainly because of the lack of a selective αvβ8 ligand.

Systematic synthesis of a 6-component organic-salt alloy of naftopidil, and pentanary, quaternary and ternary multicomponent crystals.

The single-crystal X-ray structure of a 6-component organic-salt alloy (hexanary) of naftopidil (1) (an active pharmaceutical ingredient) with benzoic acid (2) and four different hydroxy-substituted benzoic acids, salicylic acid (3), 2,3-di-hydroxybenzoic acid (4), 2,4-di-hydroxybenzoic acid (5) and 2,6-di-hydroxybenzoic acid (6), is reported. The hexanary assembly originates from the observation that the binary salts of naftopidil with the above acids are isostructural. In addition to the 6-component solid, we also describe five 5-component, ten 4-component, and ten 3-component organic-salt alloys of naftopidil (1) with carboxylic acids (2)-(6).

N-Heterocyclic Carbene-Catalyzed Michael-Michael-Lactonization Cascade for the Enantioselective Synthesis of Tricyclic δ-Lactones.

Enantioselective synthesis of tricyclic δ-lactones with three contiguous stereocenters has been demonstrated by the N-heterocyclic carbene (NHC)-catalyzed functionalization of benzylic C(sp)-H bonds. The NHC-catalyzed reaction of enals with dinitrotoluene derivatives under oxidative conditions proceeds via the chiral α,β-unsaturated acylazoliums and produces the δ-lactones in good yields and excellent diastereoselectivity and enantioselectivity. This mild and atom-economic cascade reaction takes place in a Michael/Michael/lactonization sequence and tolerates a broad range of functional groups.

From a Helix to a Small Cycle: Metadynamics-Inspired αvβ6 Integrin Selective Ligands.

The RGD-recognizing αvβ6 integrin has only recently emerged as a major target for cancer diagnosis and therapy. Thus, the development of selective, low-molecular-weight ligands of this receptor is still in great demand. Here, a metadynamics-driven design strategy allowed us to successfully convert a helical nonapeptide into a cyclic pentapeptide (6) showing remarkable potency and αvβ6 specificity.

N-Methylation of isoDGR Peptides: Discovery of a Selective α5β1-Integrin Ligand as a Potent Tumor Imaging Agent.

Specific targeting of the integrin subtype α5β1 possesses high potential in cancer diagnosis and therapy. Through sequential N-methylation, we successfully converted the biselective α5β1/αvβ6 peptide c(phg- isoDGR-k) into a potent peptidic RGD binding α5β1 subtype selective ligand c(phg- isoDGR-( NMe)k). Nuclear magnetic resonance spectroscopy and molecular modeling clarified the molecular basis of its improved selectivity profile.

Structural Insights into Selective Ligand-Receptor Interactions Leading to Receptor Inactivation Utilizing Selective Melanocortin 3 Receptor Antagonists.

Systematic N-methylated derivatives of the melanocortin receptor ligand, SHU9119, lead to multiple binding and functional selectivity toward melanocortin receptors. However, the relationship between N-methylation-induced conformational changes in the peptide backbone and side chains and melanocortin receptor selectivity is still unknown. We conducted comprehensive conformational studies in solution of two selective antagonists of the third isoform of the melanocortin receptor (hMC3R), namely, Ac-Nle-c[Asp-NMe-His-d-Nal(2')-NMe-Arg-Trp-Lys]-NH (15) and Ac-Nle-c[Asp-His-d-Nal(2')-NMe-Arg-NMe-Trp-NMe-Lys]-NH (17).

Total Synthesis of the Marine Natural Product Solomonamide B Necessitates Stereochemical Revision.

The first total synthesis of the proposed structure of solomonamide B has been achieved. However, the (1)H and (13)C NMR spectral data of the synthesized compound was not exactly matching with that of the natural solomonamide B. This prompted us to revise the originally proposed structure, in particular, the stereochemistry of the nonpeptide part, which was confirmed by its total synthesis.

Stable Peptides Instead of Stapled Peptides: Highly Potent αvβ6-Selective Integrin Ligands.

The αvβ6 integrin binds the RGD-containing peptide of the foot and mouth disease virus with high selectivity. In this study, the long binding helix of this ligand was downsized to an enzymatically stable cyclic peptide endowed with sub-nanomolar binding affinity toward the αvβ6 receptor and remarkable selectivity against other integrins. Computational studies were performed to disclose the molecular bases underlying the high binding affinity and receptor subtype selectivity of this peptide.

cis-Peptide Bonds: A Key for Intestinal Permeability of Peptides? .

Recent structural studies on libraries of cyclic hexapeptides led to the identification of common backbone conformations that may be instrumental to the oral availability of peptides. Furthermore, the observation of differential Caco-2 permeabilities of enantiomeric pairs of some of these peptides strongly supports the concept of conformational specificity driven uptake and also suggests a pivotal role of carrier-mediated pathways for peptide transport, especially for scaffolds of polar nature. This work presents investigations on the Caco-2 and PAMPA permeability profiles of 13 selected N-methylated cyclic pentaalanine peptides derived from the basic cyclo(-D-Ala-Ala4 -) template.

Systematic Backbone Conformational Constraints on a Cyclic Melanotropin Ligand Leads to Highly Selective Ligands for Multiple Melanocortin Receptors.

Human melanocortin receptors (hMCRs) have been challenging targets to develop ligands that are explicitly selective for each of their subtypes. To modulate the conformational preferences of the melanocortin ligands and improve the biofunctional agonist/antagonist activities and selectivities, we have applied a backbone N-methylation approach on Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH2 (Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2), a nonselective cyclic peptide antagonist at hMC3R and hMC4R and an agonist at hMC1R and hMC5R. Systematic N-methylated derivatives of Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2, with all possible backbone N-methylation combinations, have been synthesized and examined for their binding and functional activities toward melanocortin receptor subtypes 1, 3, 4, and 5 (hMCRs).

Enantiomeric cyclic peptides with different Caco-2 permeability suggest carrier-mediated transport.

Recently, oral absorption of cyclic hexapeptides was improved by N-methylation of their backbone amides. However, the number and position of N-methylations or of solvent exposed NHs did not correlate to intestinal permeability, measured in a Caco-2 model. In this study, we investigate enantiomeric pairs of three polar and two lipophilic peptides to demonstrate the participation of carrier-mediated transporters.

Receptor-bound conformation of cilengitide better represented by its solution-state structure than the solid-state structure.

The X-ray crystal and NMR spectroscopic structures of the peptide drug candidate Cilengitide (cyclo(RGDf(NMe)Val)) in various solvents are obtained and compared in addition to the integrin receptor bound conformation. The NMR-based solution structures exhibit conformations closely resembling the X-ray structure of Cilengitide bound to the head group of integrin αvβ3. In contrast, the structure of pure Cilengitide recrystallized from methanol reveals a different conformation controlled by the lattice forces of the crystal packing.

Tumor Targeting via Integrin Ligands.

Selective and targeted delivery of drugs to tumors is a major challenge for an effective cancer therapy and also to overcome the side-effects associated with current treatments. Overexpression of various receptors on tumor cells is a characteristic structural and biochemical aspect of tumors and distinguishes them from physiologically normal cells. This abnormal feature is therefore suitable for selectively directing anticancer molecules to tumors by using ligands that can preferentially recognize such receptors.