Triphosgene: an efficient carbonylating agent for liquid and solid-phase aza-peptide synthesis. Application to the synthesis of two aza-analogues of the AChR MIR decapeptide

Solid phase submonomer azapeptide synthesis

Azapeptides contain at least one aza-amino acid, where the α-carbon has been replaced by a nitrogen atom, and have found broad applicability in fields ranging from medicinal chemistry to biomaterials. In this chapter, we provide a step-by-step protocol for the solid phase submonomer synthesis of azapeptides, which includes three steps: (1) hydrazone activation and coupling onto a resin-bound peptide, (2) chemoselective semicarbazone functionalization for installation of the aza-amino acid side chain, and (3) orthogonal deprotection of the semicarbazone to complete the monomer addition cycle. We focus on semicarbazone functionalization by N-alkylation with primary alkyl halides, and describe conditions for coupling onto aza-amino acids. Such divergent methods accelerate the synthesis of peptidomimetics and allow the rapid introduction of a wide variety of natural and unnatural side chains directly on solid support using easily accessible submonomers.

Design, synthesis, and biological evaluation of stable β6.3-Helices: Discovery of non-hemolytic antibacterial peptides

Gramicidin A, a topical antibiotic made from alternating L and D amino acids, is characterized by its wide central pore; upon insertion into membranes, it forms channels that disrupts ion gradients. We present helical peptidomimetics with this characteristic wide central pore that have been designed to mimic gramicidin A channels. Mimetics were designed using molecular modeling focused on oligomers of heterochiral dipeptides of proline analogs, in particular azaproline (AzPro). Molecular Dynamics simulations in water confirmed the stability of the designed helices. A sixteen-residue Formyl-(AzPro-Pro)₈-NHCH₂CH₂OH helix was synthesized as well as a full thirty-two residue Cbz-(AzPro-Pro)₁₆-O^tBu channels. No liposomal lysis activity was observed suggesting lack of channel formation, possibly due to inappropriate hydrogen-bonding interactions in the membrane. These peptidomimetics also did not hemolyze red blood cells, unlike gramicidin A.

Direct extraction of carbonyl from waste polycarbonate with amines under environmentally friendly conditions: scope of waste polycarbonate as a carbonylating agent in organic synthesis

An efficient green method for converting polycarbonate into urea derivatives and bisphenol-A by sequestration of carbonyl from waste polycarbonate.

Azapeptides and their therapeutic potential

Azapeptides are peptide analogs in which one or more of the amino residues is replaced by a semicarbazide. This substitution of a nitrogen for the α-carbon center results in conformational restrictions, which bend the peptide about the aza-amino acid residue away from a linear geometry. The resulting azapeptide turn conformations have been observed by x-ray crystallography and spectroscopy, as well as predicted based on computational models. In biologically active peptide analogs, the aza-substitution has led to enhanced activity and selectivity as well as improved properties, such as prolonged duration of action and metabolic stability. In light of these characteristics, azapeptides have found important uses as receptor ligands, enzyme inhibitors, drugs, pro-drugs, probes and imaging agents. Recent improvements in synthetic methods for their procurement have ushered in a new era of azapeptide chemistry. This review aims to provide a historical look at the development of azapeptide science along with a focus on recent developments and perspectives on the future of this useful tool for medicinal chemistry.

Aza-Amino Acid Scanning of Secondary Structure Suited for Solid-Phase Peptide Synthesis with Fmoc Chemistry and Aza-Amino Acids with Heteroatomic Side Chains

Aza-peptides, peptide analogues in which the alpha-carbon of one or more of the amino acid residues is replaced with a nitrogen atom, exhibit a propensity for adopting beta-turn conformations. A general Fmoc-protection protocol for the stepwise solid-phase synthesis of aza-peptides has now been developed based on the activation of N'-alkyl fluoren-9-ylmethyl carbazates with phosgene for coupling the aza-amino acid residues. This method has proven effective for introducing aza-amino acid residues with aliphatic (Ala, Leu, Val, and Gly) and aromatic (Phe, Tyr, and Trp) side chains. Acid promoted loss of aromatic side chains was noted with aza-Trp and aza-Tyr residues during peptide cleavage and suppressed by temperature control in the case of the latter. In addition, aza-peptides with heteroatomic side chain residues (Lys, Orn, Arg, and Asp) were conveniently synthesized using this protocol. Partial aza-amino acid scans were performed on three biologically active peptides: the potent tetrapeptide melanocortin receptor agonist, Ac-His-d-Phe-Arg-Trp-NH2; the growth hormone secretagogue hexapeptide, GHRP-6, His-d-Trp-Ala-Trp-d-Phe-Lys-NH2; and the human calcitonin gene-related peptide (hCGRP) antagonist, FVPTDVGPFAF-NH2. This practical procedure for aza-amino acid scanning using Fmoc-based solid-phase synthesis should find general utility for probing the existence and importance of beta-turn conformations in bioactive peptides.

Design of selective peptidomimetic agonists for the human orphan receptor BRS-3

New tool substances may help to unravel the physiological role of the human orphan receptor BRS-3 and its possible use as a drug target for the treatment of obesity and cancer. In continuation of our work on BRS-3, the solid- and solution-phase synthesis of a library of low molecular weight peptidomimetic agonists based on the recently developed short peptide agonist 4 is described. Functional potencies of the compounds were determined measuring calcium mobilization in a fluorometric imaging plate reader (FLIPR) assay. Focusing on the N-terminus, the d-Phe-Gln moiety of 4 was modified in a combinatorial SAR-oriented medicinal chemistry approach. With the incorporation of N-arylated glycine and alanine building blocks azaglycine, piperazine, or piperidine and the synthesis of semicarbazides and semicarbazones, a number of highly potent and selective compounds with a reduced number of peptide bonds were obtained, which also should have enhanced metabolic stability.

Impact of azaproline on amide cis-trans isomerism: conformational analyses and NMR studies of model peptides including TRH analogues

The beta-turn is a well-studied motif in both proteins and peptides. Four residues, making almost a complete 180 degree-turn in the direction of the peptide chain, define the beta-turn. Several types of the beta-turn are defined according to Phi and Psi torsional angles of the backbone for residues i + 1 and i + 2. One special type of beta-turn, the type VI-turn, usually contains a proline with a cis-amide bond at residue i + 2. In an aza-amino acid, the alpha-carbon of the amino acid is changed to nitrogen. Peptides containing azaproline (azPro) have been shown to prefer the type VI beta-turn both in crystals and in organic solvents by NMR studies. MC/MD simulations using the GB/SA solvation model for water explored the conformational preferences of azPro-containing peptides in aqueous systems. An increase in the conformational preference for the cis-amide conformer of azPro was clearly seen, but the increased stability was relatively minor with respect to the trans-conformer as compared to previous suggestions. To test the validity of the calculations in view of the experimental data from crystal structures and NMR in organic solvents, [azPro(3)]-TRH and [Phe(2), azPro(3)]-TRH were synthesized, and their conformational preferences were determined by NMR in polar solvents as well as the impact of the azPro substitution on their biological activities.

Computational study of conformational preferences of thioamide-containing azaglycine peptides

The effect of thioamide substitution on the conformational stability of an azaglycine-containing peptide, For-AzaGly-NH2 (1), was investigated for the sake of finding possible applications by using ab initio and DFT methods. As model compounds, For-[psiCSNH]-AzaGly-NH2 (2), For-AzaGly-[psiCSNH]-NH2 (3), and For-[psiCSNH]-AzaGly-[psiCSNH]-NH2 (4) were used. Two-dimensional phi-psi potential energy surfaces (PESs) for 2-4 were calculated at the B3LYP/6-31G*//HF/6-31G* level in gas (epsilon = 1.0) and in water (epsilon = 78.4) by applying the isodensity polarizable continuum model (IPCM) method. On the basis of these PESs, the minimum energy conformations for 2-4 were characterized at the B3LYP level with 6-31G*, 6-311G**, and 6-31+G** basis sets. The remarkable structural effect of thioamide substitution for 2-4 is that beta-strand structure is observed as a global or local minimum. The minima of 2-4 are also compared with those for glycine and thioamide-containing glycine peptides. Our theoretical results demonstrate that compounds 2-4 would be used to design controllable secondary structures.

A theoretical study of conformational properties of N-methyl azapeptide derivatives

The conformational properties of azapeptide derivatives, Ac-azaGly-NHMe (1), Ac-azaAla-NHMe (2), Ac-NMe-azaGly-NHMe (3), Ac-NMe-azaAla-NHMe (4), Ac-azaGly-NMe(2) (5), Ac-azaAla-NMe(2) (6), Ac-NMe-azaGly-NMe(2) (7), and Ac-NMe-azaAla-NMe(2) (8), were systematically examined by using ab initio MO and DFT methods. Structural perturbations in azapeptides resulting from cyclic substitution of a methyl group at three N-positions of an azaamino acid were studied on the basis of the structure of the simplest model azapeptide, 1. Potential energy surfaces were generated at the HF/6-31G level for 1-4 and at the HF/6-31G//HF/3-21G level for 5-8 by rotating two key dihedral angles (phi, psi) in increments of 30 degrees. The backbone (phi, psi) angles of the minima for 1-4 are observed at the i + 2 position to form the betaI(I')-, betaII(II')-, betaVI-turns or the polyproline II structure according to the orientation of the acetyl group and the positions of the N-methyl groups. Compounds 5-8 coupled to a secondary amine were found to preferentially adopt polyproline II, betaI(III)-turn, or alpha-helical structure or even extended conformations depending on the orientation of the acetyl group and the positions of the N-methyl groups. Furthermore, N-methyl groups, depending on their positions, were found to affect the orientation of the amide group in the lowest energy conformations, the pyramidality of the N2 atom, and the bond length in azapeptide derivatives. These unique theoretical conformations of N-methyl azapeptide derivatives could be utilized in the definite design of secondary structure for peptides and proteins, and in the development of new drugs and molecular machines.

Solid-phase synthesis of a nonpeptide RGD mimetic library: new selective alphavbeta3 integrin antagonists

The solid-phase synthesis of a low molecular weight RGD mimetic library is described. Activities of the compounds in inhibiting the interaction of ligands, vitronectin and fibrinogen, with isolated immobilized integrins alphavbeta3 and alphaIIbbeta3 were determined in a screening assay. Highly active and selective nonpeptide alphavbeta3 integrin antagonists with regard to orally bioavailability were developed, based on the aza-glycine containing lead compound 1. An important variation is the substitution of the aspartic amide of 1 by an aromatic residue. Furthermore, different guanidine mimetics have been incorporated to improve the pharmacokinetic profile. Exchange of the beta-amino acid NH by a methylene moiety in one set of RGD mimetics leads to the azacarba analogue compounds representing a novel peptidomimetic approach, which should increase the metabolic stability.

Hydrazino peptides as foldamers: an extension of the beta-peptide concept

Replacing the C(beta) atoms in the beta-amino acid constituents of beta-peptides by nitrogen atoms leads to hydrazino peptides. A systematic conformation analysis of blocked hydrazino peptide oligomers of the general type I at the HF/6-31G, MP2/6-31G, and DFT/B3LYP/6-31G levels of ab initio MO theory and on the basis of molecular mechanics reveals a wide variety of secondary structures, as for instance various helices and sheet- and turnlike conformers. Some of them are closely related to secondary structure types found in beta-peptides; others represent novel types. Thus, a very stable, novel helix with 14-membered hydrogen-bonded pseudocycles, which occupies a conformation space different from that of helices with 14-membered rings found among the most stable conformers in beta-peptides, is indicated. The most important secondary structure elements are characterized by interactions between peptidic NH and CO groups. The additional hydrazino N(alpha)H group takes part in special structuring effects but is of lesser importance for secondary structure formation. The influence of environmental effects on the existence and stability of the various structure types is discussed. Due to the wide variety of structural possibilities, hydrazino peptides might be a useful tool for peptide and protein design.

In situ generation of Fmoc-amino acid chlorides using bis-(trichloromethyl) carbonate and its utilization for difficult couplings in solid-phase peptide synthesis

This paper reports procedures for the straightforward in situ generation of Fmoc-amino acid chlorides using bis-(trichloromethyl)carbonate (BTC) and their utilization for difficult couplings during solid-phase peptide synthesis. The BTC-mediated coupling of all Fmoc-protected proteinogenic amino acids to a large variety of N-alkylated amino acid-peptidyl-resin was studied. The majority of the couplings proceeded with quantitative conversion and without racemization. The utilization of BTC-mediated coupling for facile solid-phase synthesis of backbone cyclic peptides is presented.

Azaproline as a beta-turn-inducer residue opposed to proline

Azaproline (AzPro) is an analogue of proline containing a nitrogen atom in place of the C(alpha)H group. AzPro has been introduced in various model peptides, and especially in the Boc-Ala-AzPro-Ala-NHiPr tripeptide. The structural consequence of that modification has been investigated in solution by using IR and 1H NMR, with reference to the cognate proline-containing peptide. Contrary to proline, which induces beta-folding of the Pro-Ala sequence, azaproline apparently favors betaVI-folding of the Ala-AzPro one with high occurrence. Opening of the AzPro pyrazolidine ring to get N-methylazaalanine fundamentally does not change the structural properties of the azatripeptide, but allows the existence of open conformers to an extent depending on the solvent.