Synthesis and receptor binding of opioid peptide analogues containing β3‐homo‐amino acids

β‐Amino acids containing hybrid peptides and β‐peptides show great potential as peptidomimetics. In this paper we describe the synthesis and affinity toward the µ‐ and δ‐opioid receptors of β‐peptides, analogues of Leu‐enkephalin, deltorphin I, dermorphin and α,β‐hybrides, analogues of deltorphin I. Substitution of α‐amino acid residues with β³‐homo‐amino acid residues, in general resulted in decrease of affinity to opioid receptors. However, the incorporation β³h‐D ‐Ala in position 2 or β³hPhe in position 3 of deltorphin I resulted in potent and selective ligand for δ‐opioid receptor. The NMR studies of β‐deltorphin I analogue suggest that conformational motions in the central part of the peptide backbone are partially restricted and some conformational preferences can be expected. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Efficient one‐pot synthesis of N‐ethyl amino acids

Mono‐N‐ethylated α‐amino acid esters are obtained in high yields using reductive amination procedures. Formation of imine is achieved by excess of acetaldehyde, followed by removal of acetaldehyde and reduction by NaBH(OAc)₃. The elaborated one‐pot synthesis allows for the efficient synthesis of side‐chain protected amino acid derivatives. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.     

An amphipathic cyclic tetrapeptide scaffold containing halogenated β2,2‐amino acids with activity against multiresistant bacteria

The present study describes the synthesis and biological studies of a small series of head‐to‐tail cyclic tetrapeptides of the general structure c(Lys‐β^2,2‐Xaa‐Lys) containing one lipophilic β^2,2‐amino acid and Lys, Gly, Ala, or Phe as the Xaa residue in the sequence. The peptides were investigated for antimicrobial activity against gram‐positive and gram‐negative reference strains and 30 multiresistant clinical isolates including strains with extended spectrum β‐lactamase—carbapenemase (ESBL‐CARBA) production. Toxicity was determined against human red blood cells. The most potent peptides showed high activity against the gram‐positive clinical isolates with minimum inhibitory concentrations of 4–8 μg/mL and low haemolytic activity. The combination of high antimicrobial activity and low toxicity shows that these cyclic tetrapeptides containing lipophilic β^2,2‐amino acids form a valuable scaffold for designing novel antimicrobial agents.     

Synthesis of anticancer heptapeptides containing a unique lipophilic β2,2‐amino acid building block

We report a series of synthetic anticancer heptapeptides (H‐KKWβ^2,2WKK‐NH₂) containing eight different central lipophilic β^2,2‐amino acid building blocks, which have demonstrated high efficiency when used as scaffolds in small cationic antimicrobial peptides and peptidomimetics. The most potent peptides in the present study had IC₅₀ values of 9–23 µm against human Burkitt's lymphoma and murine B‐cell lymphoma and were all nonhaemolytic (EC₅₀ > 200 µm ). The most promising peptide 10e also demonstrated low toxicity against human embryonic lung fibroblast cells and peripheral blood mononuclear cells and exceptional proteolytic stability. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

TAPP analogs containing β3‐homo‐amino acids: synthesis and receptor binding

β‐Amino acids containing α,β‐hybrid peptides show great potential as peptidomimetics. In this paper, we describe the synthesis and affinity to μ‐opioid and δ‐opioid receptors of α,β‐hybrids, analogs of the tetrapeptide Tyr‐ d ‐Ala‐Phe‐Phe‐NH₂ (TAPP). Each amino acid was replaced with an l ‐ or d ‐β³‐h‐amino acid. All α,β‐hybrids of TAPP analogs were synthesized in solution and tested for affinity to μ‐opioid and δ‐opioid receptors. The analog Tyr‐β³h‐ d ‐Ala‐Phe‐PheNH₂ was found to be as active as the native tetrapeptide. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

A β‐amino acid modified heptapeptide containing a designed recognition element disrupts fibrillization of the amyloid β‐peptide

We study the complex formation of a peptide βAβAKLVFF, previously developed by our group, with Aβ(1–42) in aqueous solution. Circular dichroism spectroscopy is used to probe the interactions between βAβAKLVFF and Aβ(1–42), and to study the secondary structure of the species in solution. Thioflavin T fluorescence spectroscopy shows that the population of fibers is higher in βAβAKLVFF/Aβ(1–42) mixtures compared to pure Aβ(1–42) solutions. TEM and cryo‐TEM demonstrate that co‐incubation of βAβAKLVFF with Aβ(1–42) causes the formation of extended dense networks of branched fibrils, very different from the straight fibrils observed for Aβ(1–42) alone. Neurotoxicity assays show that although βAβAKLVFF alters the fibrillization of Aβ(1–42), it does not decrease the neurotoxicity, which suggests that toxic oligomeric Aβ(1–42) species are still present in the βAβAKLVFF/Aβ(1–42) mixtures. Our results show that our designed peptide binds to Aβ(1–42) and changes the amyloid fibril morphology. This is shown to not necessarily translate into reduced toxicity. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.     

α‐N‐Protected dipeptide acids: a simple and efficient synthesis via the easily accessible mixed anhydride method using free amino acids in DMSO and tetrabutylammonium hydroxide

The importance of dipeptides both in medicinal and pharmacological fields is well documented and many efforts have been made to find simple and efficient methods for their synthesis. For this reason, we have investigated the synthesis of α‐N‐protected dipeptide acids by reacting the easily accessible mixed anhydride of α‐N‐protected amino acids with free amino acids under different reaction conditions. The combination of TBA‐OH and DMSO has been found to be the best to overcome the low solubility of amino acids in organic solvents. Under these experimental conditions, the homogeneous phase condensation reaction occurs rapidly and without detectable epimerization. The present method is also applicable to side‐chain unprotected Tyr, Trp, Glu, and Asp but not Lys. This latter residue is able to engage two molecules of mixed anhydride giving the corresponding isotripeptide. Moreover, the applicability of this protocol for the synthesis of tri‐ and tetrapeptides has been tested. This approach reduces the need for protecting groups, is cost effective, scalable, and yields dipeptide acids that can be used as building blocks in the synthesis of larger peptides. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.     

Syntheses and anti‐tubercular activity of β‐substituted and α,β‐disubstituted peptidyl β‐aminoboronates and boronic acids

Tuberculosis is still affecting millions of people worldwide, and new resistant strains of Mycobacterium tuberculosis are being found. It is therefore necessary to find new compounds for treatment. In this paper, we report the synthesis and in vitro testing of peptidyl β‐aminoboronic acids and β‐aminoboronates with anti‐tubercular activity. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.     

Synthesis and reactivity of N‐protected‐α‐amino aldehydes

During the last decade α‐amino aldehydes have attracted widespread attention as the important natural source of chiral substrates useful in stereocontrolled organic synthesis. They are of special interest due to their ready availability in both enantiomeric forms from natural sources, as well as their pronounced versatility, due to the presence of both the formyl group and suitably protected amino functionality in the molecule. These bifunctional compounds exhibit a valuable dual reactivity, which has been utilized in a broad range of synthetic applications. Chirality 15:514–541, 2003. © 2003 Wiley‐Liss, Inc.     

Asymmetric Allylation of α‐Ketoester‐Derived N‐Benzoylhydrazones Promoted by Chiral Sulfoxides/N‐Oxides Lewis Bases: Highly Enantioselective Synthesis of Quaternary α‐Substituted α‐Allyl‐α‐Amino Acids

Chiral sulfoxides/N‐oxides (R)‐ 1 and (R,R)‐ 2 are effective chiral promoters in the enantioselective allylation of α‐keto ester N‐benzoylhydrazone derivatives 3a , 3b , 3c , 3d , 3e , 3f , 3g to generate the corresponding N‐benzoylhydrazine derivatives 4a , 4b , 4c , 4d , 4e , 4f , 4g , with enantiomeric excesses as high as 98%. Representative hydrazine derivatives 4a , 4b were subsequently treated with SmI₂, and the resulting amino esters 5a , 5b with LiOH to obtain quaternary α‐substituted α‐allyl α‐amino acids 6a , 6b , whose absolute configuration was assigned as (S), with fundament on chemical correlation and electronic circular dichroism (ECD) data. Chirality 25:529–540, 2013. © 2013 Wiley Periodicals, Inc.     

Synthesis of 2‐oxoamides based on sulfonamide analogs of γ‐amino acids and their activity on phospholipase A2

A variety of lipophilic 2‐oxoamides containing sulfonamide analogs of γ‐amino acids as well as acyl sulfonamides of γ‐aminobutyric acid were synthesized. Their ability to inhibit intracellular GIVA cPLA₂ and GVIA iPLA₂ as well as secreted GV sPLA₂ was evaluated. The sulfonamide group seems a bioisosteric group suitable to replace the carboxyl group in 2‐oxoamide inhibitors of GVIA cPLA₂. Copyright © 2008 European Peptide Society and John Wiley & Sons, Ltd.     

Comparative studies of Nsc and Fmoc as Nα‐protecting groups for SPPS

2‐(4‐Nitrophenyl)sulfonylethoxycarbonyl (Nsc) is an alternative base‐labile N^α‐protecting group to 9‐fluorenylmethoxycarbonyl (Fmoc) for amino acids. The UV spectrum of the Nsc group exhibits moderate absorption at 380 nm which is excellent for real‐time monitoring of the deprotection process. It also decreases the rearrangement of X‐Asp, which can be a serious problem in SPPS. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.     

Chirality in amino acids beyond the Cα configuration

Based on a CSD search, a meta‐analysis of 1179 structures of 19 natural amino acids H₃NC_αH(R)C′(O)O and their derivatives H₃NC_αH(R)C′(O)O(H/R/M), protonated, esterified, or coordinated at the carboxylic group, shows that the chirality chain with its two steps, established in the preceding paper for alanine, can be extended to natural amino acids. High diastereoselectivities are observed in the induction from the L configuration at C_α to the ?ψ and +ψ conformations, which in turn distort the planar carboxylic group C_αC′(O_cis)O_trans to asymmetric flat tetrahedra, showing that the chirality chain is an integral part of natural amino acids.     

Synthesis,binding affinities and metabolic stability of dimeric dermorphin analogs modified with β3‐homo‐amino acids

In this study, proteinogenic amino acids residues of dimeric dermorphin pentapeptides were replaced by the corresponding β³‐homo‐amino acids. The potency and selectivity of hybrid α/β dimeric dermorphin pentapeptides were evaluated by competetive receptor binding assay in the rat brain using [3H]DAMGO (a μ ligand) and [3H]DELT (a δ ligand). Tha analog containing β³‐homo‐Tyr in place of Tyr (Tyr‐d ‐Ala‐Phe‐Gly‐β³‐homo‐Tyr‐NH‐)₂ showed good μ receptor affinity and selectivity (IC₅₀ = 0.302, IC₅₀ ratio μ/δ = 68) and enzymatic stability in human plasma. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.     

Axially chiral Ni(II) complexes of α‐amino acids: Separation of enantiomers and kinetics of racemization

Herein we present design, synthesis, chiral HPLC resolution, and kinetics of racemization of axially chiral Ni(II) complexes of glycine and di‐(benzyl)glycine Schiff bases. We found that while the ortho‐fluoro derivatives are configurationally unstable, the pure enantiomers of corresponding axially chiral ortho‐chloro‐containing complexes can be isolated by preparative HPLC and show exceptional configurational stability (t_1/2 from 4 to 216 centuries) at ambient conditions. Synthetic implications of this discovery for the development of new generation of axially chiral auxiliaries, useful for general asymmetric synthesis of α‐amino acids, are discussed.     

Stereoselective synthesis of fully protected (2S,4S,6S)‐2‐amino‐6‐hydroxy‐4‐methyl‐8‐oxodecanoic acid (AHMOD)

The stereocontrolled synthesis of fully protected (2S,4S,6S)‐2‐amino‐6‐hydroxy‐4‐methyl‐8‐oxodecanoic acid was accomplished using a glutamate derivative as starting material. The key steps of this stereochemical synthetic pathway involved an Evans asymmetric alkylation, a Sharpless asymmetric epoxidation, and a Grignard reaction. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

Liquid and subcritical fluid chromatographic enantioseparation of Nα‐Fmoc proteinogenic amino acids on Quinidine‐based zwitterionic and anion‐exchanger type chiral stationary phases. A comparative study

Stereoselective high‐performance liquid chromatographic and subcritical fluid chromatographic separations of 19 N^α‐Fmoc proteinogenic amino acid enantiomers were carried out by using Quinidine ‐based zwitterionic and anion‐exchanger‐type chiral stationary phases Chiralpak ZWIX(−) and QD‐AX. For optimization of retention and enantioselectivity, the ratio of bulk solvent components (MeOH/MeCN, H₂O/MeOH, or CO₂/MeOH) and the nature and concentration of the acid and base additives (counter‐ and co‐ions) were systematically varied. The effect of column temperature on the enantioseparation was investigated and thermodynamic parameters were calculated from the van't Hoff plots ln α vs. 1/T. The thermodynamic parameters revealed that the enantioseparations were enthalpy‐driven. The elution sequence was determined in all cases and with the exception of Fmoc‐Cys(Trt)‐OH, it was identical on both chiral stationary phases whereby the L‐enantiomers eluted before the D‐enantiomers.     

β‐Amino acids containing peptides and click‐cyclized peptide as β‐turn mimics: a comparative study with ‘conventional’ lactam‐ and disulfide‐bridged hexapeptides

The increasing interest in click chemistry and its use to stabilize turn structures led us to compare the propensity for β‐turn stabilization of different analogs designed as mimics of the β‐turn structure found in tendamistat. The β‐turn conformation of linear β‐amino acid‐containing peptides and triazole‐cyclized analogs were compared to ‘conventional’ lactam‐ and disulfide‐bridged hexapeptide analogs. Their 3D structures and their propensity to fold in β‐turns in solution, and for those not structured in solution in the presence of α‐amylase, were analyzed by NMR spectroscopy and by restrained molecular dynamics with energy minimization. The linear tetrapeptide Ac‐Ser‐Trp‐Arg‐Tyr‐NH₂ and both the amide bond‐cyclized, c[Pro‐Ser‐Trp‐Arg‐Tyr‐D ‐Ala] and the disulfide‐bridged, Ac‐c[Cys‐Ser‐Trp‐Arg‐Tyr‐Cys]‐NH₂ hexapeptides adopt dominantly in solution a β‐turn conformation closely related to the one observed in tendamistat. On the contrary, the β‐amino acid‐containing peptides such as Ac‐(R)‐β³‐hSer‐(S)‐Trp‐(S)‐β³‐hArg‐(S)‐β³‐hTyr‐NH₂, and the triazole cyclic peptide, c[Lys‐Ser‐Trp‐Arg‐Tyr‐βtA]‐NH₂, both specifically designed to mimic this β‐turn, do not adopt stable structures in solution and do not show any characteristics of β‐turn conformation. However, these unstructured peptides specifically interact in the active site of α‐amylase, as shown by TrNOESY and saturation transfer difference NMR experiments performed in the presence of the enzyme, and are displaced by acarbose, a specific α‐amylase inhibitor. Thus, in contrast to amide‐cyclized or disulfide‐bridged hexapeptides, β‐amino acid‐containing peptides and click‐cyclized peptides may not be regarded as β‐turn stabilizers, but can be considered as potential β‐turn inducers. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Synthesis of linear and cyclic opioid‐based peptide analogs containing multiple N‐methylated amino acid residues

A series of six novel opioid peptide analogs containing one to three N‐methylamino acid residues, and six cyclic counterparts of these peptides were prepared by the solid‐phase method. Introduction of two consecutive N‐methylated amino acids, as well as cyclization of such conformationally constrained sequences, turned out to be challenging. The use of a recently reported triazine‐based coupling reagent, 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium toluene‐4‐sulfonate, enabled the synthesis and cyclization of the designed analogs in acceptable yields and with a lesser amount of by‐products than observed with the standard coupling reagents such as TBTU or HATU.Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.