Aza-peptides. II. X-ray structures of aza-alanine and aza-asparagine-containing peptides

Influence of Aza-Glycine Substitution on the Internalization of Penetratin

The cell-penetrating peptide (CPP) penetratin has gained much attention over many years due to its potential role as a transporter for a broad range of cargo into cells. The modification of penetratin has been extensively investigated too. Aza-peptides are peptide analogs in which one or more of the amino residues are replaced by a semicarbazide. This substitution results in conformational restrictions and modifications in hydrogen bonding properties, which affect the structure and may lead to enhanced activity and selectivity of the modified peptide. In this work, the Trp residues of penetratin were substituted by aza-glycine or glycine residues to examine the effect of these modifications on the cellular uptake and the internalization mechanism. The substitution of Trp48 or Trp48,56 dramatically reduced the internalization, showing the importance of Trp48 in cellular uptake. Interestingly, while aza-glycine in the position of Trp56 increased the cellular uptake, Gly reduced it. The two Trp-modified derivatives showed altered internalization pathways, too. Based on our knowledge, this is the first study about the effect of aza-amino acid substitution on the cell entry of CPPs. Our results suggest that aza-amino acid insertion is a useful modification to change the internalization of a CPP.

A lysine-based 2:1-[α/aza]-pseudopeptide series used as additives in polymeric membranes for CO2 capture: synthesis, structural studies, and application†

The current study presents for the first time the synthesis of a new 2:1-[α/aza]-pseudopeptide series possessing charged amino acids (i.e., lysine) and aims at studying the influences of chirality, backbone length, and the nature of the lysine side chains on the conformation of the 2:1-[α/aza]-oligomers in solution using NMR, FTIR spectroscopy and molecular dynamic calculations. The spectroscopic results emphasized the conservation of the β-turn conformation adopted by the trimers regardless of the chirality which demonstrated a noticeable effect on the conformation of homochiral hexamer (8c) compared with the hetero-analogue (8d). The molecular dynamic calculations predicted that the chirality and the side chain of the lysine residues caused a little distortion from the classical β-turn conformation in the case of short trimer sequences (7c and 7d), while the chirality and the backbone length exerted more distortion on the β-turn adopted by the longer hexamer sequences (8c and 8d). The large disturbance in hexamers from classical β-turn was attributed to increasing the flexibility and the possibility of molecules to adopt a more energetically favorable conformation stabilized by non-classical β-turn intramolecular hydrogen bonds. Thus, alternating d- and l-lysine amino acids in the 2:1-[α/aza]-hexamer (8d) decreases the high steric hindrance between the lysine side chains, as in the homo analogue (8c), and the distortion is less recognized. Finally, short sequences of aza-pseudopeptides containing lysine residues improve CO₂ separation when used as additives in Pebax® 1074 membranes. The best membrane performances were obtained with a pseudopeptidic dimer as an additive (6b′; deprotected lysine side chain), with an increase in both ideal selectivity α_CO2/N2 (from 42.8 to 47.6) and CO₂ permeability (from 132 to 148 Barrer) compared to the virgin Pebax® 1074 membrane.

A new 2:1-[α/aza]-pseudopeptide series based charged lysine amino acid was synthesized. Influences of chirality, backbone length, and lysine side chains on the oligomers conformation were investigated in solution using NMR, FTIR and MD calculations.

Thiocarbazate building blocks enable the construction of azapeptides for rapid development of therapeutic candidates

Peptides, polymers of amino acids, comprise a vital and expanding therapeutic approach. Their rapid degradation by proteases, however, represents a major limitation to their therapeutic utility and chemical modifications to native peptides have been employed to mitigate this weakness. Herein, we describe functionalized thiocarbazate scaffolds as precursors of aza-amino acids, that, upon activation, can be integrated in a peptide sequence to generate azapeptides using conventional peptide synthetic methods. This methodology facilitates peptide editing-replacing targeted amino acid(s) with aza-amino acid(s) within a peptide-to form azapeptides with preferred therapeutic characteristics (extending half-life/bioavailability, while at the same time typically preserving structural features and biological activities). We demonstrate the convenience of this azapeptide synthesis platform in two well-studied peptides with short half-lives: FSSE/P5779, a tetrapeptide inhibitor of HMGB1/MD-2/TLR4 complex formation, and bradykinin, a nine-residue vasoactive peptide. This bench-stable thiocarbazate platform offers a robust and universal approach to optimize peptide-based therapeutics.

Two consecutive aza-amino acids in peptides promote stable β-turn formation in water

Studies on the synthetic methodologies and the structural propensity of peptides containing consecutive aza-amino acids are still in their infancy. Here, details of the synthesis and conformational analysis of tripeptides containing two consecutive aza-amino acids are provided. The demonstration that the type I β-turn folding is induced, even in aqueous media, by the introduction of one or two lateral chains on the diaza-peptide unit is of particular importance for the design of peptidomimetics of biological interest.

Conformational preferences of Ac-Pro-azaXaa-NHMe (Xaa = Asn, Asp, Ala) and the effect of intramolecular hydrogen bonds on their stability in gas phase and solution

The conformational preferences of three azadipeptides Ac-Pro-azaXaa-NHMe [Xaa = Asn (1), Asp (2), Ala (3)] have been carried out in gas phase and solution (water) using the density functional method B3LYP/6-311 +  + G(d,p) to explore the effect of the change of side chain of azaamino acids at the i + 2 position on the stability of these components. The most stable conformations of compounds (1), (2), and (3) have an amid bond oriented trans, trans, and cis, respectively, in gas phase, whereas the orientation of amid bond in water solvent of compounds (2) and (3) has changed to cis and trans, respectively. We have also noticed the importance of backbone-side chain hydrogen bonds in the stabilization of the β turn motif in gas phase since this motif is more stable in the case of compounds (1) and (2) and less stable in the case of compound (3) in which these hydrogen bonds are absent. Furthermore, the βII(βII') turn structure is more stable than βI turn for all conformations of the three compounds in gas phase, while it is not true in the case of some conformations in solution. Moreover, the stability of β turn increases from azaAsn to azaAsp which could be due to the side chain's basic nature of azaAsn. In general, hydrogen bonds were found to play a key role in the stabilization of these compounds since most of conformers are lower in energy when they have more than two hydrogen bond interactions while conformations with one or no hydrogen bonds are higher in energy and thus less stable.

Stabilization of Azapeptides by Namide···H-Namide Hydrogen Bonds

An unusual N_amide···H-N_amide hydrogen bond (HB) was previously proposed to stabilize the azapeptide β-turns. Herein we provide experimental evidence for the N_amide···H-N_amide HB and show that this HB endows a stabilization of 1-3 kcal·mol^-1 and enforces the trans-cis-trans (t-c-t) and cis-cis-trans (c-c-t) amide bond conformations in azapeptides and N-methyl-azapeptides, respectively. Our results indicate that these N_amide···H-N_amide HBs can have stabilizing contributions even in short azapeptides that cannot fold to form β-turns.

Paired Utility of Aza-Amino Acyl Proline and Indolizidinone Amino Acid Residues for Peptide Mimicry: Conception of Prostaglandin F2α Receptor Allosteric Modulators That Delay Preterm Birth

Peptide mimicry employing a combination of aza-amino acyl proline and indolizidinone residues has been used to develop allosteric modulators of the prostaglandin F2α receptor. The systematic study of the N-terminal phenylacetyl moiety and the conformation and side chain functions of the central turn dipeptide residue has demonstrated the sensitive relationships between modulator activity and topology. Examination of aza-Gly-Pro and aza-Phe-Pro analogs 2a and 2b in a murine preterm labor model featuring treatment with lipopolysaccharide demonstrated their capacity to extend significantly (>20 h) the average time of delivery offering new prototypes for delaying premature birth.

Progress with peptide scanning to study structure-activity relationships: the implications for drug discovery

INTRODUCTION

Peptides have gained renewed interest as candidate therapeutics. However, to bring them to a broader clinical use, challenges such as the rational optimization of their pharmacological properties remain. Peptide scanning techniques offer a systematic framework to gain information on the functional role of individual amino acids of a peptide. Due to progress in mastering new chemical synthesis routes targeting amino acid backbone, they are currently diversified. Structure-activity relationship (SAR) analyses such as alanine- or enantioneric- scanning can now be supplemented by N-substitution, lactam cyclisation- or aza-amino scanning procedures addressing not only SAR considerations but also the peptide pharmacological properties.

AREAS COVERED

This review highlights the different scanning techniques currently available and illustrates how they can impact drug discovery.

EXPERT OPINION

Progress in peptide scanning techniques opens new perspectives for peptide drug development. It comes with the promise of a paradigm change in peptide drug design in which peptide drugs will be closer to the parent peptides. However, scanning still remains assimilable to a trial and error strategy that could benefit from being combined with specific in silico approaches that start reaching maturity.

Investigation of the active turn geometry for the labour delaying activity of indolizidinone and azapeptide modulators of the prostaglandin F2α receptor

On pursuing molecules that delay labour, so-called tocolytics, the prostaglandin F2α receptor (FP) was targeted, because of its role in the stimulation of uterine contractions leading to birth and preterm birth. Previously, both the indolizidinone PDC-113.824 (5) and the aza-glycinyl-proline analog 6 were shown to delay labour in mice by modulating the FP function, likely by an allosteric mechanism, which features biased signalling. The crystal structure and computational analyses of the indolizidin-2-one amino acid and aza-glycinyl-proline components of 5 and 6 in model peptides have shown them to adopt a geometry that mimics ideal type I and II'β-turns. To elucidate the precise turn geometry for receptor recognition, analogs 1-4 have now been synthesized: macrocycle and pyrroloazepinone mimics 1 and 2 to mimic type I, and glycinyl-proline and d-alaninyl-proline analogs 3 and 4 to favour type II'β-turn geometry. Notably, transannular cyclization of peptide macrocycle 13 has provided diastereoselectively pyrroloazepinone 15 by a novel route that provides effective access to mimics 1 and 2 by way of a common intermediate. Among the four analogs, none exhibited efficacy nor potency on par with 5 and 6; however, d-alaninyl-proline analog 4 proved superior to the other analogs in reducing PGF2α-induced myometrial contractions and inhibiting FP modulation of cell ruffling, a response dependent on the Gα12/RhoA/ROCK signaling pathway. Furthermore Gly-Pro analog 3 potentiated the effect of PGF2α on Gαq mediated ERK1/2 activation. Evidence that 4 adopted turn geometry was obtained by conformational analysis using NMR spectroscopy to characterize respectively the influence of solvent and temperature on the chemical shifts of the amide NH protons. Although mimicry of the type II' geometry by 3, 4, 5 and 6 may favour activity, distortion from ideal geometry by the indolizidinone and aza-glycinyl residues of the latter appears to enhance their biological effects.

β-Turn structure in glycinylphenylalanine dipeptide based N-amidothioureas

Transforming the C-terminal amide of a glycinylphenylalanine dipeptide into N-amidothiourea affords a β-turn structure in the formed dipeptide based N-amidothioureas, which can be readily identified by an induced CD signal from the achiral phenylthiourea chromophore.

Phenylthiocarbamate or N-carbothiophenyl group chemistry in peptide synthesis and bioconjugation

The design of novel chemoselective and site-specific ligation methods provides new tools for obtaining complex scaffolds, peptidomimetics, and peptide conjugates. The chemistry of the N-phenylthiocarbonyl group has led to several developments in peptide ligation chemistry and peptide bioconjugation during the last 10 years. The aim of this review is to provide an overview of this emerging field.

Peptide scanning for studying structure-activity relationships in drug discovery

Peptide-based therapeutics have grown in importance over the last few decades. Furthermore, peptides have been extensively used as lead compounds in the drug discovery process to investigate the nature of chemical space required for molecular recognition and activity at a variety of targets. This critical commentary reviews scanning techniques, which employ natural and non-proteinogenic amino acids to facilitate understanding of structural requirements for peptide biological activity. The value of sequence analysis by such methods is highlighted by examples, in which the elements for peptide affinity and activity have been elucidated and employed to prepare peptidomimetic leads for drug development.

N-Amino-imidazolin-2-one peptide mimic synthesis and conformational analysis

Base-promoted 5-exo-dig cyclizations of aza-propargylglycinamides provided N-amino-imidazolin-2-one peptide mimics, which exhibited turn geometry in X-ray crystallographic and NMR spectroscopic analyses. Sonogashira coupling prior to cyclization afforded N-amino-imidazolin-2-ones with diverse 4-position aromatic substituents with potential to serve as Phe and Trp mimics.

Serum stability of selected decapeptide agonists of KISS1R using pseudopeptides

Metastin/kisspeptin, a 54-amino acid peptide, is the ligand of the G-protein-coupled receptor KISS1R which plays a key role in pathways that regulate reproduction and cell migration in many endocrine and gonadal tissues. The N-terminally truncated decapeptide, metastin(45-54), has 3-10 times higher receptor affinity and intracellular calcium ion-mobilizing activity but is rapidly inactivated in serum. In this study we designed and synthesized stable KISS1R agonistic decapeptide analogs with selected substitutions at positions 47, 50, and 51. Replacement of glycine with azaglycine (azaGly) in which the α-carbon is replaced with a nitrogen atom at position 51 improved the stability of amide bonds between Phe(50)-Gly(51) and Gly(51)-Leu(52) as determined by in vitro mouse serum stability studies. Substitution for tryptophan at position 47 with other amino acids such as serine, threonine, β-(3-pyridyl)alanine, and D-tryptophan (D-Trp), produced analogs that were highly stable in mouse serum. D-Trp(47) analog 13 showed not only high metabolic stability but also excellent KISS1R agonistic activity. Other labile peptides may have increased serum stability using amino acid substitution.

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.

Calcitonin gene-related peptide analogues with aza and indolizidinone amino acid residues reveal conformational requirements for antagonist activity at the human calcitonin gene-related peptide 1 receptor

Calcitonin gene-related peptide antagonists have potential for the treatment and prevention of disease states such as non-insulin-dependent diabetes mellitus, migraine headache, pain, and inflammation. To gain insight into the spatial requirements for CGRP antagonism, three strategies were employed to restrict the conformation of the potent undecapeptide antagonist, [D31,P34,F35]CGRP27-37. First, aza-amino acid scanning was performed, and ten aza-peptide analogues were synthesized and examined for biological activity. Second, (3S,6S,9S)-2-oxo-3-amino-indolizidin-2-one amino acid (I2aa) and (2S,6S,8S)-9-oxo-8-amino-indolizidin-9-one amino acid (I9aa) both were introduced at positions 31-32, 32-33, 33-34, and 34-35, regions of the backbone expected to adopt turns. Finally, the conformation of the backbone and side-chain of the C-terminal residue, Phe35-Ala36-Phe37-NH2, was explored employing (2S,4R,6R,8S)-9-oxo-8-amino-4-phenyl-indolizidin-9-one amino acid (4-Ph-I9aa) as a constrained phenylalanine mimic. The structure-activity relationships exhibited by our 26 analogues illustrate conformational requirements important for designing CGRP antagonists and highlight the importance of beta-turns centered at Gly33-Pro34 for potency.

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.

Aza-Amino Acid Scan for Rapid Identification of Secondary Structure Based on the Application ofN-Boc-Aza1-Dipeptides in Peptide Synthesis

Azapeptides, peptide analogues in which the alpha-carbon of one or more of the amino acid residues is replaced with a nitrogen atom, exhibit propensity for adopting beta-turn conformations. A general protocol for the synthesis of azapeptides without racemization on solid phase has now been developed by introducing the aza-amino acid residue as an N-Boc-aza(1)-dipeptide. This approach has been validated by the synthesis of six N-Boc-aza(1)-dipeptides and their subsequent introduction into analogues of the C-terminal peptide fragment of the human calcitonin gene-related peptide (hCGRP). By performing an aza-amino acid scan of such antagonist peptides, a set of aza-hCGRP analogues was synthesized to examine the relationship between turn secondary structure and biological activity.

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.

Torsion angle based design of peptidomimetics: a dipeptidic template adopting beta-I turn (Ac-Aib-AzGly--NH(2))

We have attempted to design a model dipeptide (acetyl dipeptide amide, Ac-CA1-CA2--NH(2)) that can adopt specifically typical torsion angles of the beta-I turn (phi(i+1), psi(i+1), phi(i+2), psi(i+2)=-60 degrees, -30 degrees, -90 degrees, 0 degrees ). The key of the design is the combination of constrained amino acids that prefer to adopt the desired torsion angles. We chose Aib (aminoisobutyric acid) as the first residue of which phi and psi angles must be -60 degrees and -30 degrees, respectively. Then, we selected an azaamino acid as the second residue since previous studies have indicated that they prefer to adopt +/-90 degrees of phi angle and 0 degrees or 180 degrees of psi angle. The conformational preference of the resulting Ac-Aib-AzGly--NH(2) is investigated using ab initio methods. The conformations implying beta-I and beta-I' turns are energetically most favorable, as we expected. Thus, we synthesized the designed molecule on the solid phase considering the future generation of combinatorial libraries using an automatic peptide synthesizer. Then, NMR spectroscopy was carried out to confirm their conformational preference in solution was carried out. The results indicated that the Ac-Aib-AzGly--NH(2) adopt beta-I or beta-I' turns in solution forming an intramolecular hydrogen bonding between Ac--C(O) and terminal NH(2). We believe that such a small peptidomimetic template is highly useful for the design of drug candidates and molecular devices.

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.

Role of azaamino acid residue in beta-turn formation and stability in designed peptide

The structural perturbation induced by C(alpha)-->N(alpha) exchange in azaamino acid-containing peptides was predicted by ab initio calculation of the 6-31G* and 3-21G* levels. The global energy-minimum conformations for model compounds, For-azaXaa-NH2 (Xaa=Gly, Ala, Leu) appeared to be the beta-turn motif with a dihedral angle of phi= +/- 90 degrees, psi=0 degrees. This suggests that incorporation of the azaXaa residue into the i+2 position of designed peptides could stabilize the beta-turn structure. The model azaLeu-containing peptide, Boc-Phe-azaLeu-Ala-OMe, which is predicted to adopt a beta-turn conformation was designed and synthesized in order to experimentally elucidate the role of the azaamino acid residue. Its structural preference in organic solvents was investigated using 1H NMR, molecular modelling and IR spectroscopy. The temperature coefficients of amide protons, the characteristic NOE patterns, the restrained molecular dynamics simulation and IR spectroscopy defined the dihedral angles [ (phi i+1, psi i+1) (phi i+2, psi i+2)] of the Phe-azaLeu fragment in the model peptide, Boc-Phe-azaLeu-Ala-OMe, as [(-59 degrees, 127 degrees) (107 degrees, -4 degrees)]. This solution conformation supports a betaII-turn structural preference in azaLeu-containing peptides as predicted by the quantum chemical calculation. Therefore, intercalation of the azaamino acid residue into the i+2 position in synthetic peptides is expected to provide a stable beta-turn formation, and this could be utilized in the design of new peptidomimetics adopting a beta-turn scaffold.

Conformational Preference of Acetyl-Azaalanine-N-Methyl Amide

Conformational preference of Ac-AzAla-NHMe has been investigated using ab initio calculations. Azaalanine in peptides prefer to adopt two conformational families including four conformations in which (φ,ψ) are (-90°, -13°), (90°, 13°), (-70°, 164°) and (70°, -164°). The stability at φ = ±90° is explained by repulsion of lone pairs on the nitrogens and hydrogen bonding between Ac-N and terminal N(Me)H. On the other hand, the angle ψ adopts only ~0° or 180° since the N(Me)-C(O) bond has double bond character. One conformational family in which the (φ,ψ) torsion angles are (-90°, -13°) or (90°, 13°), is similar to the i+2 position of typical β-I or β-II turns. The other conformation in which the (φ,ψ) torsion angles are (-70°, 164°) or (70°, -164°), is similar to the polyproline II structure appearing in collagen. We believe that these results are useful in designing constrained peptidomimetics for drug discovery and peptide engineering.

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.

X-ray structures of aza-proline-containing peptides

The aza-analogue of proline (AzPro) contains a nitrogen atom in place of the CH alpha of the cognate residue. The resolution of the crystal structures of seven AzPro-containing peptides, presenting a set of ten AzPro motifs, reveals the structural properties of this particular aza-residue. Because of sterical hindrances, both nitrogen atoms are out of planarity, and the reduced electronic conjugation in the two AzPro-adjacent amide groups probably explains the longer amide bond distances and the weak proton-accepting character of the two pyrazolidine nitrogens. The absolute configuration of both AzPro nitrogens depends on the chemical nature of the sequence. In all cases, the AzPro residue assumes the same intrinsic three-dimensional structure and presents folding tendencies opposed to those induced by proline.

Aza-peptides. III. Experimental structural analysis of aza-alanine and aza-asparagine-containing peptides

To determine the structural perturbations induced by the C alpha H-->N alpha exchange in aza-peptides, we have examined by 1H NMR and IR spectroscopy various derivatives of the aza-analogues of alanine, aspartic acid and asparagine in different organic solvents with increasing polarity. Their general formulas are: R1-AzXaa-NR2R3, R1-Pro-AzXaa-NR2R3 and R1-AzXaa-Pro-NR2R3 (where AzXaa denotes the aza-analogue of the amino acid residue Xaa = Ala, Asp, Asn; R1 = Boc, Z; R2, R3 = H, Me, iPr). The aza-analogue of an amino acid residue appears to be a strong beta-turn-inducing motif, and the AzAsn carboxamide side-chain is capable of interacting, as a proton donor, with the preceding peptide carbonyl group.