β‐Peptide Conjugates: Syntheses and CD and NMR Investigations of β/α‐Chimeric Peptides,of a DPA‐β‐Decapeptide,and of a PEGylated β‐Heptapeptide

β³‐Peptides consisting of six, seven, and ten homologated proteinogenic amino acid residues have been attached to an α‐heptapeptide (all d‐ amino acid residues; 4 ), to a hexaethylene glycol chain (PEGylation; 5c ), and to dipicolinic acid (DPA derivative 6 ), respectively. The conjugation of the β‐peptides with the second component was carried out through the N‐termini in all three cases. According to NMR analysis (CD₃OH solutions), the (M)‐3₁₄‐helical structure of the β‐peptidic segments was unscathed in all three chimeric compounds (Figs. 2, 4, and 5). The α‐peptidic section of the α/β‐peptide was unstructured, and so was the oligoethylene glycol chain in the PEGylated compound. Thus, neither does the appendage influence the β‐peptidic secondary structure, nor does the latter cause any order in the attached oligomers to be observed by this method of analysis. A similar conclusion may be drawn from CD spectra (Figs. 1, 3, and 5). These results bode well for the development of delivery systems involving β‐peptides.     

New Open‐Chain and Cyclic Tetrapeptides,Consisting of α‐, β2‐, and β3‐Amino‐Acid Residues,as Somatostatin Mimics – A Survey

Cyclo‐β‐tetrapeptides are known to adopt a conformation with an intramolecular transannular hydrogen bond in solution. Analysis of this structure reveals that incorporation of a β²‐amino‐acid residue should lead to mimics of ‘α‐peptidic β‐turns’ (cf. A, B, C ). It is also known that short‐chain mixed β/α‐peptides with appropriate side chains can be used to mimic interactions between α‐peptidic hairpin turns and G protein‐coupled receptors. Based on these facts, we have now prepared a number of cyclic and open‐chain tetrapeptides, 7 – 20 , consisting of α‐, β²‐, and β³‐amino‐acid residues, which bear the side chains of Trp and Lys, and possess backbone configurations such that they should be capable of mimicking somatostatin in its affinity for the human SRIF receptors (hsst_1–5). All peptides were prepared by solid‐phase coupling by the Fmoc strategy. For the cyclic peptides, the three‐dimensional orthogonal methodology (Scheme 3) was employed with best success. The new compounds were characterized by high‐resolution mass spectrometry, NMR and CD spectroscopy, and, in five cases, by a full NMR‐solution‐structure determination (in MeOH or H₂O; Fig. 4). The affinities of the new compounds for the receptors hsst_1–5 were determined by competition with [¹²⁵I]LTT‐SRIF₂₈ or [¹²⁵I] [Tyr¹⁰]‐CST₁₄. In Table 1, the data are listed, together with corresponding values of all β‐ and γ‐peptidic somatostatin/Sandostatin^® mimics measured previously by our groups. Submicromolar affinities have been achieved for most of the human SRIF receptors hsst_1–5. Especially high, specific binding affinities for receptor hsst₄ (which is highly expressed in lung and brain tissue, although still of unknown function!) was observed with some of the β‐peptidic mimics. In view of the fact that numerous peptide‐activated G protein‐coupled receptors (GPCRs) recognize ligands with turn structure (Table 2), the results reported herein are relevant far beyond the realm of somatostatin: many other peptide GPCRs should be ‘reached’ with β‐ and γ‐peptidic mimics as well, and these compounds are proteolytically and metabolically stable, and do not need to be cell‐penetrating for this purpose (Fig. 5).     

NMR‐Solution Structures of Fluoro‐Substituted β‐Peptides: A 314‐Helix and a Hairpin Turn. The First Case of a 90° OC?C?F Dihedral Angle in an α‐Fluoro‐Amide Group

To further study the preference of the antiperiplanar (ap) conformation in α‐fluoro‐amide groups, two β‐peptides, 1 and 2 , containing a (2‐F)‐β³hAla and a (2‐F)‐β²hPhe residue, have been synthesized. Their NMR‐solution structures in CD₃OH were determined and compared with those of non‐F‐substituted analogs, 3 and 4a . While we have found in a previous investigation (Helv. Chim. Acta 2005 , 88, 266) that a stereospecifically introduced F‐substituent in the central position of a β‐heptapeptide is capable of ‘breaking’ the 3₁₄‐helical structure by enforcing the F? C? C?O ap‐conformation, we could now demonstrate that the same procedure leads to a structure with the unfavorable ca. 90° F? C? C?O dihedral angle, enforced by the 3₁₄‐helical folding in a β‐tridecapeptide (cf. 1 ; Fig. 4). This is interpreted as a consequence of cooperative folding in the longer β‐peptide. A F‐substituent placed in the turn section of a β‐peptidic hairpin turn was shown to be in an ap‐arrangement with respect to the neighboring C?O bond (cf. 2 ; Fig. 7). Analysis of the non‐F‐substituted β‐tetrapeptides (with helix‐preventing configurations of the two central β²/β³‐amino acid residues) provides unusually tight hairpin structural clusters (cf. 3 and 4a ; Figs. 8 and 9). The skeleton of the β‐tetrapeptide H‐(R)β³hVal‐(R)β²hVal‐(R)β³hAla‐(S)β³hPhe‐OH ( 4a ) is proposed as a novel, very simple backbone structure for mimicking α‐peptidic hairpin turns.     

Linear,Peptidase‐Resistant β2/β3‐Di‐ and α/β3‐Tetrapeptide Derivatives with Nanomolar Affinities to a Human Somatostatin Receptor,Preliminary Communication

N‐Acyl‐β²/β³‐dipeptide‐amide somatostatin analogs, 5 – 8 , with β²‐HTrp‐β³‐HLys ('natural' sequence) and β²‐HLys‐β³‐HTrp (retro‐sequence) have been synthesized (in solution). Depending on their relative configurations and on the nature of the terminal N‐acyl and terminal C‐amino group, the linear β‐dipeptide derivatives have affinities for the human receptor hsst 4, ranging from 250 to >10000 nanomolar (Fig. 3). Also, N‐Ac‐tetrapeptide amides 9 and 10 , which contain one α‐ and three β‐amino acid residues (N‐β‐α‐β‐β‐C), have been prepared (solid‐phase synthesis), with the natural (Phe, Trp, Lys, Thr) and the retro‐sequence (Thr, Lys, Trp, Phe) of side chains and with two different configurations, each, of the two central amino acid residues. The novel ‘mixed', linear α/β‐peptides have affinities for the hsst 4 receptor ranging from 23 to >10000 nanomolar (Fig. 4), and, like ‘pure' β‐peptides, they are completely stable to a series of proteolytic enzymes. Thus, the peptidic turn of the cyclic tetradecapeptide somatostatin (Fig. 1) can be mimicked by simple linear di‐ and tetrapeptides. The tendency of β‐dipeptides for forming hydrogen‐bonded rings is confirmed by calculations at the B3LYP/6‐31G(d,p) level (Fig. 2). The reported results open new avenues for the design of low‐molecular‐weight peptidic drugs.     

Effect of β3‐Amino Acids on the Performance of the Peptidic Catalyst H‐dPro‐Pro‐Glu‐NH2

The effect of β³‐amino acids on the conformation and catalytic performance of the peptidic catalyst H‐d Pro‐Pro‐Glu‐NH₂ was investigated. Analogues of the peptidic catalyst bearing instead of the α‐amino acids the respective β³‐amino acids were prepared and their reactivity and stereoselectivity was investigated in conjugate addition reactions of aldehydes to nitroolefins. Additional computational studies provided insights into the preferred conformations of the peptidic catalysts. The results show that conformational flexibility at the N‐terminus has a severe effect on the stereoselectivity but is tolerated at the C‐terminus.     

Solid‐Phase Synthesis and Characterization of N‐Terminally Elongated Aβ−3–x‐Peptides

In addition to the prototypic amyloid‐β (Aβ) peptides Aβ_1–40 and Aβ_1–42, several Aβ variants differing in their amino and carboxy termini have been described. Synthetic availability of an Aβ variant is often the key to study its role under physiological or pathological conditions. Herein, we report a protocol for the efficient solid‐phase peptide synthesis of the N‐terminally elongated Aβ‐peptides Aβ_?3–38, Aβ_?3–40, and Aβ_?3–42. Biophysical characterization by NMR spectroscopy, CD spectroscopy, an aggregation assay, and electron microscopy revealed that all three peptides were prone to aggregation into amyloid fibrils. Immunoprecipitation, followed by mass spectrometry, indicated that Aβ_?3–38 and Aβ_?3–40 are generated by transfected cells even in the presence of a tripartite β‐site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitor. The elongated Aβ peptides starting at Val(?3) can be separated from N‐terminally‐truncated Aβ forms by high‐resolution isoelectric‐focusing techniques, despite virtually identical isoelectric points. The synthetic Aβ variants and the methods presented here are providing tools to advance our understanding of the potential roles of N‐terminally elongated Aβ variants in Alzheimer's disease.     

Histidine‐Rich Oligopeptides To Lessen Copper‐Mediated Amyloid‐β Toxicity

Brain copper imbalance plays an important role in amyloid‐β aggregation, tau hyperphosphorylation, and neurotoxicity observed in Alzheimer's disease (AD). Therefore, the administration of biocompatible metal‐binding agents may offer a potential therapeutic solution to target mislocalized copper ions and restore metallostasis. Histidine‐containing peptides and proteins are excellent metal binders and are found in many natural systems. The design of short peptides showing optimal binding properties represents a promising approach to capture and redistribute mislocalized metal ions, mainly due to their biocompatibility, ease of synthesis, and the possibility of fine‐tuning their metal‐binding affinities in order to suppress unwanted competitive binding with copper‐containing proteins. In the present study, three peptides, namely HWH , HK^CH , and HAH , have been designed with the objective of reducing copper toxicity in AD. These tripeptides form highly stable albumin‐like complexes, showing higher affinity for Cu^II than that of Aβ(1‐40). Furthermore, HWH , HK^CH , and HAH act as very efficient inhibitors of copper‐mediated reactive oxygen species (ROS) generation and prevent the copper‐induced overproduction of toxic oligomers in the initial steps of amyloid aggregation in the presence of Cu^II ions. These tripeptides, and more generally small peptides including the sequence His‐Xaa‐His at the N‐terminus, may therefore be considered as promising motifs for the future development of new and efficient anti‐Alzheimer drugs.     

Interaction of PiB‐Derivative Metal Complexes with Beta‐Amyloid Peptides: Selective Recognition of the Aggregated Forms

Metal complexes are increasingly explored as imaging probes in amyloid peptide related pathologies. We report the first detailed study on the mechanism of interaction between a metal complex and both the monomer and the aggregated form of Aβ_1–40 peptide. We have studied lanthanide(III) chelates of two PiB‐derivative ligands (PiB=Pittsburgh compound B), L¹ and L², differing in the length of the spacer between the metal‐complexing DO3A macrocycle (DO3A= 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid) and the peptide‐recognition PiB moiety. Surface plasmon resonance (SPR) and saturation transfer difference (STD) NMR spectroscopy revealed that they both bind to aggregated Aβ_1–40 (K_D=67–160 μM ), primarily through the benzothiazole unit. HSQC NMR spectroscopy on the ¹⁵N‐labeled, monomer Aβ_1–40 peptide indicates nonsignificant interaction with monomeric Aβ. Time‐dependent circular dichroism (CD), dynamic light scattering (DLS), and TEM investigations of the secondary structure and of the aggregation of Aβ_1–40 in the presence of increasing amounts of the metal complexes provide coherent data showing that, despite their structural similarity, the two complexes affect Aβ fibril formation distinctly. Whereas GdL¹, at higher concentrations, stabilizes β‐sheets, GdL² prevents aggregation by promoting α‐helical structures. These results give insight into the behavior of amyloid‐targeted metal complexes in general and contribute to a more rational design of metal‐based diagnostic and therapeutic agents for amyloid‐ associated pathologies.     

Modification of a Small β‐Barrel Protein,To Give Pseudo‐Amyloid Structures,Inhibits Amyloid β‐Peptide Aggregation

Aggregation of amyloid β‐peptide (Aβ) is closely related to the pathogenesis of Alzheimer’s disease (AD). Although much effort has been devoted to the construction of molecules that inhibit the aggregation of Aβ1‐42, high doses are needed for the inhibition of Aβ aggregation in many cases. Previously, we reported that designed green fluorescent protein (GFP) analogues that gives pseudo‐Aβ β‐sheet structures can work as an aggregation inhibitor against Aβ. To further test this design strategy, we constructed protein analogues that mimic Aβ β‐sheet structures of amyloids by using insulin‐like growth factor 2 receptor domain 11 (IGF2R‐d11) as a scaffold. A designed protein, named IG11KK, which has a parallel configuration of Aβ‐like β sheets, can bind more preferentially to oligomeric Aβ1‐42 than the monomer. Moreover, IG11KK suppressed the aggregation of Aβ1‐42 efficiently, even though lower concentrations of IG11KK than Aβ were used. The aggregation kinetics of Aβ in the presence of the designed proteins revealed that IG11KK can work as an inhibitor not only for the early to middle stages, but also in the latter stage of Aβ aggregation owing to its favorable binding to oligomeric structures of Aβ. The design strategy using β‐barrel proteins such as IGF2R‐d11 and GFP is useful in generating excellent inhibitors of protein misfolding and amyloid formation.     

Organoplatinum‐Substituted Polyoxometalate Inhibits β‐amyloid Aggregation for Alzheimer's Therapy

Aggregated β‐amyloid (Aβ) is widely considered as a key factor in triggering progressive loss of neuronal function in Alzheimer's disease (AD), so targeting and inhibiting Aβ aggregation has been broadly recognized as an efficient therapeutic strategy for curing AD. Herein, we designed and prepared an organic platinum‐substituted polyoxometalate, (Me₄N)₃[PW₁₁O₄₀(SiC₃H₆NH₂)₂PtCl₂] (abbreviated as Pt^II‐PW₁₁) for inhibiting Aβ₄₂ aggregation. The mechanism of inhibition on Aβ₄₂ aggregation by Pt^II‐PW₁₁ was attributed to the multiple interactions of Pt^II‐PW₁₁ with Aβ₄₂ including coordination interaction of Pt²⁺ in Pt^II‐PW₁₁ with amino group in Aβ₄₂, electrostatic attraction, hydrogen bonding and van der Waals force. In cell‐based assay, Pt^II‐PW₁₁ displayed remarkable neuroprotective effect for Aβ₄₂ aggregation‐induced cytotoxicity, leading to increase of cell viability from 49 % to 67 % at a dosage of 8 μm . More importantly, the Pt^II‐PW₁₁ greatly reduced Aβ deposition and rescued memory loss in APP/PS1 transgenic AD model mice without noticeable cytotoxicity, demonstrating its potential as drugs for AD treatment.     

2‐amino‐4‐oxo‐6‐substituted‐pyrrolo[2,3‐d]pynmidines as potential inhibitors of thymidylate synthase

Classical, antifolate inhibitors of thymidylate synthase often suffer from a number of potential disadvantages when used as antitumor agents. These include impaired uptake due to an alteration of the active transport system required for cellular uptake, as well as the formation of long acting, non‐effluxing polygluta‐mates via folypolyglutamate synthetase, which are responsible for toxicity to normal cells. To overcome some of the disadvantages of classical thymidylate synthase inhibitors, there has been considerable interest in the synthesis and evaluation of nonclassical inhibitors, which could enter cells via passive diffusion and are not substrates for folypolyglutamate synthetase. A series of eight nonclassical 6‐substituted 2‐amino‐4‐oxo‐pyrrolo[2,3‐d]pyrimidines 2a‐2h were designed as potential inhibitors of thymidylate synthase. The synthesis of the target compounds 2a‐2h was achieved via regioselective iodination at the 6‐position of 5 , palladium‐catalyzed coupling with the appropriate phenylacetylenes, reduction of the C8‐C9 triple bond followed by saponification. Preliminary biological results indicated that none of the target compounds showed inhibitory activities against thymidylate synthase from Escherichia coli, Lactobacillus casei, rat or human thymidylate synthase at the concentrations tested. None of the target compounds showed inhibitory activity against dihydrofolate reductase from Escherichia coli, Lactobacillus casei, rat or human at 3.0 × 10^?5 M. However, 50% inhibition of dihydrofolate reductase from Pneumocystis carinii and from Toxoplasma gondii was achieved with compound 2d and with compound 2g at 3.0 × 10^?5 M.     

Tetrapeptidic Molecular Hydrogels: Self‐assembly and Co‐aggregation with Amyloid Fragment Aβ1‐40

A new family of isomeric tetrapeptides containing aromatic and polar amino acid residues that are able to form molecular hydrogels following a smooth change in pH is described. The hydrogels have been studied by spectroscopic and microscopic techniques showing that the peptide primary sequence has an enormous influence on the self‐assembly process. In particular, the formation of extended hydrophobic regions and the appearance of π‐stacking interactions have been revealed as the driving forces for aggregation. Moreover, the interaction of these compounds with amyloid peptidic fragment Aβ1‐40 has been studied and some of them have been shown to act as templates for the aggregation of this peptide into non‐β‐sheet fibrillar structures. These compounds could potentially be used for the capture of toxic, soluble amyloid oligomers.     

Attenuation of the Aggregation and Neurotoxicity of Amyloid‐β Peptides by Catalytic Photooxygenation

Alzheimer’s disease (AD), a progressive severe neurodegenerative disorder, is currently incurable, despite intensive efforts worldwide. Herein, we demonstrate that catalytic oxygenation of amyloid‐β peptides (Aβ) might be an effective approach to treat AD. Aβ1–42 was oxygenated under physiologically‐relevant conditions (pH 7.4, 37 °C) using a riboflavin catalyst and visible light irradiation, with modifications at the Tyr¹⁰, His¹³, His¹⁴, and Met³⁵ residues. The oxygenated Aβ1–42 exhibited considerably lower aggregation potency and neurotoxicity compared with native Aβ. Photooxygenation of Aβ can be performed even in the presence of cells, by using a selective flavin catalyst attached to an Aβ‐binding peptide; the Aβ cytotoxicity was attenuated in this case as well. Furthermore, oxygenated Aβ1–42 inhibited the aggregation and cytotoxicity of native Aβ.     

Synthesis of β3‐Peptides and Mixed α/β3‐Peptides by Thioligation

Five β‐peptide thioesters ( 1 – 5 , containing 3, 4, 10 residues) were prepared by manual solid‐phase synthesis and purified by reverse‐phase preparative HPLC. A β‐undecapeptide ( 6 ) and an α‐undecapeptide ( 7 ) with N‐terminal β³‐HCys and Cys residues were prepared by manual and machine synthesis, respectively. Coupling of the thioesters with the cysteine derivatives in the presence of PhSH (Scheme and Fig. 1) in aqueous solution occurred smoothly and quantitatively. Pentadeca‐ and heneicosapeptides ( 8 – 10 ) were isolated, after preparative RP‐HPLC purification, in yields of up to 60%. Thus, the so‐called native chemical ligation works well with β‐peptides, producing larger β³‐ and α/β³‐mixed peptides. Compounds 1 – 10 were characterized by high‐resolution mass spectrometry (HR‐MS) and by CD spectroscopy, including temperature and concentration dependence. β‐Peptide 9 with 21 residues shows an intense negative Cotton effect near 210 nm but no zero‐crossing above 190 nm, (Figs. 2–4), which is characteristic of β‐peptidic 3₁₄‐helical structures. Comparison of the CD spectra of the mixed α/β‐pentadecapeptide ( 10 ) and a helical α‐peptide (Fig. 5) indicate the presence of an α‐peptidic 3.6₁₃ helix.     

Synthesis and High‐Resolution NMR Structure of a β3‐Octapeptide with and without a Tether Introduced by Olefin Metathesis

Bridging between (i)‐ and (i+3)‐positions in a β³‐peptide with a tether of appropriate length is expected to prevent the corresponding 3₁₄‐helix from unfolding (Fig. 1). The β³‐peptide H‐β³hVal‐β³hLys‐β³hSer(All)‐β³hPhe‐β³hGlu‐β³hSer(All)‐β³hTyr‐β³hIle‐OH ( 1 ; with allylated βhSer residues in 3‐ and 6‐position), and three tethered β‐peptides 2 – 4 (related to 1 through ring‐closing metathesis) have been synthesized (solid‐phase coupling, Fmoc strategy, on chlorotrityl resin; Scheme). A comparative CD analysis of the tethered β‐peptide 4 and its non‐tethered analogue 1 suggests that helical propensity is significantly enhanced (threefold CD intensity) by a (CH₂)₄ linker between the β³hSer side chains (Fig. 2). This conclusion is based on the premise that the intensity of the negative Cotton effect near 215 nm in the CD spectra of β³‐peptides represents a measure of ‘helical content’. An NMR analysis in CD₃OH of the two β³‐octapeptide derivatives without (i.e., 1 ) and with tether (i.e., 4 ; Tables 1–6, and Figs. 4 and 5) provided structures of a degree of precision (by including the complete set of side chain–side chain and side chain–backbone NOEs) which is unrivaled in β‐peptide NMR‐solution‐structure determination. Comparison of the two structures (Fig. 5) reveals small differences in side‐chain arrangements (separate bundles of the ten lowest‐energy structures of 1 and 4 , Fig. 5, A and B ) with little deviation between the two backbones (superposition of all structures of 1 and 4 , Fig. 5, C ). Thus, the incorporation of a CH₂? O? (CH₂)₄? O? CH₂ linker between the backbone of the β³‐amino acids in 3‐ and 6‐position (as in 4 ) does accurately constrain the peptide into a 3₁₄‐helix. The NMR analysis, however, does not suggest an increase in the population of a 3₁₄‐helical backbone conformation by this linkage. Possible reasons for the discrepancy between the conclusion from the CD spectra and from the NMR analysis are discussed.     

Folding–Unfolding Equilibrium of a Methylidene‐Substituted β‐Peptide

β‐Peptides possess the ability to fold into secondary structure elements, and this property, together with resistance to biodegradation, makes these compounds interesting for pharmaceutical applications. Recently, a novel class of β‐peptides containing methylidene moieties was described. The GROMOS 53A6 force field was used to simulate the folding equilibrium of a β³‐hexapeptide with methylidene (CH₂?) groups at all six CA‐atoms. Due to the rotational barriers induced by these methylidene groups, the helical secondary‐structure elements, normally found in β³‐peptides, are disfavored in this molecule. Simulations, started from fully extended and 3₁₄‐helical conformations, showed that the molecule adopts a complete 2₈‐helix for ca. 5% of the time and partial 2₈‐helical conformations for ca. 20% of the time. Yet, as suggested by experiments, the folding equilibrium is dominated by unfolded conformations.     

The Cyclo‐β‐Tetrapeptide (β‐HPhe‐β‐HThr‐β‐HLys‐β‐HTrp): Synthesis,NMR Structure in Methanol Solution,and Affinity for Human Somatostatin Receptors

The known solid‐state structure (Fig. 1, top) of cyclo(β‐HAla)₄ was used to model the structure of the title compound 1 as a prospective somatostatin mimic (Fig. 1, bottom). The synthesis started with the N‐protected natural amino acids Boc‐Phe‐OH, Boc‐Trp‐OH, Boc‐Lys(2‐Cl‐Z)‐OH, and Boc‐Thr(OBn)‐OH, which were homologated to the corresponding β‐amino‐acid derivatives (Scheme 1) and coupled to the β‐tetrapeptide Boc‐β‐HTrp‐β‐HPhe‐β‐HThr(OBn)‐β‐HLys(2‐Cl‐Z)‐OMe ( 16 ); the (N‐Me)‐β‐HThr‐(N‐Me)‐β‐HPhe analog 17 was also prepared. C‐ and N‐terminal deprotection and cyclization through the pentafluorophenyl ester gave the insoluble β‐tetrapeptide with protected Thr and Lys side chains ( 18 ). Solubilization and debenzylation could only be effected in LiCl‐containing THF (ca. 10% yield; with ca. 55% recovery). HPLC Purification provided a sample of the title compound 1 , the structure of which, as determined by NMR‐spectroscopy (Fig. 2, left) was drastically different from the `theoretical' model (Fig. 1). There is a transannular H‐bond dividing the macrocyclic 16‐membered ring, thus forming a ten‐ and a twelve‐membered H‐bonded ring, the former mimicking, or actually being superimposable on, an α‐peptidic so‐called β‐turn. Still, the four side chains occupy equatorial positions on the ring, as planned, albeit with somewhat different geometry as compared to the `original'. The cyclo‐β‐tetrapeptide has micromolar affinities to the human somatostatin receptors (hsst 1 – 5). Thus, we have demonstrated for the first time that it is possible to mimic a natural peptide hormone with a small β‐peptide. Furthermore, we have discovered a simple way to construct the ubiquitous β‐turn motif with β‐peptides (which are known to be stable to mammalian peptidases).     

Experimental and Theoretical Studies of 4‐(1‐benzyl‐5‐methyl‐1H‐1,2,3‐triazol‐4‐yl)‐6‐(2,4‐dichlorophenyl)pyrimidin‐2‐amine: A Potential Antibacterial Agent

The title compound ( 1 ), 4‐(1‐benzyl‐5‐methyl‐1H‐1,2,3‐triazol‐4‐yl)‐6‐(2,4‐dichlorophenyl)pyrimidin‐2‐amine (C₂₀H₁₆Cl₂N₆), was synthesized and structurally characterized by elemental analysis, ¹H NMR and ¹³C NMR and single crystal X‐ray diffraction. The compound crystallizes as a colourless needle shaped in the triclinic system, space group P‐1 with cell constants: a = 10.7557(11) Å, b = 12.7078(17) Å, c = 15.511(2) Å, α = 68.029(4)0, β = 86.637(5)0, γ = 87.869(4)0; V = 1962.4 (4) ų, Z = 4. There are two structurally similar but crystallographically independent molecules (A and B) in the asymmetric unit of the title compound, which is linked via N‐H…Cl hydrogen bond. An intramolecular C‐H…N hydrogen also occurs in each molecule. In the crystal, each of independent molecules forms a centrosymmetric dimer with an R²₂(8) ring motifs through a pair of N‐H…N hydrogen bonds. These dimers are further connected by intermolecular N‐H…Cl and C‐H…Cl hydrogen bonds, forming an infinite two dimensional supramolecular network lying parallel to the [010] plane. The molecular geometry was also optimized using density functional theory (DFT/B3LYP) method with the 6‐311G (d, p) basis set and compared with the experimental data. Mulliken population analyses on atomic charges, HOMO‐LUMO energy levels, Molecular electrostatic potential and chemical reactivity of the title compound were investigated by theoretical calculations. The thermo dynamical properties of the title compound at different temperature have been calculated and corresponding relations between the properties and temperature have also been obtained. The in vitro antibacterial activity has been screened against Gram‐positive (Bacillus cerus and Staphylococcus epidermidis) and Gram‐Negative (Escherichia coli, Acinetobacter baumannii and Proteus vulgaris). The results revealed that the compound exhibited good to moderate antibacterial activity.     

The Effect of Fluoro Substitution upon the β‐Hairpin Fold of a β‐Tetrapeptide in Methanol

The importance of β‐peptides lies in their ability to mimic the conformational behavior of α‐peptides, even with a much shorter chain length, and in their resistance to proteases. To investigate the effect of substitution of β‐peptides on their dominant fold, we have carried out a molecular‐dynamics (MD) simulation study of two tetrapeptides, Ac‐(2R,3S)‐β^2,3hVal(αMe)‐(2S)‐β²hPhe‐(R)‐β³hLys‐(2R,3S)‐β^2,3‐Ala(αMe)‐NH₂, differing in the substitution at the C_α of Phe2 (pepF with F, and pepH with H). Three simulations, unrestrained (UNRES), using ³J‐coupling biasing with local elevation in combination with either instantaneous (INS) or time‐averaging (AVE) NOE distance restraining, were carried out for each peptide. In the unrestrained simulations, we find three (pepF) and two (pepH) NOE distance bound violations of maximally 0.22 nm that involve the terminal residues. The restrained simulations match both the NOE distance bounds and ³J‐values derived from experiment. The fluorinated peptide shows a slightly larger conformational variability than the non‐fluorinated one.     

N‐[tert‐Butoxy­carbonyl­glycyl‐(Z)‐α,β‐dehydro­phenyl­alanylglycyl‐(E)‐α,β‐dehydro­phenyl­alanylphenyl­alanyl]‐4‐nitro­aniline ethanol solvate

The α,β‐dehydro­phenyl­alanine residues influence the conformation of the title penta­peptide Boc⁰–Gly¹–Δ^ZPhe²–Gly³–Δ^EPhe⁴–l ‐Phe⁵–p‐NA ethanol solvate, C₄₂H₄₃N₇O₉·C₂H₅OH. The first unsaturated phenyl­alanyl (Δ^ZPhe²) and the third glycyl (Gly³) residues form a type I β turn, while the second unsaturated phenyl­alanyl (Δ^EPhe⁴) and the last phenyl­alanyl (l ‐Phe⁵) residues are part of a type II β turn. All the amino acids in the peptide are linked trans to one another. The crystal structure is stabilized by intra‐ and inter­molecular hydrogen bonds.