Effect of obligatory replacement and conformational restriction on psychotropic activity of thyroliberin analogs

Easy lactamization of Gln(Asn)−Pro−NH₂ with the formation of cyclic dipeptides with the diketopiperazine structure (mimetics of the conformational fragments of linear tripeptides with the X−Protrans-bond) was observed in the synthesis of tripeptide Glp−Gln−Pro−NH₂ modified by the replacement of histidine with obligatory similar glutamine in thyroliberin (Glp−His−Pro−NH₂, TRH) and in the synthesis of its structural analog [Asn²]TRH. Ion peaks corresponding to the Glp and Pro amino acid residues were revealed in the mass spectra of the peptides synthesized. The biological properties of the compounds obtained were determined indicating that the obligatory replacement resulted in an increased physiological specificity of [Gln²]TRH. The enhanced activity of conformationally restricted cyclic peptides compared to linear ones suggests that the biologically active conformation responsible for the antidepressant activity of linear TRH analogs is the conformation with X−Protrans-bond. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2015–2020, October, 1998.     

A Genetically Encoded,Phage‐Displayed Cyclic‐Peptide Library

Superior to linear peptides in biological activities, cyclic peptides are considered to have great potential as therapeutic agents. To identify cyclic‐peptide ligands for therapeutic targets, phage‐displayed peptide libraries in which cyclization is achieved by the covalent conjugation of cysteines have been widely used. To resolve drawbacks related to cysteine conjugation, we have invented a phage‐display technique in which its displayed peptides are cyclized through a proximity‐driven Michael addition reaction between a cysteine and an amber‐codon‐encoded N^?‐acryloyl‐lysine (AcrK). Using a randomized 6‐mer library in which peptides were cyclized at two ends through a cysteine–AcrK linker, we demonstrated the successful selection of potent ligands for TEV protease and HDAC8. All selected cyclic peptide ligands showed 4‐ to 6‐fold stronger affinity to their protein targets than their linear counterparts. We believe this approach will find broad applications in drug discovery.     

A Genetically Encoded,Phage‐Displayed Cyclic‐Peptide Library

Superior to linear peptides in biological activities, cyclic peptides are considered to have great potential as therapeutic agents. To identify cyclic‐peptide ligands for therapeutic targets, phage‐displayed peptide libraries in which cyclization is achieved by the covalent conjugation of cysteines have been widely used. To resolve drawbacks related to cysteine conjugation, we have invented a phage‐display technique in which its displayed peptides are cyclized through a proximity‐driven Michael addition reaction between a cysteine and an amber‐codon‐encoded N^?‐acryloyl‐lysine (AcrK). Using a randomized 6‐mer library in which peptides were cyclized at two ends through a cysteine–AcrK linker, we demonstrated the successful selection of potent ligands for TEV protease and HDAC8. All selected cyclic peptide ligands showed 4‐ to 6‐fold stronger affinity to their protein targets than their linear counterparts. We believe this approach will find broad applications in drug discovery.     

Columnar liquid crystalline tris-(ether)triazines with pendant 1,3,4-thiadiazole groups: synthesis,mesomorphic, luminescence,solvatofluorochromic and electrochemical properties

Novel columnar liquid crystals whose molecular structures consist of a C₃ star-shaped 1,3,5-triazine unit as a central core, and three pendant 2-phenyl-5-(di-, and/or tri-n-alkoxyphenyl)-1,3,4-thiadiazole arms, containing ether connecting groups, variable number and positions of linear alkoxy chains were synthesised and their mesomorphic properties were studied by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction. The mesomorphic properties were found to be dependent on the length, position and number of the peripheral alkoxy chains. Most compounds form enantiotropic hexagonal columnar phases. These compounds also show photoluminescent properties in the visible region with good quantum yields. Photophysical studies were realised in solution and in solid state. Also, solvatofluorochromism and cyclic voltammetry studies were performed.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

Proton affinities of the N- and C-terminal segments arising upon the dissociation of the amide bond in protonated peptides

Dissociation of the amide bonds in a protonated peptide leads to N-terminal sequence fragments with cyclic structures and C-terminal sequence fragments with linear structures. The ionic fragments containing the N-terminus (b _n ) have been shown to be protonated oxazolones, whereas those containing the C-terminus (y _n ) are protonated linear peptides. The coproduced neutral fragments are cyclic peptides from the N-terminus and linear peptides from the C-terminus. A likely determinant of these structural choices is the proton affinity (PA) of the described peptide segments. This study determines the PA values of such segments (Pep), i.e., cyclic and linear dipeptides and a relevant oxazolone, based on the dissociations of proton-bound dimers [Pep + B _i ]H⁺ in which B _i is a reference base of known PA value (Cooks kinetic method). The dissociations are assessed at different internal energies to thereby obtain both proton affinities as well as entropies of protonation. For species with comparable amino acid composition, the proton affinity (and gas phase basicity) follows the order cyclic peptide ≪ oxazolone ≈ linear peptide. This ranking is consistent with dissociation of the protonated peptide via interconverting proton-bound complexes involving N-terminal oxazolone (O) or cyclopeptide (C) segments and C-terminal linear peptide segments (L), viz. O ⋯ H⁺ ⋯ L ⇄ C ⋯ H⁺ ⋯ L. N-terminal sequence ions (b _n ) are formed with oxazolone structures which can efficiently compete for the proton with the linear segments. On the other hand, N-terminal neutral fragments detach as cyclic peptides, with H⁺ now being retained by the more basic linear segment from the C-terminus to yield y _n .     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

ortho‐Phenylene‐Bridged Cyclic Oligopyrroles: Conformational Flexibilities and Optical Properties

ortho‐Phenylene‐bridged cyclic trimeric oligopyrrole C3 and hexameric oligopyrrole C6 were synthesized by Suzuki–Miyaura coupling reactions. The twisted structures of C3 and C6 were unambiguously revealed by X‐ray diffraction analysis. The optical properties of these cyclic oligopyrroles were compared with linear oligopyrrole L3 and cyclic tetramer C4 . The cyclic oligopyrroles exhibited large Stokes shifts and blue fluorescence with high quantum yields in solution and in the solid state. In addition, selective N‐methylation and N‐tolylation of C3 were used to tune the optical and electrochemical properties by changing the molecular twists and conformational flexibilities. Throughout these studies, the structure–property relationship of these cyclic strained oligopyrroles has been illustrated as an interesting molecular motif for novel cyclic π‐conjugated systems.     

Synthesis and properties of cyclic ethylene-bridged 3,6-fluorene dimer and its linear analogues

Three oligomers of ethylene-bridged 3,6-fluorene were synthesized starting from phenanthrenequinone. McMurry reaction was applied to synthesize the cyclic compound. Horner-Emmons reaction was used to synthesize the linear compounds with all-trans configuration. The crystal structure of the cyclic compound was described. The optical and electronic properties of these compounds were fully characterized and compared.     

Synthesis of Cyclic Depsipeptides and Peptides via Direct Amide Cyclization

The 2,2-disubstituted 2H-azirin-3-amines 7 (2,2-disubstituted 3-amino-2H-azirines) were used as amino-acid synthons in the preparation of medium-sized cyclic depsipeptides and peptides derived from salicylic acids 6 and anthranilic acid 19 , respectively (Schemes 2--4 and 5, resp.). The combination of the ‘azirine/oxazolone method’ for the synthesis of linear peptides containing α,α-disubstituted α-amino acids and the acid-catalyzed amide cyclization in DMF at 60° proved to be an excellent preparative route to ten-membered cyclic depsipeptides and peptides. In the case of the anthranilic-acid derivative, a transannular ring-closure reaction was observed ( 24 → 25 ). Larger rings proved to be extremely sensitive to hydrolysis.     

Fragmentation and sequencing of cyclic peptides by matrix-assisted laser desorption/ionization post-source decay mass spectrometry.

A series of synthetic cyclic decapeptides and other smaller cyclic peptides were analyzed using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The investigated compounds were cyclized in a head-to-tail manner and contained non-proteinaceous amino acids, such as D-phenylalanine, D,L-4-carboxyphenylalanine, epsilon-aminocaproic acid, and gamma-aminobutyric acid, and were synthesized in a program to develop inhibitors of pp60(c-src) (Src), a tyrosine kinase that is involved in signal transduction and growth regulation. Post-source decay (PSD) spectra of the cyclic peptides featured abundant sequence ions. Two preferential ring opening reactions were detected resulting in linear fragment ions with an N-terminus of proline and a C-terminus of glutamic acid, respectively. MALDI-PSD spectra even permitted de novo sequencing of some cyclic peptides. Systematic studies on cyclic peptides using this method of fragmentation have not been reported to date. This work presents an easy mass spectrometric method, MALDI-PSD, for the characterization and identification of cyclic peptides.     

新型抗菌肽的设计、活性研究及与磷脂相互作用的计算模拟

设计合成了多个具有2个活性序列的线性和环状多肽及具有单个活性序列的短链多肽, 研究了它们的杀菌活性, 发现其杀菌活性顺序为长链肽>环状肽>短链肽, 特别是线性的Linear-KT和Linear-KS对多种革兰氏阴性菌和阳性菌均具有较高的杀菌活性. 采用MTT法考察了Linear-KT和Linear-KS对正常细胞的毒性, 其中Linear-KS表现出较低的细胞毒性, 优于阳性对照多粘菌素B. 利用计算模拟的方法计算了多肽与细菌细胞膜中磷脂酰甘油(DMPG)的相互作用. 结果表明, 多肽和DMPG的结合能也表现出长链肽>环状肽>短链肽的规律, 特别是Linear-KT和Linear-KS具有较高的结合能. 长链肽含有2个活性序列, 可提供多个荷正电的氨基酸与荷负电的磷脂结合, 结合能较大, 杀菌活性较强. 同时, 柔性的结构及Linear-KT和Linear-KS中丝氨酸和苏氨酸的β碳上的羟基可与磷脂上的羰基形成多个氢键, 进一步增大了结合能. 计算模拟的方法为抗菌肽的杀菌活性从理论上提供了一定的依据.     

Adsorption behavior of linear and cyclic genetically engineered platinum binding peptides

Recently, phage and cell-surface display libraries have been adapted for genetically selecting short peptides for a variety of inorganic materials. Despite the enormous number of inorganic-binding peptides reported and their bionanotechnological utility as synthesizers and molecular linkers, there is still a limited understanding of molecular mechanisms of peptide recognition of and binding to solid materials. As part of our goal of genetically designing these peptides, understanding the binding kinetics and thermodynamics, and using the peptides as molecular erectors, in this report we discuss molecular structural constraints imposed upon the quantitative binding characteristics of peptides with an affinity for inorganics. Specifically, we use a high-affinity seven amino acid Pt-binding sequence, PTSTGQA, as we reported in earlier studies and build two constructs: one is a Cys-Cys constrained "loop" sequence (CPTSTGQAC) that mimics the domain used in the pIII tail sequence of the phage library construction, and the second is the linear form, a septapeptide, without the loop. Both sequences were analyzed for their adsorption behavior on Pt thin films by surface plasmon resonance (SPR) spectroscopy and for their conformational properties by circular dichroism (CD). We find that the cyclic peptide of the integral Pt-binding sequence possesses single or 1:1 Langmuir adsorption behavior and displays equilibrium and adsorption rate constants that are significantly larger than those obtained for the linear form. Conversely, the linear form exhibits biexponential Langmuir isotherm behavior with slower and weaker binding. Furthermore, the structure of the cyclic version was found to adopt a random coil molecular conformation, whereas the linear version adopts a polyproline type II conformation in equilibrium with the random coil. The 2,2,2-trifluoroethanol titration experiments indicate that TFE has a different effect on the secondary structures of the linear and cyclic versions of the Pt binding sequence. We conclude that the presence of the Cys-Cys restraint affects both the conformation and binding behavior of the integral Pt-binding septapeptide sequence and that the presence or absence of constraints could be used to tune the adsorption and structural features of inorganic binding peptide sequences.     

High-throughput sequence determination of cyclic peptide library members by partial Edman degradation/mass spectrometry

Cyclic peptides provide attractive lead compounds for drug discovery and excellent molecular probes in biomedical research. Large combinatorial libraries of cyclic peptides can now be routinely synthesized by the split-and-pool method and screened against biological targets. However, post-screening sequence determination of hit peptides has been problematic. In this report, a high-throughput method for the sequence determination of cyclic peptide library members has been developed. TentaGel microbeads (90 mum) were spatially segregated into outer and inner layers; cyclic peptides were displayed on the bead surface, whereas the inner core of each bead contained the corresponding linear peptide as the encoding sequence. After screening of the cyclic peptide library against a macromolecular target, the identity of hit peptides was determined by sequencing the linear encoding peptides inside the bead using a partial Edman degradation/mass spectrometry method. On-bead screening of an octapeptide library (theoretical diversity of 160 000) identified cyclic peptides that bind to streptavidin. A 400-member library of tyrocidine A analogues was synthesized on TentaGel macrobeads and solution-phase screening of the library directly against bacterial cells identified a tyrocidine analogue of improved antibacterial activity. Our results demonstrate that the new method for cyclic peptide sequence determination is reliable, operationally simple, rapid, and inexpensive and should greatly expand the utility of cyclic peptides in biomedical research.     

A Unique Tryptophan C‐Prenyltransferase from the Kawaguchipeptin Biosynthetic Pathway

Cyanobactins are a rapidly growing family of linear and cyclic peptides produced by cyanobacteria. Kawaguchipeptins A and B, two macrocyclic undecapeptides reported earlier from Microcystis aeruginosa NIES‐88, are shown to be products of the cyanobactin biosynthetic pathway. The 9 kb kawaguchipeptin (kgp) gene cluster was identified in a 5.26 Mb draft genome of Microcystis aeruginosa NIES‐88. We verified that this gene cluster is responsible for the production of the kawaguchipeptins through heterologous expression of the kgp gene cluster in Escherichia coli. The KgpF prenyltransferase was overexpressed and was shown to prenylate C‐3 of Trp residues in both linear and cyclic peptides in vitro. Our findings serve to further enhance the structural diversity of cyanobactins to include tryptophan‐prenylated cyclic peptides.     

Three novel anabaenopeptins from the cyanobacterium Anabaena sp.

Three new cyclic peptides, anabaenopeptins NZ825, NZ841, and NZ857, were isolated from the hydrophilic extract of the cultured cyanobacterium Anabaena sp. The planar structure of the compounds was determined by homonuclear and inverse-heteronuclear 2D-NMR techniques as well as high-resolution mass spectrometry. The absolute configuration of the asymmetric centers was studied using Marfey's method for HPLC. This is the first report of anabaenopeptins that contain N-methyl glycine instead of the common N-methyl alanine. The incorporation of N-methyl glycine into the cyclic portion of the compounds results in their appearance as a mixture of two, equally stable, conformers, instead of the one distinct conformer in anabaenopeptins that contain N-methyl alanine or N-methyl homotyrosine. The three compounds were tested for inhibition of serine proteases and found to be not active.     

Photophysical,Electrochemical, Thermal and Morphological Properties of Polyurethanes Containing Azomethine Bonding

In this paper, three new low band gap Schiff bases were prepared by using 3-etoxy-4-hydroxybenzaldehyde and different o-phylene diamines. Then, these Schiff bases were converted to low band gap polyurethane derivatives, and their photophysical, electrochemical, thermal, mechanical and morphological properties were investigated. Photophysical properties of the compounds were investigated by using UV-Vis and photoluminescence (PL) spectroscopy. Electrochemical properties of Schiff bases and polyurethanes containing azomethine were investigated by using cyclic voltammetry (CV). Thermal decomposition and transitions were determined by using TG-DTA, DMA and DSC techniques, respectively. Morphological properties of the compounds were also determined by using scanning electron microscopy (SEM). SEM images showed that polyurethanes containing azomethine consist of semi-crystalline particles.     

Cyclic and hairpin peptide complexes of heme

We have synthesized and characterized a new class of heme-peptide complexes using disulfide-linked hairpin-turn and cyclic peptides and compared these to their linear analogues. The binding affinities, helicities, and mechanism of binding of linear, hairpin, and cyclic peptides to [FeIII(coproporphyrin-I)]+ have been determined. In a minimalist approach, we utilize amphiphilic peptide sequences (15-mers), where a central histidine provides heme ligation, and the hydrophobic effect is used to optimize heme-peptide complex stability. We have incorporated disulfide bridges between amphiphilic peptides to make hairpin and even cyclic peptides that bind heme extremely well, roughly 5 x 106 times more strongly than histidine itself. CD studies show that the cyclic peptide heme complexes are completely alpha-helical. NMR spectra of paramagnetic complexes of the peptides show that the 15-mer peptides bind sequentially, with an observable monopeptide, high-spin intermediate. In contrast, the cyclic peptide complexes ligate both imidazoles cooperatively to the heme, producing only a low-spin complex. Electrochemical measurements of the E1/2 of the FeIII(coproporphyrin-I)+ complexes of these peptides are all at fairly low potentials, ranging from -215 to -252 mV versus NHE at pH 7.     

Isolation,Structure Determination,and Synthesis of Allo‐RA‐V and Neo‐RA‐V,RA‐Series Bicyclic Peptides from Rubia cordifolia L.

Two bicyclic hexapeptides, allo‐RA‐V ( 4 ) and neo‐RA‐V ( 5 ), and one cyclic hexapeptide, O‐seco‐RA‐V ( 6 ), were isolated from the roots of Rubia cordifolia L. Their gross structures were elucidated on the basis of spectroscopic analysis and X‐ray crystallography of compound 5 . The absolute stereochemistry of compounds 4 and 5 were established by their total syntheses, and the absolute stereochemistry of compound 6 by chemical correlation with deoxybouvardin ( 3 ). Comparison of the 3D structures of highly active RA‐VII ( 1 ) with less‐active compounds 4 and 5 suggests that the orientation of the Tyr‐5 and/or Tyr‐6 phenyl rings plays a significant role in their biological activity. The isolation of peptides 4 – 6 , along with compound 3 , and the comparison of their structures seem to indicate that peptide 6 may be the common precursor to bicyclic peptides 3 – 5 in the plant.     

1,2,3-Triazolodiazepines. I. Preparation and benzodiazepine receptor binding of 1-benzyl- and 1-phenethyl-1,2,3-triazolo-[4,5-b][1,4]diazepines

Several new 1,2,3-triazolo[4,5-b][1,4]diazepines were prepared starting from 1-benzyl-1 and 1-phenethyl-4,5-diamino-1,2,3-triazole 2 (Scheme 1), by condensation reactions with β-diketones (Scheme 2), β-ketoesters (Scheme 3), and diethyl malonates (Scheme 4). In the first case we obtained compounds 3 and 4 with basic properties, while the ester function condensations gave cyclic amide derivatives 7, 8, 10, 12 and 13 with acid properties. Some N-methyl derivatives 11, 14 and 15 were obtained from the cyclic amide compounds. Most of compounds were tested for their ability to displace [³H]flunitrazepam from bovine brain membranes but no compound showed benzodiazepine receptor binding affinity.