Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “macrocycles”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6Linked to original sources
PubMed2000

Binding of phosphate with a simple hexaaza polyammonium macrocycle.

A mixture of dihydrogen phosphate and phosphoric acid has been crystallized with a hexaprotonated 26-membered polyammonium macrocycle, 1,4,7,14,17,20-hexaazacyclohexacosane, as the counterion. The complex crystallizes in the monoclinic space group P2(1)/c with unit cell parameters of a = 10.006(2) A, b = 12.525(1) A, c = 19.210(2) A, beta = 102.91(1) degrees, and V = 2346.6(5) A3. The hexaprotonated macrocycle is located on a crystallographic center of inversion and is surrounded by eight phosphate anions. Six of the phosphates are dihydrogen phosphates (H2PO4-), and the other two are neutral phosphoric acid molecules. Intricate hydrogen-bonding networks, involving the anionic and neutral phosphates and the protonated macrocycle, dominate the crystal lattice. Potentiometric studies using NaCl as the supporting electrolyte indicate high formation constants for the triprotonated macrocycle, H3L3+, with PO4(3-) at pH approximately 9.5 (log K = 4.55(4)), for the tetraprotonated macrocycle, H4L4+, with monohydrogen phosphate, HPO4(2-), at pH approximately 8.0 (log K = 6.01(3)), and for ditopic complexes with H5L5+ and H6L6+ and dihydrogen phosphate, H2PO4-, at pH approximately 4.0 (log K = 6.16(6)) and pH approximately 2.5 (log K = 6.44(5)), respectively. The ditopic behavior in the simple polyazamacrocycle receptor is a somewhat unusual occurrence, as is the finding of phosphoric acid species in the crystal structure.

Journal Article↗
PubMed2002

m-Diethynylbenzene macrocycles: syntheses and self-association behavior in solution.

m-Diethynylbenzene macrocycles (DBMs), buta-1,3-diyne-bridged [4(n)]metacyclophanes, have been synthesized and their self-association behaviors in solution were investigated. Cyclic tetramers, hexamers, and octamers of DBMs having exo-annular octyl, hexadecyl, and 3,6,9-trioxadecyl ester groups were prepared by intermolecular oxidative coupling of dimer units or intramolecular cyclization of the corresponding open-chain oligomers. The aggregation properties were investigated by two methods, the (1)H NMR spectra and the vapor pressure osmometry (VPO). Although some discrepancies were observed between the association constants obtained from the two methods, the qualitative view was consistent with each other. The analysis of self-aggregation by VPO revealed unique aggregation behavior of DBMs in acetone and toluene, which was not elucidated by the NMR method. Namely, the association constants for infinite association are several times larger than the dimerization constant, suggesting that the aggregation is enhanced by the formation of dimers (a nucleation mechanism). In polar solvents, DBMs aggregate more strongly than in chloroform due to the solvophobic interactions between the macrocyclic framework and the solvents. Moreover, DBMs self-associate in aromatic solvents such as toluene and o-xylene more readily than in chloroform. In particular, the hexameric DBM having a large macrocyclic cavity exhibits extremely large association constants in aromatic solvents. By comparing the aggregation properties of DBMs with the corresponding acyclic oligomers, the effect of the macrocyclic structure on the aggregation propensity was clarified. Finally, it turned out that DBMs tend to aggregate more readily than the corresponding phenylacetylene macrocycles, acetylene-bridged [2(n)]metacyclophanes, owing to the withdrawal of the electron density from the aromatic rings by the butadiyne linkages which facilitates pi-pi stacking interactions.

Journal Article↗
PubMed2003

Selective recognition of G-qQuadruplex telomeric DNA by a bis(quinacridine) macrocycle.

The interaction of G-quadruplex DNA with the macrocyclic compound BOQ1, which possesses two dibenzophenanthroline (quinacridine) subunits, has been investigated by a variety of methods. The oligonucleotide 5'-A(GGGT(2)A)(3)G(3), which mimics the human telomeric repeat sequence and forms an intramolecular quadruplex, was used as one model system. Equilibrium binding constants measured by biosensor surface plasmon resonance (SPR) methods indicate a high affinity of the macrocycle for the quadruplex conformation (K > 1 x 10(7) M(-)(1)) with two equivalent binding sites. The affinity of BOQ1 for DNA duplexes is at least 1 order of magnitude lower. In addition, the macrocycle is more selective than the monomeric control compound (MOQ2), which is not able to discriminate between the two DNA structures (K(duplex) approximately K(quadruplex) approximately 10(6) M(-)(1)). Strong binding of BOQ1 to G4 DNA sequences was confirmed by fluorometric titrations with a tetraplex-forming oligonucleotide. Competition dialysis experiments with a panel of different DNA structures, from single strands to quadruplexes, clearly established the quadruplex binding specificity of BOQ1. Fluorescence resonance energy transfer (FRET) T(m) experiments with a doubly labeled oligonucleotide also revealed a strong stabilization of the G4 conformation in the presence of BOQ1 (DeltaT(m) = +28 degrees C). This DeltaT(m) value is one of the highest values measured for a G-quadruplex ligand and is significantly higher than observed for the monomer control compounds (DeltaT(m) = +10-12 degrees C). Gel mobility shift assays indicated that the macrocycle efficiently induces the formation of G-tetraplexes. Strong inhibition of telomerase was observed in the submicromolar range (IC(50) = 0.13 microM). These results indicate that macrocycles represent an exciting new development opportunity for targeting DNA quadruplexes.

Acridines↗
PubMed2002

Conformationally constrained macrocycles that mimic tripeptide beta-strands in water and aprotic solvents.

The beta-strand conformation is unknown for short peptides in aqueous solution, yet it is a fundamental building block in proteins and the crucial recognition motif for proteolytic enzymes that enable formation and turnover of all proteins. To create a generalized scaffold as a peptidomimetic that is pre-organized in a beta-strand, we individually synthesized a series of 15-22-membered macrocyclic analogues of tripeptides and analyzed their structures. Each cycle is highly constrained by two trans amide bonds and a planar aromatic ring with a short nonpeptidic linker between them. A measure of this ring strain is the restricted rotation of the component tyrosinyl aromatic ring (DeltaG(rot) 76.7 kJ mol(-1) (16-membered ring), 46.1 kJ mol(-1) (17-membered ring)) evidenced by variable temperature proton NMR spectra (DMF-d(7), 200-400 K). Unusually large amide coupling constants ((3)J(NH-CHalpha) 9-10 Hz) corresponding to large dihedral angles were detected in both protic and aprotic solvents for these macrocycles, consistent with a high degree of structure in solution. The temperature dependence of all amide NH chemical shifts (Deltadelta/T 7-12 ppb/deg) precluded the presence of transannular hydrogen bonds that define alternative turn structures. Whereas similar sized conventional cyclic peptides usually exist in solution as an equilibrium mixture of multiple conformers, these macrocycles adopt a well-defined beta-strand structure even in water as revealed by 2-D NMR spectral data and by a structure calculation for the smallest (15-membered) and most constrained macrocycle. Macrocycles that are sufficiently constrained to exclusively adopt a beta-strand-mimicking structure in water may be useful pre-organized and generic templates for the design of compounds that interfere with beta-strand recognition in biology.

Carboxylic Acids↗
PubMed2003

Rigid macrocyclic triamides as anion receptors: anion-dependent binding stoichiometries and 1H chemical shift changes.

The cyclic triamide of 3'-amino-3-biphenylcarboxlic acid is readily synthesized in a stepwise manner and represents a novel class of anion receptors with a large central cavity. This macrocycle binds more strongly to tetrahedral anions than spherical or planar anions in organic solvents. The binding stoichiometries for anions with symmetrical charge distribution depend on the solvent polarity, while tetrahedral p-tosylate binds to the macrocycle with 1:1 stoichiometry in all solvents studied. The (1)H NMR chemical shift changes of the protons lining the interior of the macrocycle's central cavity also depend on the geometry of the bound anion. The importance of the convergent array of hydrogen bond donors for anion binding by the macrocycle was confirmed by control studies with an acyclic triamide and a macrocycle with intramolecular hydrogen bonds.

Amides↗
PubMed2006

Liquid crystalline order from ortho-phenylene ethynylene macrocycles.

Triangular ortho-phenylene ethynylene (o-PE) cyclic trimers represent a novel member of shape-persistent macrocycles. Shape-persistent cyclic structures remain of great interest as molecular components in the fields of supramolecular materials, host-guest chemistry, and materials science. Novel discotic liquid crystalline properties are reported from triangular-shaped o-PE macrocycles containing branched alkoxy- and/or triethylene glycol (TEG) side chains using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The macrocycles self-assemble into thermotropic rectangular columnar (Colr) (for M1), hexagonal columnar (Colh) (for M2), and discotic nematic (for M3) mesophases at room temperature. This work shows clearly that electron-rich PE macrocycles can form LC materials. Alkyl side chains in M1 promote order, while hydrophilic side chains of M2 generate an amphiphilic structure that provides a different driving force for organization. The ability to create ordered self-assembling materials from these novel electron-rich macrocycles is important in nanotechnology.

Journal Article↗
PubMed1990

Lipophilic 1,3-xylyl-21-crown-6 macrocyclic polyether 2-carboxylic acids as biological mimics of the ionophore antibiotics.

Twelve lipophilic 1,3-xylyl-21-crown-6 macrocyclic polyether 2-carboxylic acids, two lariat ether 1,3-xylyl-21-crown-6 macrocyclic polyether 2-carboxylic acids, and two 1,3-xylyl-28-crown-8 macrocyclic polyether 2-carboxylic acids were synthesized and tested for in vitro antibacterial activity, in vitro stimulation of rumen propionic acid production, and in vivo anticoccidial activity in chickens. These are biological screens relevant to animal health areas where the ionophore antibiotics such as monensin have found application. While the parent structure 1 without lipophilic substituents was biologically inactive, the lipophilic macrocycles were active in the two in vitro tests but not against chicken coccidiosis. One compound was tested in cattle and was found to increase levels of propionic acid in the rumen fermentation. This effect is considered an important factor for increasing the efficiency of feed utilization in cattle exhibited by the ionophore antibiotic monensin. The alkali ion salts of these lipophilic macrocyclic polyether carboxylic acids are very soluble in organic solvents and insoluble in water. These compounds are proposed to act as ion-transport agents and functional mimics of the ionophore antibiotics in the biological systems described above.

Animals↗
PubMed2004

Macrocyclic inhibitors for peptide deformylase: a structure-activity relationship study of the ring size.

Peptide deformylase (PDF) catalyzes the removal of the N-terminal formyl group from newly synthesized polypeptides in eubacteria. Its essential role in bacterial cells but not in mammalian cells makes it an attractive target for antibacterial drug design. We have previously reported an N-formylhydroxylamine-based, metal-chelating macrocyclic PDF inhibitor, in which the P(1)' and P(3)' side chains are covalently joined. In this work, we have carried out a structure-activity relationship study on the size of the macrocycle and found that 15-17-membered macrocycles are optimal for binding to the PDF active site. Unlike the acyclic compounds, which are simple competitive inhibitors, the cyclic compounds all act as slow-binding inhibitors. As compared to their acyclic counterparts, the cyclic inhibitors displayed 20-50-fold higher potency against the PDF active site (K(I) as low as 70 pM), improved selectivity toward PDF, and improved the metabolic stability in rat plasma. Some of the macrocyclic inhibitors had potent, broad spectrum antibacterial activity against clinically significant Gram-positive and Gram-negative pathogens. These results suggest that the macrocyclic scaffold provides an excellent lead for the development of a new class of antibiotics.

Amidohydrolases↗
PubMed2004

Macrocyclic diacylglycerol-bis-lactones as conformationally constrained analogues of diacylglycerol-lactones. Interactions with protein kinase C.

A series of macrocyclic diacylglycerol (DAG)-bis-lactones were investigated as extreme conformationally constrained analogues of DAG-lactones in order to seek more potent protein kinase C (PKC) ligands with higher binding affinities and less lipophilicity than previous compounds. The additional constraint achieved the desired objective as exemplified by the macrocyclic DAG-bis-lactone 57, which exhibited a 6-fold higher binding affinity for PKCalpha (K(i) = 6.07 nM) than the corresponding nonmacrocyclic 3-alkylidene DAG-lactone 6. A structure-activity relationship (SAR) analysis of the macrocyclic DAG-bis-lactones demonstrated a parabolic relationship between activity and lipophilicity, as well as a predilection for the Z-alkylidene isomers as the preferred ligands. Molecular docking studies revealed that macrocyclic DAG-bis-lactone 57 bound to the C1b domain of PKCalpha exclusively in the sn-1 binding mode in contrast to DAG-lactone 6, which showed both sn-1 and sn-2 binding modes. It is proposed that the high potency displayed by these macrocyclic DAG-bis-lactones results from a set of more favorable hydrogen bonding and hydrophobic interactions with PKCalpha as well as from a reduced entropy penalty due to conformational restriction.

Diglycerides↗
PubMed2001

Supramolecular assistance to regioselectivity in the reactions of chlorocyclophosphazenes with sodium oxyanions: macrocyclic effect and anion dependence.

Comparative studies of the naphtholysis of crown-bearing tetrachlorocyclotriphosphazene 2 and its acyclic analogue, nongem-diethoxytetrachlorocyclotriphosphazene 5 (which was synthesized and isolated in the pure cis form for the purpose of this study), reveal a significant macrocyclic effect of the substituent on both the kinetics and regiochemistry of chlorine substitution in the N(3)P(3) ring. Whereas substitution of 5 with sodium naphthoxide provided moderate yields of the respective sterically and electronically favored nongem-naphthoxydiethoxy derivative as the major product, isolated as the mixture of cis and trans isomers 8 + 9, the corresponding reaction of crown substrate 2 resulted in highly regioselective formation of the gem-to-macrocycle substituted mononaphthoxy PNP-crown derivative 11a. The importance of the PNP-crown related effects for the regio- and stereocontrol of the substitution of chlorine atoms in the N(3)P(3) ring is discussed in terms of supramolecular sodium cation-assisted interactions between the ring oxy substituent(s) and incoming oxyanions. To approach the problem of the anion-dependence of the regioselectivity, a comparison has been drawn between the reactions of the PNP-crown substrate 2 with sodium arylates derived from beta-naphthol (10a) and phenol (10b), carried out with equimolar or 100% excess sodium oxyanion with respect to 2, and the corresponding reactions of 2 with sodium monoenolates derived from the beta-dicarbonyl compounds: acetylacetone (10c) and ethyl acetylacetate (10d). The effectiveness of supramolecularly assisted transition state stabilization is found to be dependent not only on the dimensional complementarity between the cation and the macrocycle but also on the properties of the four interacting ionic centers according to the HSAB principle and on the nature of the electronic interaction of the PNP-crown substituent with the N(3)P(3) ring, the latter determining the extent of regiocontrol to either one or two macrocyclic chloride functions. The increase of the extent of regioselectivity of gem-substitution at the PNP-macrocycle when passing from arylate to beta-dicarbonyl enolate oxy anions is related to the observed trends in the (31)P NMR chemical shifts and refers to the electronic structure of the oxy substituent.

Journal Article↗
PubMed2002

Synthesis of macrocyclic, potential protease inhibitors using a generic scaffold.

A generic macrocyclic peptide structure 2 was designed as a potential inhibitor of a range of proteinases, by using as a basis for the design the known structures of a series of enzyme-inhibitor complexes. The macrocyclic nature of the target 2 was chosen so as to reduce the entropic advantage in the hydrolytic enzymatic step, and thereby to inhibit the function of the enzyme. The nature of the linking group was identified as a benzoxazole by molecular modeling, so as to preserve the recognized conformation of the peptide chain. The specificity of the potential inhibitor was tuned by variation of the P(1) group (by incorporating phenylalanine, aspartic acid, or lysine), to allow recognition by different enzyme classes. The targets were prepared from the bis-amino acid derivative 5, itself prepared using the Pd-catalyzed coupling of an organozinc reagent with the iodobenzothiazole 7 and subsequent macrocyclization of the open-chain derivatives 22-24 using HATU. None of the macrocylic compounds 25, 28-30, and 32 inhibited their target enzymes. NMR and MS studies on the interaction of macrocycle 29 and chymotrypsin established that compound 29 was in fact a substrate of the enzyme. This result indicated that while the design had been partially successful in identifying a compound that bound, the reduction in entropic advantage due to its macrocyclic nature was not sufficient to allow 29 to act as an inhibitor.

Catalysis↗
PubMed1997

Recognition of Bipyridinium-Based Derivatives by Hydroquinone- and/or Dioxynaphthalene-Based Macrocyclic Polyethers: From Inclusion Complexes to the Self-Assembly of [2]Catenanes.

A range of pi-electron-rich macrocyclic polyethers incorporating dioxybenzene (hydroquinone) and/or dioxynaphthalene units have been synthesized in good yields by simple two-step procedures. These macrocycles are able to bind bipyridinium-based guests as a result of a series of cooperative noncovalent bonding interactions. These molecular recognition events can be extended to the self-assembly of [2]catenanes incorporating the bipyridinium-based cyclophane, cyclobis(paraquat-p-phenylene), and the macrocyclic polyethers incorporating dioxybenzene and -naphthalene units. The efficiencies of these self-assembly processes were found to depend upon the stereoelectronic features of the pi-electron-rich macrocycles-namely, the nature and the substitution pattern of the aromatic units. X-ray crystallographic analysis of some of these [2]catenanes proved unequivocally the relative geometries of the interlocked components. In addition, in the case of those asymmetric [2]catenanes incorporating two different aromatic units within their macrocyclic polyether components, only one of the expected two translational isomers was observed in the solid state. In particular, in all the structures examined, the 1,4-dioxybenzene and 1,5-dioxynaphthalene units are located within the cavity of the tetracationic cyclophane component in preference to other regioisomeric dioxynaphthalene units that reside alongside. Variable-temperature (1)H NMR spectroscopic investigation of the geometries adopted by these [2]catenanes in solution revealed the same selectivity that was observed for one translational isomer over another in the solid state.

Journal Article↗
PubMed2003

Enzymology of acyl chain macrocyclization in natural product biosynthesis.

Polyketides and nonribosomal peptides constitute a large and diverse set of natural products with biological activity in microbial survival and pathogenesis, as well as broad pharmacological utility as antineoplastics, antibiotics or immunosupressants. These molecules are biosynthesized by the ordered condensation of monomer building blocks, acyl-CoAs or amino acids, leading to construction of linear acyl chains. Many of these natural products are constrained to their bioactive conformations by macrocyclization whereby, in one of the terminal steps of biosynthesis, parts of the molecule distant in the constructed linear acyl chain are covalently linked to one another. Typically, macrocyclization is catalyzed by a thioesterase domain at the C-terminal end of the biosynthetic assembly line, although alternative strategies are known. The enzymology of these macrocyclization catalysts, their structure, mechanism, and catalytic versatility, is the subject of this review. The diversity of macrocyclic structures accessed by enzyme catalyzed cyclization of linear acyl chains as well as their inherent substrate tolerance suggests their potential utility in reprogramming natural product biosynthesis pathways or accessing novel macrocyclic structures.

Animals↗
PubMed2003

Metal ion recognition via 'selective detuning'. The interaction of selected transition and post-transition metal ions with a mono-N-benzylated O2N3-donor macrocycle and its xylyl-bridged ring analogue.

The interaction of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) with symmetrical mono-N-benzylated and xylyl-linked macrocyclic ligands derived from the O2N3-macrocycle, 1,12,15-triaza-3,4:9,10-dibenzo-5,8-dioxacycloheptadecane, has been investigated. The log K values for the respective 1 : 1 complexes in 95% methanol (I= 0.1; Et4NClO4, 25 degrees C) of the mono-benzylated derivative have been determined potentiometrically and the results compared with the values obtained previously for the parent (non-benzylated) ring system as well as for related di- and tri-benzylated macrocyclic species. Mono-benzylation results in slightly enhanced stability for the 1 : 1 silver(I) complex while the values for the corresponding complexes of the other six ions are in each case decreased even though the highest stability is still maintained for the 1 : 1 copper(II) complex. The log K results are in accord with a previous proposal that N-benzylation of amine-containing macrocyclic rings of the present type will normally have only a minor (positive or negative) influence on the affinity towards silver(I) while the corresponding binding strengths towards the remaining six metal ions are significantly reduced-behaviour we term 'selective detuning'. Competitive seven-metal transport experiments across a bulk chloroform membrane have been performed using both ligand systems as ionophores. In parallel to the log K results, transport selectivity for copper(II) was exhibited by both systems, with similar transport efficiencies being evident when compared on a 'per macrocyclic cavity' basis.

Journal Article↗
PubMed2004

Tren centered tris-macrocycles as polytopic ligands for Cu(II) and Ni(II).

Two novel symmetric polytopic ligands L(1) and L(2) have been synthesized. They are composed of three 1,4,8,11-tetraazacyclotetradecane macrocycles which are connected to a central tren moiety via an ethylene and a trimethylene bridge, respectively. The complexation potential and the speciation diagrams of L(1) and L(2) towards Cu(2+) and Ni(2+) were determined by spectrophotometric and potentiometric titrations. Insight into the geometry of the Cu(2+) complexes is provided by UV-VIS and EPR spectroscopy. The simplified ligands L(3) and L(4) are utilized as references for an aminoethyl- and a tren-substituted tetraaza macrocycle to help assign the EPR spectra of the polytopic ligands L(1) and L(2). At a metal-to-ligand ratio of 3 : 1, the metal cations are preferentially bound to the tetraaza macrocycles of L(1) and L(2) in a square planar geometry. At high pH values, a nitrogen atom of the tren moiety in L(1) serves as an additional ligand in an axial position leading to a square pyramidal coordination around Cu(2+), whereas in L(2) no such geometry change is observed. At a metal-to-ligand ratio of 4 : 1, the additional metal cation resides in the central tren moiety of L(1) and L(2). However, in contrast to the typical trigonal bipyramidal geometry found in the [Cutren](2+) complex, the fourth Cu(2+) has a square pyramidal coordination caused by the interaction with the Cu(2+) cations in the macrocycles (as evidenced by EPR spectra). Since the sequence of metal complexation is such that the first three metal ions always bind to the three macrocycles of L(1) and L(2) and the fourth to the tren unit, it is possible to prepare heteronuclear complexes such as [Cu(3)NiL](8+) or [Ni(3)CuL](8+), which can be unambiguously identified by their spectral properties.

Journal Article↗
PubMed2005

Iron(III) activation hits a [4 + 4] macrocycle.

The tetranuclear complex [Fe(III)2(L')(OH)(CH3O)]2, 1, has been synthesised from the reaction of either ferrous [in excess as 4:1 or stoichiometric 2:1 iron(II) : H4L] or ferric ions [4:1 iron(III) : H4L] with the large macrocycle, H4L, using aerobic conditions in methanol in the presence of triethylamine. The structure of 1 was determined by single-crystal X-ray diffraction. These reaction conditions lead to the modification of the original macrocycle through the incorporation of a methylene group between two amine groups to give an imidazolidine ring in (L')4-. The controlled addition of formaldehyde into the reaction system results in a significantly improved yield of 1, suggesting that it is involved in the reaction mechanism. The (L')4- macrocycle binds to two, well-separated, iron(III) centres [Fe(1)...Fe(1a) > 8 A]. Each iron(III) centre is further linked via hydroxy and methoxy bridges to equivalent iron(iii) centres contained in a second macrocycle. Overall this gives a structure containing two {Fe(OH)(CH(3)O)Fe} dimers [Fe(1)...Fe(2)ca. 3.2 A] sandwiched by two (L')4- macrocycles. The complex was further characterised by SQUID magnetic measurements and can be interpreted in terms of two isolated antiferromagnetically coupled Fe(III) dimers (J=-23.75 K).

Journal Article↗
PubMed2005

Kinetics and mechanism of the stepwise complex formation of Cu(II) with tren-centered tris-macrocycles.

The stepwise complexation kinetics of Cu2+ with three tetratopic ligands L1, L2 and L3, tren-centred macrocycles with different bridges connecting the 14-membered macrocycles with the tren unit, have been measured by stopped-flow photodiode array techniques at 25 degrees C, I= 0.5 M (KNO3), and pH = 4.96. The reaction between the first Cu2+ and the ligand consists of several steps. In a rapid reaction Cu2+ first binds to the flexible and more reactive tren-unit. In this intermediate a translocation from the tren unit to the macrocyclic ring, which forms the thermodynamic more stable complex, takes place. This species can react further with a second Cu2+ to give a heterotopic dinuclear species with one Cu2+ bound by the tren-unit and the other coordinated by the macrocycle. A further translocation occurs to give the homoditopic species with two Cu2+ in the macrocycles. Finally a slow rearrangement of the dinuclear complex gives the final species. The rates of the translocation are dependent on the length and rigidity of the bridge, whereas the complexation rates with the tren unit are little affected by it. VIS spectra of the species obtained by fitting the kinetic results, EPR-spectra taken during the reaction, and ES mass spectra of the products confirm the proposed mechanism. The addition of a second, third and fourth equivalent of Cu2+ proceeds in an analogous way, but is complicated by the fact that we start and end with a mixture of species. These steps were evaluated in a qualitative way only.

Journal Article↗
PubMed1999

An unusual structural motif of antimicrobial peptides containing end-to-end macrocycle and cystine-knot disulfides.

Four macrocyclic cystine-knot peptides of 29-31 residues, kalata, circulin A and B (CirA and CirB), and cyclopsychotride, have been isolated from coffee plants but have undetermined physiological functions. These macrocycles and 10 of their analogs prepared by chemical synthesis were tested against nine strains of microbes. Kalata and CirA were specific for the Gram-positive Staphylococcus aureus with a minimum inhibition concentration of approximately 0.2 microM. They were relatively ineffective against Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. However, CirB and cyclopsychotride were active against both Gram-positive and Gram-negative bacteria. In particular, CirB showed potent activity against E. coli with a minimum inhibitory concentration of 0.41 microM. All four cyclic peptides were moderately active against two strains of fungi, Candida kefyr and Candida tropicalis, but were inactive against Candida albicans. These macrocycles are cytotoxic and lysed human red blood cell with a lethal dose 50% of 400 microM. Modifying the Arg residue in kalata with a keto aldehyde significantly reduced its activity against S. aureus whereas blocking the arg in CirA produced no significant effect. The two-disulfide variants and their scrambled disulfide isomers exhibited antimicrobial profiles and potency similar to their native peptides. However, in high-salt assays (100 mM NaCl), few of these macrocyclic peptides, natives or analogs, retained antimicrobial activity. These results show that the macrocyclic peptides possess specific and potent antimicrobial activity that is salt-dependent and that their initial interactions with the microbial surfaces may be electrostatic, an effect commonly found in defensin antimicrobial peptides. Furthermore, their end-to-end cyclic structure with a cystine-knot motif represents a molecular structure of antimicrobials and may provide a useful template for the design of novel peptide antibiotics.

Amino Acid Sequence↗