Identification and Characterization of a Periplasmic Aminoacyl-phosphatidylglycerol Hydrolase Responsible for Pseudomonas aeruginosa Lipid Homeostasis

Specific aminoacylation of the phospholipid phosphatidylglycerol (PG) with alanine (or with lysine) was shown to render various organisms less susceptible to antimicrobial agents and environmental stresses. In this study, we make use of the opportunistic pathogen Pseudomonas aeruginosa to decode ORF PA0919-dependent lipid homeostasis. Analysis of the polar lipid content of the deletion mutant ΔPA0919 indicated significantly enlarged levels of alanyl-PG. The resulting phenotype manifested an increased susceptibility to several antimicrobial compounds when compared with the wild type. A pH-dependent PA0919 promoter located within the upstream gene PA0920 was identified. Localization experiments demonstrated that the PA0919 protein is anchored to the periplasmic surface of the inner bacterial membrane. The recombinant overproduction of wild type and several site-directed mutant proteins in the periplasm of Escherichia coli facilitated a detailed in vitro analysis of the enzymatic PA0919 function. A series of artificial substrates (p-nitrophenyl esters of various amino acids/aliphatic acids) indicated enzymatic hydrolysis of the alanine, glycine, or lysine moiety of the respective ester substrates. Our final in vitro activity assay in the presence of radioactively labeled alanyl-PG then revealed hydrolysis of the aminoacyl linkage, resulting in the formation of alanine and PG. Consequently, PA0919 was termed alanyl-PG hydrolase. The elucidated enzymatic activity implies a new regulatory circuit for the appropriate tuning of cellular alanyl-PG concentrations.     

The Human Antimicrobial Peptide LL-37 Binds Directly to CsrS,a Sensor Histidine Kinase of Group A Streptococcus,to Activate Expression of Virulence Factors

Group A Streptococcus (GAS) responds to subinhibitory concentrations of LL-37 by up-regulation of virulence factors through the CsrRS (CovRS) two-component system. The signaling mechanism, however, is unclear. To determine whether LL-37 signaling reflects specific binding to CsrS or rather a nonspecific response to LL-37-mediated membrane damage, we tested LL-37 fragments for CsrRS signaling and for GAS antimicrobial activity. We identified a 10-residue fragment (RI-10) of LL-37 as the minimal peptide that retains the ability to signal increased expression of GAS virulence factors, yet it has no detectable antimicrobial activity against GAS. Substitution of individual key amino acids in RI-10 reduced or abrogated signaling. These data do not support the hypothesis that CsrS detects LL-37-induced damage to the bacterial cell membrane but rather suggest that LL-37 signaling is mediated by a direct interaction with CsrS. To test whether LL-37 binds to CsrS, we used the purified CsrS extracellular domain to pull down LL-37 in vitro, a result that provides further evidence that LL-37 binds to CsrS. The dissociation of CsrS-mediated signaling from membrane damage by LL-37 fragments together with in vitro evidence for a direct LL-37-CsrS binding interaction constitute compelling evidence that signal transduction by LL-37 through CsrS reflects a direct ligand/receptor interaction.     

A Sensory Complex Consisting of an ATP-binding Cassette Transporter and a Two-component Regulatory System Controls Bacitracin Resistance in Bacillus subtilis

Resistance against antimicrobial peptides in many Firmicutes bacteria is mediated by detoxification systems that are composed of a two-component regulatory system (TCS) and an ATP-binding cassette (ABC) transporter. The histidine kinases of these systems depend entirely on the transporter for sensing of antimicrobial peptides, suggesting a novel mode of signal transduction where the transporter constitutes the actual sensor. The aim of this study was to investigate the molecular mechanisms of this unusual signaling pathway in more detail, using the bacitracin resistance system BceRS-BceAB of Bacillus subtilis as an example. To analyze the proposed communication between TCS and the ABC transporter, we characterized their interactions by bacterial two-hybrid analyses and could show that the permease BceB and the histidine kinase BceS interact directly. In vitro pulldown assays confirmed this interaction, which was found to be independent of bacitracin. Because it was unknown whether BceAB-type transporters could detect their substrate peptides directly or instead recognized the peptide-target complex in the cell envelope, we next analyzed substrate binding by the transport permease, BceB. Direct and specific binding of bacitracin by BceB was demonstrated by surface plasmon resonance spectroscopy. Finally, in vitro signal transduction assays indicated that complex formation with the transporter influenced the autophosphorylation activity of the histidine kinase. Taken together, our findings clearly show the existence of a sensory complex composed of TCS and ABC transporters and provide the first functional insights into the mechanisms of stimulus perception, signal transduction, and antimicrobial resistance employed by Bce-like detoxification systems.     

Defining the Structure and Receptor Binding Domain of the Leaderless Bacteriocin LsbB

LsbB is a class II leaderless lactococcal bacteriocin of 30 amino acids. In the present work, the structure and function relationship of LsbB was assessed. Structure determination by NMR spectroscopy showed that LsbB has an N-terminal α-helix, whereas the C-terminal of the molecule remains unstructured. To define the receptor binding domain of LsbB, a competition assay was performed in which a systematic collection of truncated peptides of various lengths covering different parts of LsbB was used to inhibit the antimicrobial activity of LsbB. The results indicate that the outmost eight-amino acid sequence at the C-terminal end is likely to contain the receptor binding domain because only truncated fragments from this region could antagonize the antimicrobial activity of LsbB. Furthermore, alanine substitution revealed that the tryptophan in position 25 (Trp²⁵) is crucial for the blocking activity of the truncated peptides, as well as for the antimicrobial activity of the full-length bacteriocin. LsbB shares significant sequence homology with five other leaderless bacteriocins, especially at their C-terminal halves where all contain a conserved KXXXGXXPWE motif, suggesting that they might recognize the same receptor as LsbB. This notion was supported by the fact that truncated peptides with sequences derived from the C-terminal regions of two LsbB-related bacteriocins inhibited the activity of LsbB, in the same manner as found with the truncated version of LsbB. Taken together, these structure-function studies provide strong evidence that the receptor-binding parts of LsbB and sequence-related bacteriocins are located in their C-terminal halves.     

Structural Basis for the Recognition of Peptide RJPXD33 by Acyltransferases in Lipid A Biosynthesis

UDP-N-acetylglucosamine acyltransferase (LpxA) and UDP-3-O-(acyl)-glucosamine acyltransferase (LpxD) constitute the essential, early acyltransferases of lipid A biosynthesis. Recently, an antimicrobial peptide inhibitor, RJPXD33, was identified with dual affinity for LpxA and LpxD. To gain a fundamental understanding of the molecular basis of inhibitor binding, we determined the crystal structure of LpxA from Escherichia coli in complex with RJPXD33 at 1.9 Å resolutions. Our results suggest that the peptide binds in a unique modality that mimics (R)-β-hydroxyacyl pantetheine binding to LpxA and displays how the peptide binds exclusive of the native substrate, acyl-acyl carrier protein. Acyltransferase binding studies with photo-labile RJPXD33 probes and truncations of RJPXD33 validated the structure and provided fundamental insights for future design of small molecule inhibitors. Overlay of the LpxA-RJPXD33 structure with E. coli LpxD identified a complementary peptide binding pocket within LpxD and serves as a model for further biochemical characterization of RJPXD33 binding to LpxD.     

Synthesis, spectral (UV-Vis, IR, ESI-MS), magnetic and structural characterizations, and the antimicrobial effect of potassium isothiocyanato-(N-salicylidene-amino-acidato)cuprates

A series of potassium isothiocyanato-(N-salicylidene-amino acidato)cuprates with the general formulas of K₂[Cu₂(sal-aa)₂(μ-NCS)₂]·nH₂O, where n = 0 or 4 and (sal-aa) stands for the dianion of N-salicylideneamino acid derived from glycine (I), dl-α-alanine (II), dl-valine (III), dl-phenylalanine (IV), and {K[Cu(sal-β-ala)(μ-NCS)]}_n for β-alanine (V), respectively, was synthesized and fully characterized by elemental analysis, UV-Vis and IR spectroscopy, ESI-MS spectrometry, magnetic measurements, and X-ray structural analysis (II and IV). It has been found that the copper(II) atom adopts a distorted square-pyramidal surrounding in the dimeric complexes I-IV, while the geometry in the polymeric complex V can be described as distorted square-bipyramidal. The analysis of magnetic properties revealed weak antiferromagnetic exchanges in the dinuclear species I-IV and an alternating ferro/antiferromagnetic exchange in the case of 1D-polymeric compound V. Moreover, the complexes were tested for their antibacterial activity against the G+ bacteria Staphylococcus aureus, G− bacteria Escherichia coli, filamentous fungi Microsporum gypseum, and yeast Candida albicans. The best results were achieved with G+ bacteria S. aureus with MIC values in the range of 0.22-0.57 mmol L⁻¹. It may be concluded that both the antimicrobial and antifungal activity decreased within the tested group of cuprates derived from α-amino acids with the increasing lipophility of the complexes, i.e.I → IV.     

Novel families of antimicrobial peptides with multiple functions from skin of Xizang plateau frog, Nanorana parkeri

Xizang plateau frog (Nanorana parkeri) captured in Lhasa, Tibet, China, solely lives in the subtropical plateau, where there is strong ultraviolet radiation and long duration of sunshine. Considering its harsh living environment, the frog's innate defense against microbes and environmental stress was investigated. In current study, three antimicrobial peptides (AMPs) were purified and characterized from the skin secretion of N. parkeri. The coding cDNA sequences were also cloned from the skin cDNA library of N. parkeri. By structural characterization, two peptides were identified belonging to Japonicin-1 family, and named as Japonicin-1Npa (FLLFPLMCKIQGKC) and Japonicin-1Npb (FVLPLVMCKILRKC). The third peptide isolated named Parkerin with a unique sequence of GWANTLKNVAGGLCKITGAA did not show similarity to any known amphibian AMPs. Multi-functions of three AMPs were examined (antioxidant, MCD, hemolytic etc). Their solution structures determined by CD and antimicrobial mechanisms investigated by SEM are very well consistent with their functional characters. Current result suggests that these novel multi-functional AMPs could play an important role in defending N. parkeri against environmental oxidative stress and pathogenic microorganisms, which may partially reveal the ecological adaptation of these plateau-living amphibians.     

Hainanenins: a novel family of antimicrobial peptides with strong activity from Hainan cascade-frog, Amolops hainanensis

Antimicrobial peptides (AMPs) secreted by amphibian skin represent an important innate immune defense strategy. There are more than 340 species in the family of Ranidae worldwidely, and from which nearly 100 families of AMPs comprising between 8 and 48 amino acid (aa) residues have been characterized. In current work, two novel AMPs were purified from the skin secretion of Hainan cascade-frog, Amolops hainanensis, and 31 cDNA sequences encoding 10 novel AMPs belonging to 4 families were cloned from the constructed skin cDNA library of A. hainanensis. Among these 10 AMPs, 5 peptides represent the prototypes of a novel amphibian AMP family. According to the generic name of the species of origin, they were designated as hainanenin-1-5. Each of them consists of 21 aa residues with a C-terminal disulphide loop of 7 residues between Cys(15) and Cys(21). Two of them (hainanenin-1 and 5) were then synthesized and their in vitro activities were screened, including antimicrobial, hemolytic and antioxidant activities. The results showed that hainanenin-1 and 5 possessed strong and broad-spectrum antimicrobial activities against Gram-positive, Gram-negative bacteria and fungi, including a large number of clinically isolated drug-resistant pathogenic microorganisms, and slight antioxidant activity. Undesirably, hainanenin-1 and 5 exhibited strong hemolytic activity on human erythrocytes. The discovery of hainanenins and their great antimicrobial potency provides new templates for anti-infective agent design.     

Design,Synthesis, Structural and Functional Characterization of Novel Melanocortin Agonists Based on the Cyclotide Kalata B1

Obesity is an increasingly important global health problem that lacks current treatment options. The melanocortin receptor 4 (MC4R) is a target for obesity therapies because its activation triggers appetite suppression and increases energy expenditure. Cyclotides have been suggested as scaffolds for the insertion and stabilization of pharmaceutically active peptides. In this study, we explored the development of appetite-reducing peptides by synthesizing MC4R agonists based on the insertion of the His-Phe-Arg-Trp sequence into the cyclotide kalata B1. The ability of the analogues to fold similarly to kalata B1 but display MC4R activity were investigated. Four peptides were synthesized using t-butoxycarbonyl peptide chemistry with a C-terminal thioester to facilitate backbone cyclization. The structures of the peptides were found to be similar to kalata B1, evaluated by Hα NMR chemical shifts. KB1(GHFRWG;23–28) had a K_i of 29 nm at the MC4R and was 107 or 314 times more selective over this receptor than MC1R or MC5R, respectively, and had no detectable binding to MC3R. The peptide had higher affinity for the MC4R than the endogenous agonist, α-melanocyte stimulation hormone, but it was less potent at the MC4R, with an EC₅₀ of 580 nm for activation of the MC4R. In conclusion, we synthesized melanocortin analogues of kalata B1 that preserve the structural scaffold and display receptor binding and functional activity. KB1(GHFRWG;23–28) is potent and selective for the MC4R. This compound validates the use of cyclotides as scaffolds and has the potential to be a new lead for the treatment of obesity.     

Synthesis, characterization, and anion selectivity of copper(II) complexes with a tetradentate Schiff base ligand

A new mononuclear Cu(II) complex, [CuL(ClO₄)₂] (1) has been derived from symmetrical tetradentate di-Schiff base, N,N′-bis-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine (L) and characterized by X-ray crystallography.The copper atom assumes a tetragonally distorted octahedral geometry with two perchlorate oxygens coordinated very weakly in the axial positions.Reactions of 1 with sodium azide, ammonium thiocyanate or sodium nitrite solution yielded compounds [CuL(N₃)]ClO₄ (2), [CuL(SCN)]ClO₄ (3) or [CuL(NO₂)]ClO₄ (4), respectively, all of which have been characterized by X-ray analysis.The geometries of the penta-coordinated copper(II) in complexes 2-4 are intermediate between square pyramid and trigonal bipyramid (tbp) having the Addition parameters (τ) 0.47, 0.45 and 0.58, respectively.In complex 4, the nitrite ion is coordinated as a chelating ligand and essentially both the O atoms of the nitrite occupy one axial site.Complex 1 shows distinct preference for the anion in the order in forming the complexes 2-4 when treated with a mixture. Electrochemical electron transfer study reveals Cu^IICu^I reduction in acetonitrile solution.     

Synthesis, characterisation and crystal structures of a few coordination complexes of nickel(II), cobalt(III) and zinc(II) with N′-[(2-pyridyl)methylene]salicyloylhydrazone Schiff base

Four new coordination complexes, Ni^II(L)₂ (1), [Co^III(L)₂]ClO₄ (2), [Zn(HL)(L)]ClO₄ · H₂O (3) and [Zn(L)₂][Zn(L)(HL)]ClO₄ · 7H₂O (4) (where L is a monoanion of a Schiff base ligand, N′-[(2-pyridyl)methylene]salicyloylhydrazone (HL) with NNO tridentate donor set), have been synthesised and systematically characterised by elemental analysis, spectroscopic studies and room temperature magnetic susceptibility measurements. Single crystal X-ray diffraction analysis reveals that 1 is a neutral complex, while 2-4 are cationic complexes. Among them, 4 is a rare type of cationic complex with two molecules in the asymmetric unit. The ligand chelates the metal centre with two nitrogen atoms from the pyridine and imino moieties and one oxygen atom coming from its enolic counterpart. All the reported complexes show distorted octahedral geometry around the metal centres, with the two metal-N (imino) bonds being significantly shorter than the two metal-N (Py) bonds.     

Synthesis, structure and ferromagnetic properties of a copper(II) coordination polymer with azide and 4-pyridylacrylate

A Cu(II) complex with azide and 4-pyridylacrylic acid (4-Hpya), [Cu₂(4-pya)₂(N₃)₂(DMF)₂] (1) has been synthesized and characterized crystallographically and spectroscopically. This compound consists of binuclear units in which Cu(II) ions are connected through two equatorial-equatorial end-on azido bridges. The Cu(II) dimers are interlinked by 4-pya to generate two-dimensional coordination polymers. Magnetic investigations revealed a relatively strong ferromagnetic interaction through the azide bridges with J = 145 cm⁻¹, and a weak ferromagnetic interdimer interaction through the long but π-conjugated 4-pya ligands. The magneto-structural correlations have been discussed in comparison with other Cu(II) dimers with the same bridging motif.     

Cardiac troponin T, a sarcomeric AKAP, tethers protein kinase A at the myofilaments

Efficient and specific phosphorylation of PKA substrates, elicited in response to β-adrenergic stimulation, require spatially confined pools of PKA anchored in proximity of its substrates. PKA-dependent phosphorylation of cardiac sarcomeric proteins has been the subject of intense investigations. Yet, the identity, composition, and function of PKA complexes at the sarcomeres have remained elusive. Here we report the identification and characterization of a novel sarcomeric AKAP (A-kinase anchoring protein), cardiac troponin T (cTnT). Using yeast two-hybrid technology in screening two adult human heart cDNA libraries, we identified the regulatory subunit of PKA as interacting with human cTnT bait. Immunoprecipitation studies show that cTnT is a dual specificity AKAP, interacting with both PKA-regulatory subunits type I and II. The disruptor peptide Ht31, but not Ht31P (control), abolished cTnT/PKA-R association. Truncations and point mutations identified an amphipathic helix domain in cTnT as the PKA binding site. This was confirmed by a peptide SPOT assay in the presence of Ht31 or Ht31P (control). Gelsolin-dependent removal of thin filament proteins also reduced myofilament-bound PKA-type II. Using a cTn exchange procedure that substitutes the endogenous cTn complex with a recombinant cTn complex we show that PKA-type II is troponin-bound in the myofilament lattice. Displacement of PKA-cTnT complexes correlates with a significant decrease in myofibrillar PKA activity. Taken together, our data propose a novel role for cTnT as a dual-specificity sarcomeric AKAP.     

Co(II), Zn(II) and Cu(II) compounds with 1,4; 1,3 or 1,2-bis-(benzimidazole-1-yl-methylene)-benzene as a flexible spacer ligand: Synthesis, characterization and X-ray structures of some polynuclear species

New dinuclear and polynuclear Co(II), Zn(II) and Cu(II) compounds with the ligands 1,4-bis-(benzimidazole-1-yl-methylene)-benzene (bbpx), 1,3-bis-(benzimidazole-1-yl-methylene)-benzene (bbmx) and 1,2-bis-(benzimidazole-1-yl-methylene)-benzene (bbox) are reported. With Co(II) and Zn(II) and the ligands bbpx, bbmx and bbox six new dinuclear and polynuclear compounds are described, i.e. [CoCl₂(bbpx)]_n(DMF)_2n (1), [CoCl₂(bbmx)]₂(DMF)₂ (2), [ZnCl₂(bbmx)]₂(DMF)₂ (3), [CoCl₂(bbmx)]_n(DMF)_n/2(CH₃OH)_n/4 (4), [CoCl₂(bbox)]_n(DMF)_3n/2 (5) and [ZnCl₂(bbox)]_n(DMF)_3n/2 (6). Also one polynuclear Cu(II) compound is reported, i.e. [Cu(bbpx)₂(C₂N₃)_1.5(OH)_0.5]₂(DMF)_4.15(CH₃OH)_1.75(H₂O)_3.5. The X-ray structures, physical and electronic properties are discussed.     

Synthesis, solid-state characterization and antimicrobial activities of three different polymorphs of a copper(II) complex with 4-isopropyltropolone (hinokitiol)

There exist at least three different polymorphs in the copper(II) complex [Cu(hino)₂] with a hinokitiol ligand (Hhino; 4-isopropyltropolone¹). In addition to deep-green plate crystals 1a and deep-green rod crystals 1b, whose crystal structures have been recently reported, novel green needle crystals 1c of [Cu(hino)₂] were found, the crystal structure of which was here determined by single-crystal X-ray analysis. Since only one crystal structure has been reported for the copper(II) complex [Cu(trop)₂] with a tropolone ligand (Htrop), the polymorphism found in the crystals of [Cu(hino)₂] would be due to the presence of the isopropyl group on the tropolone ring. The synthetic conditions giving the three polymorphs in good yields were found and the crystals were characterized with elemental analysis, FT-IR, TG/DTA and X-ray powder diffraction (XPD) measurements, as well as solution molecular weight measurements for 1a. The solid-state magnetic behaviors or the temperature-dependent magnetic susceptibilities were measured with Superconductivity Quantum Interference Devices (SQUID): 1a showed a weak ferromagnetic interaction, 1b showed a paramagnetic nature with S=1/2, while 1c showed a weak antiferromagnetic interaction. The antimicrobial activities for selected bacteria, yeasts and molds were also measured in the water-suspension system: 1a and 1b showed no activity, while 1c showed modest activities, and these activities were compared with those of the neutral Hino and the anionic hino⁻ ligands.     

Allosteric modulation of the calcium-sensing receptor by gamma-glutamyl peptides: inhibition of PTH secretion, suppression of intracellular cAMP levels, and a common mechanism of action with L-amino acids

γ-Glutamyl peptides were identified previously as novel positive allosteric modulators of Ca(2+)(o)-dependent intracellular Ca(2+) mobilization in HEK-293 cells that bind in the calcium-sensing receptor VFT domain. In the current study, we investigated whether γ-glutamyl-tripeptides including γ-Glu-Cys-Gly (glutathione) and its analogs S-methylglutathione and S-propylglutathione, or dipeptides including γ-Glu-Ala and γ-Glu-Cys are positive allosteric modulators of Ca(2+)(o)-dependent Ca(2+)(i) mobilization and PTH secretion from normal human parathyroid cells as well as Ca(2+)(o)-dependent suppression of intracellular cAMP levels in calcium-sensing receptor (CaR)-expressing HEK-293 cells. In addition, we compared the effects of the potent γ-glutamyl peptide S-methylglutathione, and the amino acid L-Phe on HEK-293 cells that stably expressed either the wild-type CaR or the double mutant T145A/S170T, which exhibits selectively impaired responses to L-amino acids. We find that γ-glutamyl peptides are potent positive allosteric modulators of the CaR that promote Ca(2+)(o)-dependent Ca(2+)(i) mobilization, suppress intracellular cAMP levels and inhibit PTH secretion from normal human parathyroid cells. Furthermore, we find that the double mutant T145A/S170T exhibits markedly impaired Ca(2+)(i) mobilization and cAMP suppression responses to S-methylglutathione as well as L-Phe indicating that γ-glutamyl peptides and L-amino acids activate the CaR via a common mechanism.     

Synthesis, structures, and magnetic properties of dicopper(II) and dinickel(II) complexes with a new bis(macrocycle), 7,7-(2-hydoxypropane-1,3-diyl)bis{3,7,11,17-tetraazabicyclo[11.3.1]- heptadeca-1(17),13,15-triene}

A new bis(macrocycle) ligand, 7,7^′-(2-hydoxypropane-1,3-diyl)-bis{3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} (HL), and its dicopper(II) ([Cu₂(HL)Cl₂](NO₃)₂ · 4H₂O (4a), [Cu₂(HL)I₂]I₂ · H₂O (4b)) and dinickel(II) ([Ni₂(L)(OH₂)](ClO₄)₃ (5a), [Ni₂(L)(OH₂)]I₃ · 2H₂O (5b), [Ni₂(L)N₃](N₃)₂ · 7H₂O (5c)) complexes have been synthesized. The alkoxide bridged face-to-face structure of the dinickel(II) complex 5c has been revealed by X-ray crystallography, as well as the “half-opened clamshell” form of the bis(macrocyclic) dicopper(II) complex 4b. Variable temperature magnetic susceptibility studies have indicated that there exists intramolecular antiferromagnetic coupling (J=−33.8 cm⁻¹ (5a), −32.5 cm⁻¹ (5b), and −29.7 cm⁻¹ (5c)) between the two nickel(II) ions in the nickel(II) complexes.     

Synthesis, X-ray structures and properties of the first tris-dioximate cobalt clathrochelates with nonequivalent chelate ribbed fragments

One-pot macrocyclization and reduction of the kinetically inert nonmacrocyclic cobalt(III) bis-α-benzyldioximate and dimethylglyoximate with BF₃·O(C₂H₅)₂ and metallic silver followed by cycloaddition of the corresponding α-dioxime to the generated insitu macrocyclic cobalt(II) bis-dioximates afforded the cobalt(II) clathrochelates with nonequivalent α-dioximate fragments. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV-Vis, ¹H, ¹³C{¹H} and ¹⁹F NMR spectroscopies, magnetochemistry, EPR, and X-ray crystallography. The coordination polyhedra of an encapsulated in a three-dimensional macrobicyclic ligand cavity cobalt(II) ion have a distorted trigonal prismatic geometry. The displacement of a caged metal ion from the centers of these polyhedra is caused mainly by the Jahn-Teller effect. Magnetochemical data for cobalt(II) clathrochelates obtained characterize them as the low-spin complexes in the temperature range of 2-400 K. The cyclic voltammograms of the synthesized clathrochelates contain the one-electron oxidation and reduction waves. The increase of the electron-donating properties of the ribbed substituents causes the negative shift of these waves. A comparative analysis of the reduction and oxidation potentials allowed to assign these processes to the cobalt-centered reduction and oxidation. The “electrochemical gap” values for clathrochelates studied are very small and characteristic of the complexes with the redox processes localized on the molecular orbitals which are close in energy.     

Synthesis, structure, spectroscopic and magnetic characterization of a novel spin-crossover iron(II) complex with 1-cyclopropyltetrazole ligands

For the first time a 1-cyclopropyl substituted tetrazole (C₃tz) has been used as a potential ligand for iron(II) spin-transition complexes. The complexation of 1-cyclopropyltetrazol with iron(II) tetrafluoroborate yielded a fine powdered product of [Fe(C₃tz)₆](BF₄)₂ being poorly soluble in most common solvents. Single crystals of complex were grown in situ from a solution of ligand and iron(II) hexafluorophosphate, which yielded a hexagonal prismatic crystalline product of [Fe(C₃tz)₆](PF₆)₂. A comparison of XRPD data of the homologues [Fe(C₃tz)₆](BF₄)₂ and [Fe(C₃tz)₆](PF₆)₂ proves them to be homeotypic. The thermally induced spin-crossover phenomenon of [Fe(C₃tz)₆](BF₄)₂ complex shows very abrupt spin transitions, with a spin-crossover temperature T_1/2 ≈ 180 K which is found to be ≈50 K above the T_1/2 of all known iron(II) complexes with n-alkyltetrazoles as ligands. The T_1/2 was determined by temperature-dependent ⁵⁷Fe-Mössbauer, far FT-IR and UV-Vis-NIR spectroscopy as well as temperature dependent magnetic susceptibility measurements (SQUID).