Complexes produced by associated forms of porphyrin bound with hydrophobic-hydrophilic copolymer and transition metal ions

Properties of protonated dimeric forms of meso-tetraphenylporphine (TPP) and meso-tetra(p-aminophenyl)porphine (TAPP) bound with copolymer and also complexes produced by associated TAPP bound with copolymer, Mn2+, and Fe3+ are investigated by absorption, luminescence, and Raman spectroscopy. According to absorption spectra of protonated dimers of TPP, three dimeric forms of the porphyrin are observed in the ground state. However, selective excitation of these forms according to the fluorescence spectra reveals only two dimeric forms in the excited state. In contrast, similar selective excitation of TAPP bound with copolymer in aqueous-dioxane solution results in weak changes in the fluorescence spectra, nevertheless, there is strong interaction between porphyrin and macromolecular carboxyl groups in the ground state. In the case of the formation of the complexes between associated TAPP bound with copolymer, Mn2+ and Fe3+, a new band in the near IR region with a maximum at 840 nm is built up in the fluorescence spectrum. However, this near IR emission is completely quenched when new strong vibrational bands at approximately 1800 and 1900 cm-1 are revealed in the resonance Raman spectra of the complexes. The observed effects are explained in terms of direct participation of water molecules involved in the water-porphyrin dimeric complex in the processes of transformation of excitation energy. The involvement of water in this dimeric complex can lead to redistribution of flows of the energy degradation when transition metal ions play a role of the agent which enhances the trapping properties of the porphyrin-metal-ions complexes.     

Transition metal complexes IX. Nickel complexes with a chiral azaphosphole ligand

The reaction of (−)-(R)-myrtenal and (+)-(R)-phenylethylamine gave a Schiff base 1 which was reacted with MePBr₂ in the presence of a base to give under dehydrohalogenation of an intermediate McCormack product a salt 2. Treatment of 2 with sodium led to the formation of the azaphosphole 4. η³-C₃H₅NiCl and 4 gave a 1:1 adduct 5 and nickel(0) gave a 1:4 complex 6. Compounds 4–6 were characterized by NMR spectroscopy as well as by single crystal X-ray structure determination.     

Transition metal ion complexes of 2-thio-6-picoline N-oxide

Complexes of the anion 2-thio-6-picoline N-oxide (6MOS) have been isolated with the following stoichiometry: M(6MOS)₃ (M = Cr, Fe, and Co) and M(6MOS)₂ (M = Co, Ni, Cu and Zn). The spectral properties of these complexes are compared with those of 2-thiopyridine N-oxide in order to determine the stereochemical effect of the 6-methyl substituent. The nature of the Ni(I) species formed on exposure to high energy radiation, and the nature of the heterocyclic amine adducts to both the Ni(II) and Cu(II) complexes are also reported.     

Transition metal complexes with pyrazole derivatives as ligands

Degradation reactions of scorpionates were observed in the presence of transition metal salts MX₂ to give complexes of transition metal and pyrazole derivatives. Otherwise, pyrazolato complexes of transition metals and weakly coordinating anions such as nitrates have been synthesized from transition metal nitrates and 3-phenyl- and 4-phenyldiazo-pyrazole. A number of complexes with pyrazole derivatives as ligands, [Zn(3-tBupzH)₂Cl₂], [Fe₂(3-Phpz)₆Cl₄], [Cu(pzH)₄Br₂], [Ni(py)₂(pzH)₂Cl₂], [Li(THF)₄][Ti₂(μ-pz)₃Cl₄(NMe₂)₂], [Zn₂(μ-3-Phpz)₂(3-PhpzH)₂][(NO₃)₂], [M(3-PhpzH)₄(NO₃)₂] (M = Co, Ni, Cu, Zn, Cd), [Zn(3-PhpzH)₂(NO₃)₂], [Zn(4-PhNNpzH)₂(NO₃)₂](H₂O), and [Cd(4-PhNNpzH)₂(NO₃)₂(H₂O)₂], have been crystallized and characterized by single-crystal X-ray diffraction.     

Circular dichroism spectroscopy of a cationic porphyrin bound to DNA

Recently, the porphyrin photosensitizer meso-tetra(4-N-methyl-pyridyl)-porphine was identified as a DNA-reactive agent demonstrating both electrostatic and intercalative binding. A series of porphyrin derivatives were synthesized and studied to see if similar compounds manifested identical behavior. One derivative, meso-tetra(p-N-trimethylanilinium) porphine did not exhibit intercalation behavior but did show avid binding and novel circular dichroic features when bound to B-form DNA. At an ionic strength of 1.02, the binding constant was found to be on the order of 10⁴ and higher at lower ionic strength. The large binding constants and induced optical activity suggest that at large porphyrin/DNA ratios the final porphyrin · DNA complex may take the form of a suprahelical helix.     

Transition metal complexes of diclofenac with potentially interesting anti-inflammatory activity

An overview is given of the results of metal ion-diclofenac interactions. Several complexes have been synthesized at the University of Ioannina. Binuclear complexes, [Cu(L)2(H2O)]2 x 2H2O and [CuL2(S)]2 where S is H2O, EtOH, DMSO, (CH3)2CO and DMF, and mononuclear complexes, [MnL2(H2O)], [FeL2(H2O)2], [CoL2(H2O)2] x 0.5H2O, [CoL2(H2O)], [NiL2(H2O)2] x 2H2O, [NiL2] and [PdL2] x 2H2O, have been characterized by spectroscopy, X-ray crystallography and electrochemical studies. The catalytic activity of these complexes was correlated to the reduction potential. Some of the complexes of diclofenac exhibit very promising anti-inflammatory activity and act as antioxidant compounds, a property that is absent from diclofenac.     

The metal complexes of N-confused porphyrin as heme model compounds

Recently, metal complexes of the isomers and analogs of porphyrin have become important model compounds for heme enzymes and proteins. While the chemistry of metalloporphyrins as heme models still attracts attention, the isomers and analogs of porphyrins provide insight into the biological choice of porphine as the macrocycle of choice and also help model reactive intermediates, such as high valent oxidation states. In this mini-review, we discuss the heme-relevant chemistry of N-confused porphyrin, an isomer of porphyrin with an inverted pyrrole ring, and focus on the chemistry of manganese, iron, and cobalt. The metallation chemistry of this macrocycle is more diverse than normal porphyrin, and involves tautomerization, C-H bond activation, the Lewis basicity of the external nitrogen, and issues with nucleophilic sensitivity. Despite the challenges posed by N-confused porphyrin, significant progress has been made toward generating heme-model complexes with this macrocycle.     

Transition metal half-sandwich complexes as redox mediators to glucose oxidase

Chromium and manganese half-sandwich complexes are evaluated as mediators to glucose oxidase (GOx) since they are of similar size to ferrocene derivatives (sandwich complexes) and contain a single pi-ligand for interaction with the enzyme co-factor. A series of seven amino derivatives of [(eta-C(6)H(6))Cr(CO)(3)] were investigated of which only [[eta-C(6)Me(4)(NH(2))(2)]Cr(CO)(3)] (7), with the lowest oxidation potential of +40 mV (versus SCE), was found to display reversible electrochemistry. Small catalytic currents were recorded in the presence of GOx and glucose when complex (7) was incorporated in a screen-printed carbon electrode. Manganese cyclopentadienyl (Cp) half-sandwich complexes were found to be more effective GOx mediators and comparable in efficacy to ferrocene derivatives. A mediator rate constant k(M) of 2.1 x 10(5)M(-1)s(-1) was determined for the water-soluble complex [(eta-MeC(5)H(4))Mn(NO)(CN)(2)]Na (11) compared to a range of 3 x 10(4) to 8 x 10(6)M(-1)s(-1) previously determined for ferrocenes under the same experimental conditions. beta-Cyclodextrin (beta-cd) was found to be helpful in solubilising hydrophobic complexes such as [(eta-MeC(5)H(4))Mn(NO)(S(2)CNMe(2))] (15) and the neutral oxidised form of [MeCpMn(NO)[(SCCN)(2)]]NEt(4) (14), either directly as an inclusion adduct or in situ during cyclic voltammetry. Screen-printed amperometric electrodes, containing a mediator and GOx immobilised in an organic conducting carbon layer, were useful in assessing the mediation ability of complex (15) where aqueous insolubility precluded any kinetic studies with GOx in solution. This work was briefly extended to other oxidoreductase enzymes apart from GOx. Thus, rotating ring-disk voltammetry demonstrated that the beta-cd complex of compound (15) is also a useful mediator to Horseradish peroxidase (HRP) since it displays an identical catalytic current to the ferrocene ethanolamine derivative (1) used in the MediSense ExacTech and Precision QID blood glucose biosensor electrodes.     

Transition metal complexes of L-cysteine containing di- and tripeptides

Nickel(II), cobalt(II), zinc(II), and cadmium(II) complexes of Ala-Cys, Phe-Cys, and Ala-Ala-Cys were studied by potentiometric and spectroscopic methods. Ni(II) induces deprotonation and coordination of the amide nitrogens, and the stable monomeric or oligomeric complexes are formed, depending on the metal to ligand molar ratios. Formation of the stable bis-complexes with [S,O] coordination mode is characteristic for cobalt(II), zinc(II), and cadmium(II) ions.     

AFM analysis of DNA-protamine complexes bound to mica.

A novel method for reconstituting sperm chromatin was used to investigate how protamine 1 condenses DNA. Complexes formed in vitro using linearized plasmid DNA were imaged and measured by atomic force microscopy (AFM). The structures formed were found to be highly dependent on the sample preparation method used for reconstitution. Interstrand, side-by-side fasiculation of DNA and toroidal-like structures only 1-2 DNA diameters thick were observed for complexes formed in solution following direct mixing of the DNA and protamine. Large chromatin aggregates were also observed on the mica. However, if the DNA was first allowed to attach to the mica prior to addition of the protamine, well-defined toroidal complexes were formed without any observed DNA fasiculation or aggregate formation. The diameter of the toroids measured 30.6-50.2 nm (mean 39.4 nm). The dimensions of these structures indicate that the condensed DNA is stacked vertically by four to five turns, with each coil containing as little as 360-370 bp of 'B'-form DNA. This approach for preparing and imaging DNA-protamine complexes permits the analysis of intermediate structures 'trapped' on the mica as partially formed toruses of nucleoprotamine.     

Stability of metal ion complexes formed with methyl phosphate and hydrogen phosphate

 The acidity constants of methyl phosphoric acid, CH₃OPO(OH)₂, and orthophosphoric acid, HOPO(OH)₂, and the stability constants of the 1 : 1 complexes formed between Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, or Cd²⁺ and methyl phosphate, CH₃OPO₃ ^2–, or hydrogen phosphate, HOPO₃ ^2–, were determined by potentiometric pH titration in aqueous solution (25  °C;I = 0.1 M, NaNO₃). On the basis of previously established log K versus pK _a straight-line plots for the complexes of simple phosphate monoesters and phosphonate derivatives, R-PO₃ ^2–, where R is a noncoordinating residue, it is shown that the stability of the M(CH₃OPO₃) complexes is solely determined (as one might expect) by the basicity of the –PO₃ ^2– residue. It is emphasized that the mentioned reference lines may also be used to reveal increased complex stabilities, for example, for certain complexes formed with 8-quinolyl phosphate the occurrence of 7-membered chelates can be proven in this way; the same procedure is also applicable to complexes of nucleotides, etc. The M(HOPO₃) complexes are slightly more stable (on average by 0.08 log unit) than it is expected from the basicity of HPO₄ ^2–; this observation is attributed to a more effective solvation, including hydrogen bonding, than is possible with CH₃OPO₃ ^2– species. Received: 9 November 1995 / Accepted: 5 February 1996     

Biphasic kinetics of metal ion reactivation of trypsin-thiol complexes

This report describes biphasic kinetic data obtained when trypsin was inhibited by a thiol-containing inhibitor present in Ehrlich ascites tumour cells and then subjected to addition of Hg2+, Cu2+ or Ag+. This resulted in an initial re-activation of the trypsin, followed by inhibition of the enzyme with the addition of higher concentrations of these ions. The significance of these observations is 2-fold: (i) help to elucidate the mechanism of metal ion activation of latent enzymes, and (ii) also indicate that, in certain circumstances, the concentration of added metal ion determines whether the metal acts as an activator or an inhibitor of enzyme activity.     

Nucleolin associates with a subset of the human Ro ribonucleoprotein complexes

YL37a is an essential yeast ribosomal protein that has a C(2)-C(2) zinc finger motif. Replacement of the cysteine residues had yielded variants that lacked the capacity to bind zinc but still supported cell growth. In a continuation of an examination of the relation of the structure of YL37a to its function, the contribution of amino acid residues in the intervening sequence between the internal cysteine residues of the motif was evaluated. Substitutions of alanine for the lysine residues at positions 44, 45, or 48, or for arginine 49 slowed cell growth. The most severe effect was caused by a double-mutation, K48A-R49A. A mutation of tryptophan 55 to alanine was lethal. Mutations to alanine of six conserved residues (K6, K7, K13, Y14, R17, and Y18) in the amino-terminal region decreased cell growth; the Y14 mutation was lethal. An in vitro assay for binding of YL37a to individual 26 S rRNA domains was developed. Binding of the recombinant fusion protein MBP-YL37a was to domains II and III; the K(d) for binding to domain II was 79 nM; for domain III it was 198 nM. There was a close correspondence between the effect of mutations in YL37a on cell growth and on binding to 26 S rRNA. In the atomic structure of the 50 S subunit of Haloarcula marismortui, the archaebacteria homolog of yeast YL37a, L37ae, coordinates a zinc atom and the finger motif is folded and interacts mainly with domain III of 23 S rRNA; whereas the amino-terminal region of L37ae interacts primarily with domain II. The biochemical and genetic experiments complement the three-dimensional structure and define for the first time the functional importance of a subset of the residues in close proximity to nucleotides.     

Phosphorus nuclear magnetic resonance studies of phosphoglucomutase and its metal ion complexes.

The ³¹P nuclear magnetic resonance of the covalently bound phosphate group at the active site of phosphoglucomutase has been examined by means of Fourier transform nuclear magnetic resonance spectroscopy. At a pD of 7.9, the chemical shift of the ³¹P nucleus is 3.8 ± 0.1 ppm downfield from 85% H₃PO₄; this shift is close to that of phosphoserine (dianionic form). Proton decoupling experiments suggest that the phosphorus of the enzymic phosphate group is coupled to protons with chemical shifts similar to those of phosphoserine. In D₂O, with proton decoupling, the ratio of the longitudinal and transverse diamagnetic relaxation times in solutions of 1.6 mm phosphoenzyme yields an approximate correlation time of 10^?7s for the ³¹P nucleus of the enzyme. This is within the range of values expected for tumbling of the entire protein molecule and suggests that the covalently attached phosphate group is immobilized or “frozen” at the active site of the enzyme by means of noncovalent interactions with adjacent groups. Consistent with this, the pK_a of the enzymic phosphate is significantly lower than that of phosphoserine. Binding of the diamagnetic activator, Mg²⁺, causes little or no change in the chemical shift of the resonance of the enzymic phosphorus from pD = 5.3 to 7.6, a downfield shift (?0.5 ± 0.1 ppm) at pD = 8.6, but an upfield shift (0.8 ±0.1 ppm) for that of phosphoserine, suggesting that bound Mg²⁺ is not coordinated to the enzymic phosphate. Independent evidence against direct coordination is provided by the paramagnetic effects of Ni²⁺ bound at the active site on the relaxation rates of the enzymic phosphorus. By assessing the paramagnetic effect of bound Ni²⁺ on both the longitudinal and transverse relaxation rates of the observed resonance, and by using correlation times determined for water proton relaxation induced by the Ni²⁺ complex, a range of Ni²⁺ to phosphorus distances of 4 to 6 Å is calculated. These distances suggest a second sphere interaction between the enzyme-bound metal and the enzymic phosphate group. Bound Ni²⁺ also markedly decreases the integrated intensity of the ³¹P resonance. Although the reason for this intensity decrease is incompletely explained, the present data establish the close proximity of the bound metal ion and the active site phosphoserine on phosphoglucomutase.     

Transition metal complexes of terminally protected peptides containing histidyl residues

Histidine-containing peptide fragments of prion protein are efficient ligands to bind various transition metal ions and they have high selectivity in metal binding. The metal ion affinity follows the order: Pd(II)>Cu(II)>Ni(II)Zn(II)>Cd(II) approximately Co(II)>Mn(II). The high selectivity of metal binding is connected to the involvement of both imidazole and amide nitrogen atoms in metal binding for Pd(II), Cu(II) and Ni(II), while only the monodentate N(im)-coordination is possible with the other metal ions. The stoichiometry and binding mode of palladium(II) complexes show great variety depending on the metal ion to ligand ratio, pH and especially the presence of coordinating donor atoms in the side chains of peptide fragments. It is also clear from our data that the peptide fragments containing histidine outside the octarepeat (His96, His111 and His187) are more efficient ligands than the monomer peptide fragments of the octarepeat domain.