Metal Ion-Biomolecule Interactions. VIII: Methylmercury(II) Complexes of 8-Thioguanosine. Synthesis and Spectroscopic Investigations

The reaction of 8-thioguanosine (8-thioGuo^H₂ with methylmercury(II) has been shown to give rise to 1:1 (neutral and cationic), 1:2 (neutral and cationic), and 1:3 (cationic) complexes of the type [CH₃Hg(8-thioGuoH)], [(CH₃Hg(8-thioGuoH₂)]NO₃, [(CH₃Hg)₂(8-thioGuo)], [(CH₃Hg)₂(8-thioGuoH)]NO₃ and [(CH₃Hg)₃(8-thioGuo)]NO₃, depending upon the reactant stoichiometry and pH. ¹H NMR, ¹³C NMR, and IR, as well as analytical data were used to characterize the complexes. Coupling of methylmercury(II)-protons to mercury-199 has been observed in all compounds. The magnitude of the coupling, ²J(¹H-¹⁹⁹Hg), is strongly dependent on the nature of the ligand bonded to the methylmercury(II) group and correlates with the ¹³C chemical shifts of coordinated CH₃Hg(II) at the different binding sites.     

Metal ion-biomolecule interactions. XII. 1H and 13C NMR evidence for the preferred reaction of thymidine over guanosine in exchange and competition reactions with Mercury(II) and Methylmercury(II)

Mercury(II) bridge complexes of the type [Nuc-Hg-Nuc] (Nuc = thymidine or guanosine), and methylmercury(II) complexes of thymidine and guanosine of the type [CH₃Hg(Nuc)], have been prepared under appropriate conditions of pH and reactant's stochiometry in acqueous soluton. The various complexes have been characterized by ¹H and ¹³C NMR and used as probes, in competition and exchange studies, to establish the relative affinities of Hg(II) and CH₃Hg(II) towards the nucleosides guanosine and thymidine. These studies have confirmed that Hg(II) and CH₃Hg(II) bind to N₃ of thymidine in preference to N₁ of guanosine. The studies further show that reactions of mercury(II) with the nucleosides are thermodynamically controlled; the preperential binding reflects the relative stabilities of the respective complexes.     

Characterisation of metal binding sites for 8-azaadenine. Formation and X-ray structural analysis of methylmercury(II) complexes

The compounds [(CH₃Hg)AAdH]NO₃ (1) and [(CH₃Hg)AAd]·4H₂O (2) have been isolated from aqueous 1:1 solutions of CH₃HgOH and 8-azaadenine (AAdH) at respective pH values of 2 and 5. Their structures have been established by X-ray structural analysis. N9 is the metal binding site in both complexes. Alteration of the metal to ligand ratio to 2:1 at a pH of 5 allows the preparation of [(CH₃Hg)₂AAd]NO₃·H₂O (3) in which the base is coordinated at both N3 and N9. The compound [(CH₃Hg)₃AAdH₋₁]NO₃ (4), in which N1, N6 and N9 function as binding sites for the CH₃Hg⁺ cation, is formed in a 3:1 solution at a pH of 6.5. X-ray structural analyses have been performed on 3 and 4. N8 takes part in weak intermolecular secondary bonds to symmetry related Hg9 atoms in all four complexes. The relevance of the structures to an understanding of the basicities of the nitrogen atoms in 8-azaadenine and their alteration upon metal coordination of N9 and N6 is discussed.     

The effect of methylmercury (II) binding on the conformation of poly(L-glutamic acid) and poly(L-lysine: a Raman spectroscopic study

The binding of the methylmercury cation CH₃Hg⁺ by poly(L -glutamic acid) (PGA) and by poly(L -lysine) (PLL) has been investigated by Raman spectroscopy. Coordination on the side-chain COO^? and NH groups of these polypeptides gave characteristic ligand–Hg stretching modes at ca. 505 and 450 cm^?1, respectively. Precipitation generally occurred upon formation of the complexes and changes of conformation were common. The solid complex obtained from PGA at pH 4.6 was found to have a mostly disordered conformation, which differed from the respective α-helical and β-sheet structures of the dissolved and precipitated uncomplexed polypeptide in the same conditions. An α-helical structure was generally adopted by the complex formed with PLL, even in pH and temperature conditions where the free polypeptide normally exists in another conformation. The addition of a stronger complexing agent, glutathione, to the PLL/CH₃Hg⁺ complex caused a migration of the bound cations and a restoration of the polypeptide to its original state.     

Dissolved and suspended mercury. species in the Wabigoon River (Ontario,Canada): seasonal and regional variations

Seasonal and regional variations in the speciation, sediment-water partitioning, and dynamics of mercury (Hg) were studied at selected sites along the Hg-polluted Wabigoon River, and at unpolluted headwater and tributary sites, during April–September, 1979. ‘Dissolved’ and ‘particulate’ forms of Hg in the water were separated by continuous-flow centrifugation in the field. The Hg and other pollutants such as wood chips and salt had been discharged from a chlor-alkali plant and paper mill at Dryden, Ontario. Concentrations and loadings of particulate methyl mercury (CH₃Hg⁺) and total particulate Hg (and loadings of total ‘dissolved’ Hg) were greatest during the spring flood (April-May) owing to accelerated resuspension and transport of sediments. Concentrations of ‘dissolved’ CH₃Hg⁺, however, were highest in the summer (July–September), probably reflecting stimulation of microbial methylating activity by elevated temperatures, together with factors such as reduced levels of metal-scavenging particulates and minimal dilution by runoff. Total dissolved Hg concentrations were relatively high in September at polluted sites only, possibly because of desorption from sediments due to elevated concentrations of Cl⁻ ions. Loadings of dissolved CH₃Hg⁺ tended to be high in the summer but were generally depressed (suggesting sorption by suspended particles) during the major spring-flood episode in May. During July–August dissolved CH₃Hg⁺ was a function of total dissolved Hg, suggesting rapid biomethylation of desorbed inorganic Hg; but in general dissolved and suspended CH₃Hg⁺ levels depended on environmental variables and were unrelated to total Hg concentrations. In the summer only, total dissolved Hg was a function of dissolved Cl⁻. Hg species in particulates were associated with sulfides, hydrated Fe and Mn oxides, organic matter (notably high molecular weight humic and humic-Fe components), and selenium (Se); but CH₃Hg⁺ and total Hg differed in their specific preferences for binding agents, implying that binding sites discriminate between CH₃Hg⁺ and Hg²⁺ ions. CH₃Hg⁺ was associated with sulfide and (in the spring only) with Fe oxides, whereas total Hg was associated with organic matter and Se and with DTPA- and NaOH-extractable Fe in the spring but with Mn oxide and NaOH-extractable organics in the summer. Sulfides were most abundant in May, indicating that they were eroded from bottom sediments, but Fe and Mn oxides were most abundant in the summer, probably owing to activities of filamentous iron bacteria and other micro-organisms. Particulate Hg was 98–100% nonextractable by mild solvents such as Ca acetate, CaCl₂, dilute acetic acid, and (at polluted sites only) DTPA solutions, suggesting that the particulate Hg mobilized in the spring may not be readily available to organisms; association with Se and high molecular weight humic matter also supports this hypothesis. Hg probably becomes more bio-available in the summer, as suggested by the upsurge in dissolved CH₃Hg⁺ and total dissolved Hg levels, and by increases in the solubility of particulate Hg in acetic acid, DTPA, H₂O₂, and NaOH solutions, as well as an increase in the relative importance of lower molecular weight fractions of NaOH-extractable Hg (in September). Regional variations in Hg speciation and partitioning reflected a gradient in sediment composition from wood chips near Dryden to silt-clay mud further downstream. Hg in silt-clay mud relatively far (> 35 km) downstream from the source of pollution or in unpolluted areas appeared to be more readily solubilized by Cl⁻ ions or chelators such as DTPA, more readily methylated (as indicated by downstream increases in dissolved CH₃Hg⁺ levels and CH₃Hg⁺/total Hg ratios), and was to a greater degree organically bound (H₂O₂-extractable), and thus was probably more bio-available, than Hg in wood-chip deposits. Possible explanations include weaker binding of Hg by the mud, the more finely divided state of the mud, and improved microbial growth at lower concentrations of toxic pollutants. Owing to enrichment in sulfides and Fe oxides, resuspended wood-chip sediments were especially efficient scavengers of CH₃Hg⁺. The results indicate that in any pollution abatement plan aimed at lowering the Hg levels in the biota of lakes fed by the Wabigoon River, immobilization, removal, or detoxification of dissolved as well as particulate forms of Hg in the river would probably have to be considered. Possibly, Hg species could be ‘scrubbed’ from the river water by increasing the suspended load and by sedimentation and treatment with Hg-binding agents in special receiving basins.     

Synthesis,spectroscopic, and antibacterial activity of tetraazamacrocyclic complexes of trivalent chromium,manganese, and iron

A new series of macrocyclic complexes of type [M(TML)X]X₂, where M = Cr(III), Mn(III), or Fe(III), TML is tetradentate macrocyclic ligand, and X = Cl^?, NO₃^?, CH₃COO^? for Cr(III), Fe(III) and X = CH₃COO^? for Mn (III), has been synthesized by condensation of benzil and succinyldihydrazide in the presence of metal salt. The complexes have been so formulated due to the 1:2 electrolytic nature of these complexes as shown by conductivity measurements. The complexes have been characterized with the help of various physicochemical techniques such as elemental analysis, molar conductance, electronic and infrared spectral studies, and magnetic susceptibility. On the basis of these studies, a five-coordinate distorted square pyramidal geometry, in which two nitrogens and two carbonyl oxygen atoms are suitably placed for coordination toward the metal ion, has been proposed for all the complexes. The complexes have been tested for their in vitro antibacterial activity. Some of the complexes show remarkable antibacterial activities against some selected bacterial strains. The minimum inhibitory concentrations shown by these complexes have been compared with those shown by some standard antibiotics such as linezolid and cefaclor.     

Distribution of mercury, methyl mercury and organic sulphur species in soil, soil solution and stream of a boreal forest catchment

Concentrations of methyl mercury, CH₃Hg (II), total mercury, Hg_tot = CH₃Hg (II) + Hg (II), and organic sulphur species were determined in soils, soil solutions and streams of a small (50 ha) boreal forest catchment in northern Sweden. The CH₃Hg (II)/Hg_tot ratio decreased from 1.2–17.2% in the peaty stream bank soils to 0.4–0.8% in mineral and peat soils 20 m away from the streams, indicating that conditions for net methylation of Hg (II) are most favourable in the riparian zone close to streams. Concentrations of CH₃Hg (II) bound in soil and in soil solution were significantly, positively correlated to the concentration of Hg_tot in soil solution. This, and the fact that the CH₃Hg (II)/Hg_tot ratio was higher in soil solution than in soil may indicate that Hg (II) in soil solution is more available for methylation processes than soil bound Hg (II). Reduced organic S functional groups (Org-S_RED) in soil, soil extract and in samples of organic substances from streams were quantified using S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Org-S_RED, likely representing RSH, RSSH, RSR and RSSR functionalities, made up 50 to 78% of total S in all samples examined. Inorganic sulphide [e.g. FeS₂ (s)] was only detected in one soil sample out of 10, and in none of the stream samples. Model calculations showed that under oxic conditions nearly 100% of Hg (II) and CH₃Hg (II) were complexed by thiol groups (RSH) in the soil, soil solution and in the stream water. Concentrations of free CH₃Hg⁺ and Hg²⁺ ions in soil solution and stream were on the order of 10^–18 and 10^–32M, respectively, at pH 5. For CH₃Hg (II), inorganic bi-sulphide complexes may contribute to an overall solubility at concentrations of inorganic sulphides higher than 10^–9M, whereas considerably higher concentrations of inorganic sulphides (lower redox-potential) are required to increase the solubility of Hg (II).     

Speciation, stability constants and structures of complexes of copper(II), nickel(II), silver(I) and mercury(II) with PAMAM dendrimer and related tetraamide ligands

The acid-base properties and Cu(II), Ni(II), Ag(I) and Hg(II) binding abilities of PAMAM dendrimer, L, and of the simple model compounds, the tetraamides of EDTA and PDTA, L¹, were studied in solution by pH-metric methods and by ¹H NMR and UV-Vis spectroscopy. PAMAM is hexabasic and six pK_a values have been determined and assigned. PAMAM forms five identifiable complexes with copper(II), [CuLH₄]⁶⁺, [CuLH₂]⁴⁺, [CuLH]³⁺, [CuL]²⁺ and [CuLH_-1]⁺ in the pH range 2-11 and three with nickel(II), [NiLH]³⁺, [NiL]²⁺ and [NiLH_-1]⁺ in the pH range 7-11. The complex [CuLH₄]⁶⁺, which contains two tertiary nitrogen and three amide oxygen atoms coordinated to the metal ion, is less stable than the analogous EDTA and PDTA tetraamide complexes [CuL¹]²⁺, which contain two tertiary nitrogen and four amide oxygen atoms, due to ring size and charge effects. With increasing pH, [CuLH₄]⁶⁺ undergoes deprotonation of two coordinated amide groups to give [CuLH₂]⁴⁺ with a concomitant change from O-amide to N-amidate coordination. Surprisingly and in contrast to the tetraamide complexes [CuL¹]²⁺, these two deprotonation steps could not be separated. As expected the nickel(II) complexes are less stable than their copper(II) analogues. The tetra-N-methylamides of EDTA, L¹(b), and PDTA form mononuclear and binuclear complexes with Hg(II). In the case of L¹(b) these have stoichiometries HgL¹(b)Cl₂, [HgL¹(b)H₋₂Cl₂]²⁻, [Hg₂L¹(b)Cl₂]²⁺, Hg₂L¹(b)H₋₂Cl₂ and [Hg₂L¹(b)H₋₅Cl₂]³⁻. Based on ¹H NMR and pH-metric data the proposed structure for HgL¹(b)Cl₂, the main tetraamide ligand containing species in the pH range <3-6.5, contains L¹(b) coordinated to the metal ion through the two tertiary nitrogens and two amide oxygens while the structure of [HgL¹(b)H₋₂Cl₂]²⁻, the main tetraamide ligand species at pH 7.5-9.0, contains the ligand similarly coordinated but through two amidate nitrogen atoms instead of amide oxygens. The proposed structure of [Hg₂L¹(b)Cl₂]²⁺, a minor species at pH 3-6.5, also based on ¹H NMR and pH-metric data, contains each Hg(II) coordinated to a tertiary amino nitrogen, two amide oxygens and a chloride ligand while that of [Hg₂L¹(b)H₋₅Cl₂]³⁻, contains each Hg(II) coordinated to a tertiary amino nitrogen, two amidate nitrogens, a chloride and a hydroxo ligand in the case of one of the Hg(II) ions. The parent EDTA and PDTA amides only form mononuclear complexes. PAMAM also forms dinuclear as well as mononuclear complexes with mercury(II) and silver(I). In the pH range 3-11 six complexes with Hg(II) i.e. [HgLH₄Cl₂]⁴⁺, [HgLH₃Cl₂]³⁺, [Hg₂LCl₂]²⁺, [Hg₂LH₋₁Cl₂]⁺, [HgLH₋₁Cl₂]⁻ and [HgLH₋₂Cl₂]²⁻ were identified and only two with Ag(I), [AgLH₃]⁴⁺ and [Ag₂L]²⁺. Based on stoichiometries, stability constant comparisons and ¹H NMR data, structures are proposed for these species. Hence [HgLH₄Cl₂]⁴⁺ is proposed to have a similar structure to [CuLH₄]⁶⁺ while [Hg₂LCl₂]²⁺has a similar structure to [Hg₂L¹(b)H₋₅Cl₂]³⁻.     

Development of a transgenic tobacco plant for phytoremediation of methylmercury pollution

To develop the potential of plant for phytoremediation of methylmercury pollution, a genetically engineered tobacco plant that coexpresses organomercurial lyase (MerB) with the ppk-specified polyphosphate (polyP) and merT-encoding mercury transporter was constructed by integrating a bacterial merB gene into ppk/merT-transgenic tobacco. A large number of independent transgenic tobaccos was obtained, in some of which the merB gene was stably integrated in the plant genome and substantially translated to the expected MerB enzyme in the transgenic tobacco. The ppk/merT/merB-transgenic tobacco callus showed more resistance to methylmercury (CH₃Hg⁺) and accumulated more mercury from CH₃Hg⁺-containing medium than the ppk/merT-transgenic and wild-type progenitors. These results suggest that the MerB enzyme encoded by merB degraded the incorporated CH₃Hg⁺ to Hg²⁺, which then accumulated as a less toxic Hg-polyP complex in the tobacco cells. Phytoremediation of CH₃Hg⁺ and Hg²⁺ in the environment with this engineered ppk/merT/merB-transgenic plant, which prevents the release mercury vapor (Hg⁰) into the atmosphere in addition to generating potentially recyclable mercury-rich plant residues, is believed to be more acceptable to the public than other competing technologies, including phytovolatilization.     

Chromium(III) complexes and their relationship to the glucose tolerance factor. Part II. Structure and biological activity of amino acid complexes

A number of octahedral chromium complexes with amino acids are ligands have been prepared and their structures assigned on the basis of their chromatographic and spectral properties. These include complexes with the general structure Cr(AA)₂(H₂O)₂ where the amino acids glycine, glutamic acid and glutamine act as bidentate ligands. The analogous compound with cysteine as ligand is stable at low pH, but at high pH a terdentate cysteine complex, Cr(cysteine)₂^?, is formed. These complexes, as well as a solution of monodentate glycine aquo complexes, and Cr-nicotinic acid-glycine and Cr-nicotinic acid-cysteine complexes of undetermined structure, have been assayed for glucose tolerance factor activity using a yeast assay. Only Cr(glutamine)₂- (H₂O)₂⁺, Cr-nicotinic acid-glycine and the mixture of complexes Cr(glycine)_n(H₂O)_6-n⁺³ showed significant activity. It is proposed that a trans arrangement of the non-coordinated nitrogen atoms in the ligands of these complexes can mimic the structural features of the glucose tolerance factor which are essential for biological activity.     

Sequestration of organometallic compounds by synthetic and naturally occurring polycarboxylate ligands. Binding of monomethylmercury(II) by polyacrylic and alginic acids

Abstract

The sequestering capacity of synthetic and naturally occurring polycarboxylate ligands towards mono-methylmercury(II) was evaluated by stability quantitative data on the interaction of CH₃Hg⁺ with different molecular weight synthetic polyacrylates (2 and 20kDa average M.wt) and alginate (70–100 kDa) extracted from brown algae Macrocystis pyrifera. The influence of ionic medium was evaluated by measurements on the CH₃Hg⁺-polyacrylate systems in NaNO₃ medium at different ionic strengths (0.10, 0.25, 0.50 and 0.75 mol L^?1), and a Debye-HiJckel type equation was used for the dependence of complex formation constants on ionic strength. Measurements on the CH3Hg⁺ - alginate system were carried out at l = 0.10 mol L^?1 in NaNO₃ medium. By using the stability data, the sequestering capacity of both ligands towards monomethylmercury(II) was determined at different pH values. Results obtained show that the binding ability of polyacrylic ligands (PAA) is stronger than the alginate (AA), following the trend PAA (20 kDa)> PAA (2kDa)>AA.     

Mercury's neurotoxicity is characterized by its disruption of selenium biochemistry

Background

Methylmercury (CH₃Hg⁺) toxicity is characterized by challenging conundrums: 1) “selenium (Se)-protective” effects, 2) undefined biochemical mechanism/s of toxicity, 3) brain-specific oxidative damage, 4) fetal vulnerability, and 5) its latency effect. The “protective effects of Se” against CH₃Hg⁺ toxicity were first recognized >50?years ago, but awareness of Se's vital functions in the brain has transformed understanding of CH₃Hg⁺ biochemical mechanisms. Mercury's affinity for Se is ~1 million times greater than its affinity for sulfur, revealing it as the primary target of CH₃Hg⁺ toxicity.

Template synthesis of macrocyclic complexes and their spectroscopic and antibacterial studies

A new series of macrocyclic complexes of type [M(TML)X]X₂, where M = Cr(III), Fe(III), TML is tetradentate macrocyclic ligand, and X = Cl^?, NO₃^?, CH₃COO^?, have been synthesized by condensation of isatin and ethylenediamine in the presence of metal salt. The complexes were synthesized by both conventional and microwave methods. The complexes have been characterized with the help of elemental analysis, conductance measurement, magnetic measurement, and infrared, far infrared, and electronic spectral studies. Molar conductance values indicate them to be 1:2 electrolytes. Electronic spectra along with magnetic moments suggest five-coordinate square pyramidal geometry for these complexes. The complexes were also tested for their in vitro antibacterial activity. Some of the complexes showed satisfactory antibacterial activitiy.     

The chemistry and transport of mercury in a small wetland in the Adirondack region of New York, USA

The biogeochemistry of Hg was evaluated in a small wetland in the Adirondack region of New York. Concentrations of total Hg (Hg_T) in streamwater draining the wetland showed little temporal variation. The annual areal watershed flux of Hg_T (2.2 µg/m²-yr) was considerably smaller than regional inputs of atmospheric deposition of Hg_T, indicating that the terrestrial environment is a net sink for atmospheric deposition of Hg_T. Drainage inputs of Hg_T were conservatively transported through the beaver impoundment. The annual flux of total methyl mercury (CH₃Hg⁺ _T was greater than literature values of atmospheric deposition suggesting that the watershed is a net source of CH₃Hg⁺ _T . Stream concentrations of CH₃Hg⁺ _T increased during low-flow summer conditions in a riparian wetland, and particularly at the outlet of the beaver impoundment. Net production of CH₃Hg⁺ _T occurred in the beaver impoundment (0.45 µg/m²-yr). Rates of net methylation for the beaver impoundment were comparable to values reported in the literature for wetlands.     

A highly sensitive and reversible chemosensor for Hg2+ detection based on porphyrin‐thymine conjugates

In this study, we demonstrated a highly sensitive, selective, and reversible chemosensor for Hg²⁺ determination. This chemosensor was synthesized by direct condensation of thymin‐1‐ylacetic acid with zinc tetraaminoporphyrin, which has a porphyrin core as the fluorophore and four thymine (T) moieties as the specific interaction sites for Hg²⁺. The probe (4T‐ZnP) exhibited split Soret bands with a small peak at 408 nm and a strong band at 429 nm in a dimethylformamide/H₂O (7/3, v/v) mixed solvent as well as a strong emission band at 614 nm. Upon the addition of Hg²⁺, the probe displayed strong fluorescence quenching due to the formation of T‐Hg²⁺‐T complexes. With the aid of the fluorescence spectrometer, the chemosensor in the dimethylformamide/H₂O (7/3, v/v) mixed solvent (0.3 μM) exhibited a detection limit of 6.7 nM. Interferences from other common cations, such as Co²⁺, K⁺, Sn²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Na⁺, Ca²⁺, Mg²⁺, Pb²⁺, and Cd²⁺, associated with Hg²⁺ analysis were effectively inhibited. Copyright © 2015 John Wiley & Sons, Ltd.     

Crystal structure of (theophyllinato)methylmercury(II) monohydrate

The title compound belongs to space group P2₁/c, a = 10.884 Å, b = 9.187 Å, c = 14.458 Å, β = 131.02°, Z = 4. The structure was refined on 1355 nonzero reflections to an R factor of 0.059. The crystal contains discrete [CH₃Hg(theophyllinate)] molecules in which the proton initially bound to N7 is replaced by the CH₃Hg⁺ ion. The water molecule forms hydrogen bonds with both carbonyl oxygens, whereas an intermolecular contact of 2.98 Å is established between mercury and N9. The intramolecular Hg?O6 distance of 3.18 Å is consistent with the absence of significant Hg?carbonyl bonding interactions in the present structure.     

Self-exchange rates and electron transfer kinetics of horse heart ferricytochrome C with amino acid-pentacyanoferrate(II) complexes

The electron transfer reactions of horse heart cytochrome c with a series of amino acid-pentacyanoferrate(II) complexes have been studied by the stopped-flow technique, at 25°C, μ = 0.100, pH 7 (phosphate buffer). A second-order behavior was observed in the case of the Fe(CN)₅ (histidine)^3? complex, with k = 2.8 x 10⁵ M^?1 sec^?1. For the Fe(CN)₅ (alanine)^4? and Fe(CN)₅(L-glutamate)^5? complexes, only a minor deviation of the second-order behavior, close to the experimental error (k = 3.2 × 10⁵ and 1.6 x 10⁵ M^?1 sec^?1, respectively) was noted at high concentrations of the reactants (e.g., 6 × 10^?4 M). The results are in accord with recent work on the Fe(CN)₆^4?/cytochrome c system demonstrating weak association of the reactants. The calculated self-exchange rate constants including electrostatic interactions for the imidazole,L -histidine, 4-aminopyridine, glycinate, β-alaninate, andL-glutamate pentacyanoferrate(II) complexes were 3.3 × 105, 3.3 × 10⁵, 2.8 × 10⁶,4.1 × 10²,5.5 × 10², and 6.0 M^?1 sec^?1, respectively. Marcus theory calculations for the cytochrome c reactions were interpreted in terms of two nonequivalent binding sites for the complexes, with the metalloprotein self-exchange rate constants varying from 10⁴ M^?1 sec^?1 (histidine, imidazole, and 4-aminopyridine complexes) to 10⁶ M^?1 sec ^?1 (glycinate, β-alaninate, and L-glutamate complexes).     

Metal ion-biomolecule interactions. III. Versatility of metal ion binding to imidazoles. Synthesis and 1H nmr spectroscopic properties of N- and C-bound mercury(II) complexes

Methylmercury(II) and mercury(II) complexes of imidazole (1), 1-methylimidazole (2), and the 1,3-dimethylimidazolium ion (3) have been prepared in aqueous or ethanolic solution. Elemental analysis and ¹H nmr spectroscopy have been used to characterize the complexes. The MeHg (Me = methyl) binding sites have been identified as N₁, N₃ (1), N₃, C₂ (2), and C₂ (3). Reaction with HgO leads to the formation of Hg-bridged complexes of the type Im-Hg-Im, (Im = imidazole), where bonding occurs through N₁ (1) and C₂ (3); the latter is also formed as a result of symmetrization of the C₂-bound MeHg complex. The formation of the C₂-bound (carbene) complexes is discussed in terms of the increased acidity of the C₂ proton resulting from coordination of an electrophilic species at N₃. Based on electrostatic considerations, there appears to be a “minimum degree of activation” required before C₂ bonding can occur, which explains the lack of this coordination mode in 1. ¹⁹⁹Hg-¹H spin-spin coupling (⁴J) is observed for C-bound mercury, but not for N-bound mercury, which is interpreted in terms of a decreased ligand exchange rate in the former case, due to the greater stability of the Hg-C bond. ²J coupling constants measured in (CD₃)₂SO for a number of MeHg complexes of heterocyclic ligands (including the imidazoles of the present study) correlate well with the ligand pK_a (25°C, aqueous solution), according to ²J = ?3.88 pK_a + 248.5. Results in the present work are discussed in relation to our previous work with nucleosides. The significance of the results to biological systems is considered.     

Resonance Raman spectroscopy of chemically modified retinals: Assigning the carbon-methyl vibrations in the resonance Raman spectrum of rhodopsin

To assign the observed vibrationsl modes in the resonance Raman spectrum of the retinylidene chromophore of rhodopsin, we have studied chemically modified retinals. The series of analogs investigated are the n-butyl retinals substituted at C₉ and C₁₃. The results obtained for the 11-cis isomer have clearly assigned the CCH₃ vibrational frequencies observed in the spectrum of the retinylidene chromophore. The data show that the C₍₉₎CH₃ stretching vibration can be assigned to the vibrational mode observed in the 1017 cm^?1 region, and the vibration detected at 997 cm^?1 can be assigned to the C₍₁₃CH₃ vibration. The C₍₅₎CH₃ stretching mode does not contribute to the vibrations observed in this region. The splitting in the C_(n)CH₃ (n = 9, 13) vibration is characteristic of the 11-cis conformation. The results on the modified retinals do not support the hypothesis that the splitting arises from equilibrium mixtures of 11-cis, 12-s-cis and 11-cis, 12-s-trans in solution. Thus, this splitting cannot be used to determine whether the chromophore in rhodopsin is in a 12-s-cis or 12-s-trans conformation. However, our results demonstrate that there are other vibrational modes in the spectra which are sensitive to this conformational equilibrium and we use the presence of a strong ~ 1271 cm^?1 mode in bovine and squid rhodopsin spectra as an indication that the chromophore in these pigments is 11-cis, 12-s-trans.     

Inorganic mercury (Hg2+) transport through lipid bilayer membranes

Summary Diffusion of inorganic mercury (Hg²⁺) through planar lipid bilayer membranes was studied as a function of chloride concentration and pH. Membranes were made from egg lecithin plus cholesterol in tetradecane. Tracer (²⁰³Hg) flux and conductance measurements were used to estimate the permeabilities to ionic and nonionic forms of Hg. At pH 7.0 and [Cl^–] ranging from 10–1000mm, only the dichloride complex of mercury (HgCl₂) crosses the membrane at a significant rate. However, several other Hg complexes (HgOHCl, HgCl ₃ ^– and HgCl ₄ ^2– ) contribute to diffusion through the aqueous unstirred layer adjacent to the membrane. The relation between the total mercury flux (J _Hg), Hg concentrations, and permeabilities is: 1/J _Hg=1/P ^ul[Hg ^t ]+1/P ^m [HgCl₂], where [Hg ^t ] is the total concentration of all forms of Hg,P ^ul is the unstirred layer permeability, andP ^m is the membrane permeability to HgCl₂. By fitting this equation to the data we find thatP ^m =1.3×10^–2 cm sec^–1. At Cl^– concentrations ranging from 1–100mm, diffusion of Hg ^t through the unstirred layer is rate limiting. At Cl^– concentrations ranging from 500–1000mm, the membrane permeability to HgCl₂ becomes rate limiting because HgCl₂ comprises only about 1% of the total Hg. Under all conditions, chemical reactions among Hg²⁺, Cl^– and/or OH^– near the membrane surface play an important role in the transport process. Other important metals, e.g., Zn²⁺, Cd²⁺, Ag⁺ and CH₃Hg⁺, form neutral chloride complexes under physiological conditions. Thus, it is likely that chloride can facilitate the diffusion of a variety of metals through lipid bilayer and biological membranes.