Electrochemical behaviour of mono- and oligonucleotides I. Effects of charge and configuration of adenine mononucleotides on their adsorption at the mercury—solution interface

A systematic study of the adsorption and interfacial behaviour of the adenine mono-nucleotides (5′-AMP, 3′-AMP, cyclic 3′,5′-AMP, 5′-ADP and 5′-ATP) and adenosine for comparison at the HMDE has been carried out at pH 3.4 to 3.5. Thus, the N(1) of the adenine moiety is protonated to a major extent.The adsorption was followed by single sweep voltammetry (measurement of the time integral of the reduction peak of the adsorbed adenine moiety) and by a.c. voltammetry (out-of-phase component of the a.c. response being proportional to the differential double layer capacity). In this paper the situation corresponding to a “dilute” adsorption layer existing at low bulk concentrations is studied for various degrees of coverage. The potential dependence of the coverage is of bell shaped type with an extended maximum region around the potential of electrocapillary maximum (E_ecm) of the blank. For the same bulk concentration the coverage decreases in the series AMP, ADP, ATP, i.e. with increasing negative charge of the nucleotide, and at the same time the potential range of adsorption narrows. Among the monophosphates the coverage decreases in the series 3′-AMP, 5′-AMP, cyclic 3′,5′-AMP. The variations are connected with the varying charge of the mononucleotides and with the possibilities for interactions with adjacent molecules in the adsorption layer.At elevated bulk concentrations above a threshold value a substantial increase in coverage occurs around E_ecm as due to strong interactions between the adsorbed base moieties a rather compact film is formed.     

Electrochemical behaviour of natural and biosynthetic polynucleotides at the mercury electrode: VII. Studies on the adsorption and reduction of DNA and biosynthetic polynucleotides with a new potentiostatic double step-sweep method

By a new potentiostatic double step-sweep method conveniently applicable with common commercial polarographic equipment the adsorption and interfacial behaviour of DNA of different origin and of related biosynthetic polynucleotides has been studied in moderately acid solution (pH 5.6) of different ionic strengths over the whole extended potential range of adsorption up to ?1.6 V (SCE) at the HMDE. Furthermore the new method has proved to be an efficient tool to follow the kinetics of the reduction of strongly adsorbed substances to as well strongly adsorbed reduction products.Three potential ranges corresponding to different interfacial situations can be distinguished for the investigated polynucleotides. In the first range between ?0.4 and ? 1.2 V (SCE) the biopolymer is adsorbed. If the biopolymer has initially a double stranded form progressive irreversible deconformation occurs as well in this potential range furnishing further evidence for the previously developed concept of the sequence of interfacial events. In the second range between ?1.2 and ?1.6 V biopolymers are adsorbed and (in the double stranded case after prior deconformation) the adenine and cytosine moieties undergo a totally irreversible reduction forming according to the nature of the polynucleotide a more or less completely blocking film of strongly adsorbed reduced biopolymer. In the third range beyond ?1.6 V no adsorption and thus reduction occurs.The kinetic parameters k_ct and α_cn_a of the rate determining step of the charge transfer reaction in which the adsorbed biopolymer is reduced have been studied as function of the nature and structure of the polynucleotide, of its base composition and of ionic strengthIn general the measurements improve significantly the understanding of the complicated behaviour of DNA and related biosynthetic polynucleotides at charged interfaces and confirm in various ways our previous resulis and conclusions.     

Solvent effect on standard electrode potential: Part II. Silver-silver bromide electrode in methanol-water mixtures

The standard potentials of the silver-silver bromide electrode have been determined over a broad range of methanol concentrations from e.m.f. measurements of the cell Pt, H₂(g, 1 atm); HBr(m), X% methanol, Y% water; AgBr, Ag at eight temperatures ranging from 20 to 55°C. The standard e.m.f. has been expressed as a function of temperature. Thermodynamic functions of cell process, the primary medium effect of various media on hydrobromic acid, and thermodynamics of transfer of HBr from water to methanolic media have been computed and discussed in the light of ion-solvent interactions as well as the structural changes of the solvents.     

Thermochemistry of fluorine compounds: IV. Rubidium tetrafluoroiodate

From measurement of the heat of hydrolysis, at 25°C , the enthalpy of formation of rubidium tetrafluoroiodate is derived: ΔH°_f [RbIF₄, cryst.]₂₉₈= ?191.12±4.43 kJ mol^?1. Heat capacity measurements for RbIF₄ over the range 273–303 K are also reported.     

Relationship between the properties of radical cations and the rate constants and the substitution patterns in electrophilic aromatic substitution

By a charge transfer mechanism for electrophilic aromatic substitution the logarithmic plot of overall rate constants for substitution against ionization potentials is correctly predicted. Also, orientation of substitution is found to be correlated with the hyperfine coupling constants of the aromatic radical cation. The presence of radical cations under the conditions of electrophilic substitution is discussed.     

Electrochemical behavior of ellipticine derivatives: Part I. Oxidation of 9-hydroxy-ellipticine

The electrochemical one-electron oxidation of 9-hydroxy-ellipticinium cations at a platinum electrode has been examined with particular attention to the thermodynamic redox potential and to the dimerization rate of the radical species produced. Both the reactant and the product of the electrochemical reaction are strongly adsorbed at the solution/electrode interface. The initial step of the anodic process is a reversible electron transfer accompanied by a fast deprotonation; E^0′=900?53 pH mV vs. NHE. The resulting neutral radical dimerizes, the rate constant of the surface dimerization being ca. 2.5×10⁹ mol^?1 cm² s^?1.     

The Raman spectrum of tetragonal Eu3+CaF2

The Raman spectrum of a tetragonal single crystal of the compound Eu_xCa_1-(3/2)x F₂(where x = 0.002) is reported. The main features of the spectrum are the ⁷F₀ - ⁷F₁ and ⁷F₀ → ⁷F₂ electronic transitions of Eu³⁺ ions located at a site of C₂ symmetry and vibrational transitions which involve movements of the europium ion. The characteristic phonon band of cubic CaF₂ is for the rare-earth doped crystal moved to a higher frequency and the corresponding Raman tensor is that for a crystal of tetragonal symmetry.     

Substituent effects in the reaction of phenols with polyacrylonitrile radicals

Substituent effects in the reactivities of phenols towards polyacrylonitrile radicals have been studied in terms of Swain and Luptons' field (F_k) and resonance (R_k) components of the substituent parameters and the unique positional weighting factors (f_j and r_j) proposed by Williams and Norrington, with the aid of the following equation:

$\text{P}{}_{\mathrm{i}}\text{= x}{}_{\mathrm{i}}\text{?}{}_{\mathrm{i}}\text{F}{}_{\mathrm{k}}\text{+ ß}{}_{\mathrm{i}}\text{r}{}_{\mathrm{j}}\text{K}{}_{\mathrm{i}}\text{+ e}{}_{\mathrm{i}}\text{+}\text{P}\text{i}\text{o}$

P_i′s are the rate parameters, P_i⁰ being that for a standard reference compound. Two types of rate parameters have been employed—one, suggested by Simonyi, Tüdös and Pospisil (β) and another, suggested by us, (K), which is obtained from a plot of [Monomer]/(rate of polymerisation) vs [Phenol]. The correlations have been found to be quite satisfactory with both β and K. An attempt has been made to ascertain the nature of the transition state from the reaction parameters α_i and β_i; a dipolar transition state is suggested. The anomalous kinetic behaviour of hydroxy phenols has been discussed.     

Photoelectron spectroscopy of f-element organometallic compounds II. Tricyclopentadienyl derivatives of uranium(IV) and thorium(IV)

The photoelectron spectra of some cyclopentadienyI derivatives of uranium(IV) and thorium(IV), namely (C₅H₅)₃ ThCl, (C₅H₅)₃ UCl, (C₅ H₄ CH₃)₃ ThCl, (C₅ H₄ CH₃)₃ UCl, (C₅ H₄ CH₃)₃ UBr and (C₅ H₄ CH₃ )₃ UBH₄, are reported the metal 5f ionization has been detected in all the uranium derivative spectra and a simple molecular orbital scheme qualitatively accounts for the number and position of the observed low energy bands.     

Electrochemical behaviour of some neuroleptics: Haloperidol and its derivatives

The electrochemical characteristics of Haloperidol and related compounds, representative neuroleptics of the butyrophenone family, have been investigated as a function of pH and concentration by direct-current, alternating-current and differential-pulse polarography and cyclic voltammetry at a hanging mercury drop electrode. A single cathodic wave representing an irreversible two-electron reduction is obtained, and its half-wave potential differs from that characteristic of aromatic ketone reduction. Adsorption processes disturb the wave behaviour and an adsorption prewave is observed at high concentrations. Quantitative measurements were successful in the concentration range 1 x 10(-4)-1 x 10(-6)M (0.4 mg/l.), the lower concentration representing the detection limit by differential-pulse polarography.     

A preliminary investigation of ionic adsorption at the mercury—hexamethyl phosphoric triamide interface

A reference electrode is described that is reversible and reproducible in hexamethyl phosphoric triamide; it was used to study the adsorption of 1 : 1 electrolytes at the mercury—solution interface. The differential capacity curves show features similar to those found for water and the degree of specific adsorption of the anions follows the same sequence: PF^?₆ < BF^?₄, ClO^?₄ < Br^?. There appears to be no specific adsorption of the alkali metal cations at negative charge values whereas the tetraalkyl ammonium ions are strongly adsorbed.     

η-Allyl-molybdenum and -tungsten carbonyl,complexes containing aliphatic α-diimines as ligands and their solvent dependent infrared and electronic spectra

Pronounced solvent dependence is observed in the infrared and electronic spectra of the new η-allyl complexes (η-R′ C₃ H₄)M(CO)₂ L₂X (M = Mo or W; R′ = H or Me; X = Ct, By, I or NCS; L₂ = 1,2-ethanediylidenediimines or 2-pyridinaldimines).     

Operational comparison of various thermodynamic treatments of organic substance adsorption at the electrode/solution interface

Three isotherms are considered: Frumkin's Bennes' and Mohilner's. The relationship between the interaction parameters in the first two and the adsorbate activity coefficient in the last is discussed. The significance of various standard states for the definition of ΔG° _ads is analysed. The operational derivation of ΔG°_ads in the three cases and the relationship between the resulting three quantities are illustrated. The isotherms have been used to describe the adsorption of 1,4-dioxane on a polarized Hg electrode. The analysis has been carried out both at constant potential and constant charge. The picture emerging in each of the three cases is discussed in the light of the results obtained in the other two. It is concluded that the three approaches give the same qualitative information about the interfacial behaviour of dioxane, especially as far as the effect of the electric field and the particle-particle lateral interaction are concerned. The usefulness of the Frumkin isotherm for analysing promptly experimental adsorption data is thus maintained. Limitations of the present approach are discussed.     

Ion Chromatographic determination of sulfur and chlorine using milligram and submilligram sample weights

Initial results for Ion Chromatographic analyses of chlorine and sulfur in organic compounds combusted by the Schöniger technique show excellent agreement with analyses using standard methods. Samples weighing less than 1 mg can be analyzed if homogeneity is not a problem. Determination of chlorine in the presence of large amounts of bromine and the analysis of other heteroatoms is also possible by IC. The present limiting factor in the microdetermination of chlorine and sulfur by IC is the presence of these species in the filter paper flag.     

Redox reactions in nonaqueous media determination of amines via dithiocarbamate formation and iodine monobromide titrations

A simple and accurate nonaqueous oxidimetric method of wide applicability has been developed for the determination of primary and secondary amines after their quantitative conversion with carbon disulphide in acetonitrile to the alkylammonium alkyldithiocarbamates, which are titrated with iodine monobromide solution also in acetonitrile at room temperature. The endpoint is detected visually by the yellow tint imparted to the solution by the first drop of oxidant solution in excess; and potentiometrically by using a bright platinum wire indicator electrode, and antimony or modified calomel reference electrode. The method is simple, rapid, accurate, and widely applicable.     

Volatilization of trichothecene mycotoxins for analysis

T-2 toxin and diacetoxyscirpenol (DAS), two trichothecene mycotoxins containing one hydroxy group, have been volatilized by induction heating, revolatilized, and analyzed by gas chromatography (GC) and/or GC mass spectroscopy. Seventy to eighty percent of DAS was recovered by this system; 60–70% T-2 toxin was recovered. When the hydroxy group is derivatized by acetate, 90–100% recovery is obtained. Other trichothecenes of the macrocyclic ester type (e.g., Roridan A) were also tried. Ten to twenty percent of the macrocyclic ester was obtained without derivatization.     

Photo-induced homolysis of alkyltricyclopentadienyluranium(IV) complexes: an example of a successful UIV → UIII reduction

Photo-induced reduction of two organouranium complexes of the type Cp₃UR (R = CH₃ and n-C₄H₉) was studied in toluene and THF solution at various temperatures. Optimal conditions for the production of Cp₃U . THF as the final product involve UV/VIS irradiation in THF at ca. 60°C. Spin trap experiments indicate that, at least in THF, intermediate free radicals participate. The results, including gas chromatographic, mass spectroscopic and ¹H NMR spectroscopic measurements, are discussed in terms of a virtually first order multi-step process involving discrete radical pairs kept together efficiently by solvent cages so that H-atom abstraction from a Cp ligand followed by decomposition of the organometallic product is largely avoided.