Characteristics of a new catalytic hydrogen current observed with albumin in the presence of cobalt(III) or (II) at the hanging mercury drop electrode

When the hanging mercury drop electrode (HMDE) is placed in a solution which is 0.1 M in ammonia and 0.1 M in ammonium chloride and about 5 to 10×10^?4M in cobalt(III)-hexamine or cobalt(II) chloride and in very small concentrations of bovine serum albumin (BSA), the protein is slowly adsorbed. When the adsorption is highly incomplete and the HMDE is kept for 30 s at about ?1.05 V vs. SCE, “active cobalt’ is deposited as a complex (Co(0)BSA). This is anodically oxidized at about 0.0 V to unstable Co(I)BSA). When the electrode is then rapidly (500 mV s^?1) cathodized, a catalytic hydrogen current (i_c) with peak at circa ?1.45 V is observed. In this way it is even possible to detect and estimate BSA in concentrations of the order of 10^?12M. A detailed study has been made of the characteristics of i_c under several conditions. “Active cobalt” on the HMDE does not affect Brdi?ka currents. Cystine and cysteine also yield the catalytic hydrogen current i_c under the same conditions as does BSA.     

Methods for potentiostatic evaluation of heterogeneous kinetic parameters in the presence of an irreversible following chemical reaction

A theoretical study of Brdi?ka's adsorption waves and of the corresponding i-E curves in cyclic voltammetry and E-t curves in chronopotentiometry is presented under the following conditions: (a) the electrochemical reaction 0+ne?R is reversible; (b) both the oxidized and the reduced forms are strongly adsorbed; (c) the adsorption is rapid; (d) allowance is made in the isotherm for interactions between the molecules (Frumkin type isotherm). One mathematical function and its derivative are sufficient to describe the results in the three cases. The equations derived account in particular for the following experimental characteristics: (1) the linear variation of the half-wave potential of the adsorption wave (or of the chronopotentiogram) and of the voltammetric peak potential with the concentration c; (2) the changes in the slope of the wave when c varies and the discontinuous increase of current which is observed for high concentrations; (3) the changes in peak width and the hysteresis phenomenon observed in cyclic voltammetry; (4) the S-shaped experimental chronopotentiograms; (5) the appearance of prewaves (or postwaves) in cases where both O and R are equally adsorbed; (6) the fact that prewaves or postwaves usually appear for large molecules. The results show that the appearance of a Brdi?ka prewave or postwave is often caused by energy effects arising from the interactions between adsorbed molecules, rather than by energy effects caused by interactions with the electrode surface.     

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.     

Electrochemical and In Situ FTIR Studies of Adsorption and Oxidation of Dimethyl Ether on Platinum Electrode

Dissociative adsorption and electrooxidation of dimethyl ether (DME) on a platinum electrode in different pH solutions were studied using cyclic voltammetry (CV) and in situ FTIR reflection spectroscopy. The coverage of the dissociative adsorbed species was measured about 70% from hydrogen adsorption-desorption region (0.05-0.35 V (vs RHE)) of steady-state voltammogram recorded in 0.1 mol·L⁻¹ H₂SO₄ solution. It was found that the electrochemical reactivity of DME was pH dependent, i.e., the larger the pH value was, the less the reactivity of DME would be. No perceptible reactivity of DME in 0.1 mol·L⁻¹ NaOH solution could be detected. It was revealed that the protonation of the oxygen atom in the C-O-C bond played a key role in the electrooxidation of DME. In situ FTIR spectroscopic results illustrated that linearly bonded CO (CO_L) species determined at low potential region were derived from the dissociative adsorption of DME and behaved as ‘poisoning’ intermediate. The CO_L species could be oxidized to CO₂ at potential higher than 0.55 V (vs RHE) and in the potential range from 0.75 to 1.00 V (vs RHE) DME was oxidized simultaneously via HCOOH species that were identified as the reactive intermediates.     

478—Theory of Brdička current and the dependence of the current on cobalt ion concentration

An equation of Brdi?ka current and its experimental verification are presented, in which Brdi?ka current is expressed by a function of the surface concentration of protein, the bulk concentration of cobalt ion and two parameters, n_ck_c and k_f/k_d, characterizing the protein-zero-valent cobalt complex which is to catalize the hydrogen evolution, where n_c is the total number of sites on which the complex can be formed in a protein molecule, and k_c and k_d are the (average) constants representing the intrinsic catalytic activity and the lifetime respectively, of the complex.     

Polarographic catalytic hydrogen currents observed with native and modified bovine serum albumin

Catalytic hydrogen (presodium) currents (i_p.s.) obtained with bovine serum albumin (BSA), completely reduced BSA, ?(SH)₃₅, and ?(SCH₂COO)₃₅Na₃₅ have been studied in various buffers and at varying pH. At the same pH i_p.s.. of BSA increases with ammonium concentration in ammonia buffer and is considerably greater in ammonia than in borate and greater in the latter than in tris buffers. Calcium(chloride) greatly increases i_p.s. in all buffers, except in the presence of 8 M urea. Treatment of 1% BSA for 24 hours or longer with a large excess of formaldehyde in a phosphate buffer and dilution in a buffer reduces i_p.s. to very small values, but the calcium effect is now relatively much greater than before treatment. The same is true when the buffers after treatment are made 8 M in urea. Evidence is presented that the effect is due to a complexation of calcium with the reaction product of formaldehyde with amino and imino groups. With ?(SH)₃₅ practically no calcium effect on i_p.s. was observed. However, after reaction with formaldehyde in a borax buffer, i_p.s. is strongly reduced and after this treatment there is a large calcium effect. With ?(SCHC₂COO)₃₅Na₃₅i_p.s. is very poorly defined and occurs at more negative potential, and is much smaller than observed with BSA. There is again a distinct calcium effect. Presodium currents are also poorly defined with cystine and cysteine, a distinct calcium as well as formaldehyde effect being observed. Polarographic evidence is presented that sulfhydryl in cysteine reacts with formaldehyde. This accounts for the fact that after formaldehyde treatment i_p.s. with cystine is different from that with cysteine.     

The riboflavin—dihydroriboflavin system in acidic medium on mercury: Part I. A chronocoulometric investigation

The riboflavin (RF)—dihydroriboflavin (DRF) system in 0.01 M HClO₄ + 0.09 M NaClO₄ has been studied on mercury by the single-step chronocoulometric technique. At ?0.040 V/SCE, where RF is still electro-inactive, this substance is adsorbed according to a Langmuir isotherm with an adsorption coefficient K_O = 5.2 × 10⁶ 1 mol^?1, giving rise to a single adsorbed monolayer. At potentials along the plateau of the RF polarographic adsorption prewave, as well as at more negative potentials, DRF is adsorbed with formation of two overlapping monolayers. This behaviour denotes strong attractive vertical interactions between overlapping adsorbed DRF molecules. The progressive shift in the chronocoulometric Q vs. E curve for electro-oxidation of adsorbed DRF towards more positive potentials with an increase in the surface concentration of DRF confirms the strength of these vertical interactions. The simultaneous presence, with formation of a charge-transfer complex, of adsorbed RF and DRF molecules along the plateau of the polarographic RF prewave, as postulated by Tedoradze and co-workers [21,22] is excluded.     

Surface reactions of dimethyl ether on γ-Al2O3: 1. Adsorption and thermal effects

Reactions of dimethyl ether (DME) over γ-Al₂O₃ at 250°C have been investigated in a flow catalytic reactor. The main products of the interaction between DME and alumina are methanol and water. Heat evolution is observed as DME is passed over alumina, and replacing DME with nitrogen gives way to heat absorption. Calcination of alumina before the reaction considerably strengthens the exotherm, which is due to DME adsorption, while the endotherm is due to the desorption of weakly bound DME. The role of the hydroxyl groups of γ-Al₂O₃ in methanol and water formation has been elucidated. Treating alumina with water vapor after bringing it into contact with DME completely restores the hydroxyl cover and replaces strongly adsorbed DME with hydroxyl groups.     

New Electrocatalytic Reactions at a Mercury Electrode in the Presence of Homocysteine or Cysteine and Cobalt or Nickel Ions

Homocysteine (Hcy) and cysteine (Cys) mercury thiolate layers were prepared by anodic polarization of a mercury electrode in amino acid containing solutions and then investigated in the cathodic regime in the presence of Ni²⁺ or Co²⁺ ions. The sulfhydryl function in the mercury thiolate undergoes a slow disintegration resulting in surface‐attached mercury sulfide. During the cathodic scan, Hg²⁺ substitution by Ni²⁺ or Co²⁺ yields minute amounts of the relevant metal sulfide. Such a species catalyzes hydrogen evolution at ?1.3 V vs. Ag|AgCl|KCl(3 M). Hcy experiences a faster decomposition and, consequently, displays a stronger catalytic effect. Each compound catalyzes the reduction of Ni²⁺ or Co²⁺, but only Cys (bound in metal complexes) induces typical catalytic hydrogen evolution processes such as the Brdi?ka reaction (with Co²⁺; pH around 9), or the catalytic hydrogen prewave (CHP) (with Ni²⁺; pH near 7). On the other hand, Hcy catalyzes the hydrogen evolution in the presence of Co²⁺ at ?1.5 V in the same way than sulfur derivatives with no amine function do. Metal sulfide formation does not interfere with CHP and Brdi?ka processes. Correlations between the physical state of the metal sulfide (adsorbed molecule or aggregate form) and its catalytic properties are discussed and possible analytical applications suggested.     

Adsorption and electrooxidation of dimethyl ether on platinized platinum electrode in sulfuric acid solution

Adsorption and oxidation of dimethyl ether (DME) on the Pt/Pt electrode from 0.5 M H₂SO₄ is studied by measuring transients of current and potential, charging curves, and curves of electrooxidation in the adsorbed layer and also by cyclic voltammetry and steady-state polarization measurements. The DME adsorption is accompanied by dehydrogenation and destruction of its molecules to form a chemisorbed adsorbate that mainly consists of C1 species (HCO_ads and/or CO_ads) with (under certain conditions) a small amount of species that desorb at cathodic polarization. The adsorption and electrooxidation of DME are inhibited by adsorbed oxygen. The possible schemes of DME oxidation, where the reaction of DME chemisorption products with adsorbed oxygen-containing species is the limiting stage, are discussed.     

PMOXA/PAA Brushes toward on-Line Preconcentration for BSA in Capillary Electrophoresis

In this work, a binary-mixed-brushes-coated (BBC) capillary with switchable protein adsorption/desorption properties was developed and applied for on-line preconcentration of proteins. Firstly, amine-terminated poly(2-methyl-2-oxazoline) (PMOXA-NH₂) and thiolterminated poly(acrylic acid) (PAA-SH) were synthesized by using cationic ring-opening polymerization (CROP) and reversible addition fragmentation chain transfer (RAFT) polymerization, respectively. Then, the BBC capillary based on poly(2-methyl-2-oxazoline) (PMOXA) and poly(acrylic acid) (PAA) was prepared by sequentially grafting of PMOXA-NH₂ and PAA-SH onto fused-silica capillary inner surface through poly(dopamine) (PDA) as an anchor. The obtained PMOXA/PAA coating formed on the capillary or capillary's raw material was characterized in terms of the thickness, surface chemical composition by using scanning electron microscope (SEM) and X-ray photoelectron spectrum (XPS). The switchable protein adsorption/desorption performance of the BBC capillary was investigated by using fluorescence microscope under di erent solutions with certain pH and ionic strength(I). The results showed that bovine serum albumin (BSA) could be adsorbed on BBC capillary at pH=5.0 (I=10^-5 mol/L), and then the adsorbed BSA could be released at pH=9.0 (I=0.1 mol/L). This switchable protein adsorption/desorption property of coated capillary was then used to preconcentrate proteins on-line for increasing the detection sensitivity of BSA in capillary electrophoresis (CE). With this method, a sensitivity enhancement factor (SEF) more than 5000 for BSA detection was obtained.     

Alkylsulfanylphenyl derivatives of cytosine and 7-deazaadenine nucleosides, nucleotides and nucleoside triphosphates: synthesis, polymerase incorporation to DNA and electrochemical study

Aqueous Suzuki–Miyaura cross‐coupling reactions of halogenated nucleosides, nucleotides and nucleoside triphosphates derived from 5‐iodocytosine and 7‐iodo‐7‐deazaadenine with methyl‐, benzyl‐ and tritylsufanylphenylboronic acids gave the corresponding alkylsulfanylphenyl derivatives of nucleosides and nucleotides. The modified nucleoside triphosphates were incorporated into DNA by primer extension by using Vent(exo‐) polymerase. The electrochemical behaviour of the alkylsulfanylphenyl nucleosides indicated formation of compact layers on the electrode. Modified nucleotides and DNA with incorporated benzyl‐ or tritylsulfanylphenyl moieties produced signals in [Co(NH₃)₆]³⁺ ammonium buffer, attributed to the Brdi?ka catalytic response, depending on the negative potential applied. Repeated constant current chronopotentiometric scans in this medium showed increased Brdi?ka catalytic response, which suggests the deprotection of the alkylsulfanyl derivatives to free thiols under the conditions.     

A study on the adsorption voltammetry of cobalt(II)—dime thylglyoxime system

The behaviour of the cobalt complex with dimethylglyoxime (DMG), Co(II)A₂, at the mercury electrode has been investigated in details. The adsorption phenomena have been observed by both normal pulse polarography and voltammetry with linearly changing potential. Experimental results show that, under the condition of adsorption potentials ranging from ?0.60 to ?0.9 eV (vs. S.C.E.), Co(II)A₂ can be adsorbed on the surface of hanging mercury drop electrode (HMDE) very well. The superficial concentrations represents a Langmuir isotherm with both concentration of Co(II)A₂ and the preconcentration time. The superficial concentration equation for adsorption voltammetry, corresponding to the condition of the low coverage of the electrode surface, is deduced. The equation has been verified experimentally. The sensitivity of the proposed method, which has been analysed theoretically, is independent on the scan rate and the surface area of HMDE, but depends on the preconcentration time and the diffusion layer thickness. For the 120 sec accumulation, the lower limit of determination is 1.10^?9 M.     

Admittance measurements of adsorption properties of 2,2′-bipyridine complexes: Part I. Cobalt(II) and nickel(II) complexes in aqueous solutions

The complex [Co bipy₃]²⁺ in aqueous perchlorate solution yields on the d.c. polarogramthree adsorption type pre-waves and a main reduction wave at ?1.2 V (SCE) corresponding to the reduction to monovalent complex. From admittance measurements and other results it can be concluded that two most positive pre-waves at ?0.2 V and ?0.5 V (SCE) are of non-faradaic nature being related to the adsorption of the complex with specifically adsorbed anions of the supporting electrolyte with the formation of a compact adsorbed layer of much lower interfacial capacity. The most negative pre-wave at ?0.9 V just preceding the main wave corresponds to the reduction of the adsorbed complex. The main wave at ?1.2 V is distorted by the adsorption of reaction products. Very similar and also strong adsorption-coupled interaction was found with divalent nickel trisbipyridine complex.     

Application of Elimination Voltammetry to the Resolution of Adenine and Cytosine Signals in Oligonucleotides II. Hetero‐Oligodeoxynucleotides with Different Sequences of Adenine and Cytosine Nucleotides

Elimination voltammetry with linear scan (EVLS) was applied to the resolution of reduction signals of adenine (A) and cytosine (C) residues in short synthetic hetero‐oligodeoxynucleotides (ODNs) with different sequences of A and C. The EVLS evaluation required linear sweep voltammograms measured on a hanging mercury drop electrode (HMDE) at different scan rates. Compared to linear sweep voltammetry (LSV) and usual voltammetric methods the EVLS is capable of resolving the overlapped A and C signals, specifically by using the elimination function which eliminates the charging and kinetic currents (I_c, I_k) and conserves the diffusion current (I_d). Since for an adsorbed electroactive substance this elimination function yields a well readable peak–counterpeak signal, the adsorptive stripping (AdS) procedure was favorably used. The adsorption of ODNs was carried out at ?0.1 V for accumulation time of 120 s under stirring. It was found that heights and potentials of LSV signals were affected by ODNs concentrations, pH, scan rates, time of accumulation, and stirring speed during the adsorption. While on LSV curves the only one reduction peak of A and C residues was observed in all ODNs, the EVLS yielded two separated peaks in dependence on A, C sequences and pH. Our results showed that the EVLS in connection with the AdS procedure is a useful tool for qualitative and quantitative studies of short ODNs and a promising sensitive method for the development of electrochemical sensor following the ODN sequences.     

Numerical study of nitrogen desorption by rapid oxygen purge for a medical oxygen concentrator

Efficient desorption of selectively adsorbed N₂ from air in a packed column of LiX zeolite by rapidly purging the adsorbent with an O₂ enriched gas is an important element of a rapid cyclic pressure swing adsorption (RPSA) process used in the design of many medical oxygen concentrators (MOC). The amount of O₂ purge gas used in the desorption process is a sensitive variable in determining the overall separation performance of a MOC unit. Various resistances like (a) adsorption kinetics, (b) column pressure drop, (c) non-isothermal column operation, (d) gas phase mass and thermal axial dispersions, and (e) gas-solid heat transfer kinetics determine the amount of purge gas required for efficient desorption of N₂. The impacts of these variables on the purge efficiency were numerically simulated using a detailed mathematical model of non-isothermal, non-isobaric, and non-equilibrium desorption process in an adiabatic column. The purge gas quantity required for a specific desorption duty (fraction of total N₂ removed from a column) is minimum when the process is carried out under ideal, hypothetical conditions (isothermal, isobaric, and governed by local thermodynamic equilibrium). All above-listed non-idealities (a?Ce) can increase the purge gas quantity, thereby, lowering the efficiency of the desorption process compared to the ideal case. Items (a?Cc) are primarily responsible for inefficient desorption by purge, while gas phase mass and thermal axial dispersions do not affect the purge efficiency under the conditions of operation used in this study. Smaller adsorbent particles can be used to reduce the negative effects of adsorption kinetics, especially for a fast desorption process, but increased column pressure drop adds to purge inefficiency. A?particle size range of ??300?C500???m is found to require a?minimum purge gas amount for a given desorption duty. The purge gas requirement can be further reduced by employing a pancake column design (length to diameter ratio, L/D<0.2) which lowers the column pressure drop, but hydrodynamic inefficiencies (gas mal-distribution, particle agglomeration) may be introduced. Lower L/D also leads to a smaller fraction of the column volume that is free of N₂ at the purge inlet end, which is required for maintaining product gas purity. The simulated gas and solid temperature profiles inside the column at the end of the rapid desorption process show that a finite gas-solid heat transfer coefficient affects these profiles only in the purge gas entrance region of the column. The profiles in the balance of the column are nearly invariant to the values of that coefficient. Consequently, the gas-solid heat transfer resistance has a minimum influence on the overall integrated N₂ desorption efficiency by O₂ purge for the present application.     

Determination of copper in sea water by cathodic stripping voltammetry of complexes with catechol

A reduction current is obtained when an aqueous solution of copper and catechol is subjected to differential-pulse cathodic stripping voltammetry (d.p.c.s.v.) because of the reduction of copper(II)—catechol complex ions which adsorb onto the hanging mercury drop electrode (HMDE). The most likely form of the adsorbed complex ions is CuL^2?₂ (L being catechol). A.c. polarographic measurements showed that these complex ions adsorb more strongly onto the drop than free catechol ions. Monolayer adsorption density is obtained at 2.1 × 10^?10 molecules/cm², equivalent to a surface area of 78 A² complex ion, which agrees well with the molecular surface area calculated from the bond lengths. Analytically useful currents are obtained at very low metal concentrations, such as in uncontaminated sea water. The possible interference by other trace metals, major cations, and organic complexing ligands is investigated. Competition for copper ions by natural organic complexing ligands is evident at low concentrations of catechol. Analysis of the dissolved copper concentration in sea water by d.p.c.s.v. at the HMDE (at neutral pH) compares favourably with the d.p.a.s.v. technique at a rotating disk electrode (at low pH) because of the shorter collection period and greater sensitivity.     

Na+ transport across the beta alumina-propylene carbonate interface

Interfacial Na⁺ ion transport between polycrystalline beta alumina and propylene carbonate has been studied using a galvanostatic transient technique which separates interfacial overpotential from bulk resistivity effects. No interfacial polarization is detected during ion entry into beta alumina and exit from beta alumina across a dry interface from 30–1000 μA cm^?2. Transport across an interface contaminated with adsorbed water follows Tafel-type i/E behavior with a transition coefficient (α) of 0.24 and exchange current (i₀) of 3.0×10^?6 A cm^?2 at 23°C. Interfacial transport appears to take place through an intermediate state in which the mobile ion is adsorbed on the interface. Large increases in interfacial polarization occur at both dry and hydrated interfaces for ionic currents exceeding the rate of adsorption or desorption.     

Adsorption monitoring of phospho-l-serine on hydroxyapatite

The adsorption of the aminoacid phospho-l-serine (PLS) on the surface of hydroxyapatite {Ca₅(PO₄)₃OH, HAP}, was investigated using streaming potential measurements. Solutions saturated with respect to HAP, containing different concentrations of PLS, were brought in contact under carefully controlled flow conditions through plugs made of well dispersed HAP powder. The measurement of PLS adsorption during the equilibration of the solute with the HAP substrate, showed a plateau regime corresponding, according to geometrical considerations, to monolayer surface coverage. The PLS uptake measurements on HAP suggested that it is possible to monitor in situ adsorption during the monolayer surface coverage measuring the streaming potential of HAP. Analysis of the surface potential measurements suggested that during the monolayer surface coverage step, the negatively charged (HL^2?) PLS species were adsorbed. The adsorbed HL^2? was located at the inner Helmholtz plane of the electrical double layer, forming surface complexes with the positively charged ≡CaOH ₂ ⁺ sites on the surface of HAP. The rate constants of adsorption and desorption were calculated from the kinetics of adsorption of PLS on HAP.     

Cathodic adsorption stripping analysis of vitamin B12

Amalgam formation preconcentration is usually performed in electrochemical stripping analysis, but adsorption also can be employed for preconcentration. Vitamin B₁₂ is adsorbed on a mercury electrode in the potential region between ca. ?0.3 and ?1.5 V vs. SCE. Vitamin B₁₂ also yields a polarographic catalytic wave at ca. ?1.6 V. It is considered that the adsorbed protonated reduced form of vitamin B₁₂ plays an important role in producing the catalytic current. Stripping analysis for vitamin B₁₂ employing adsorption preconcentration was found to be possible. Ammonium acetate was used as the supporting electrolyte. It was found that the peak for vitamin B₁₂ was maximal in 0.3 M ammonium acetate. The suitable preconcentration potential was ?1.45 V. The preconcentration time was 5 min while stirring the solution. The scan rate used was 50 mV/s, considering the pen speed of the X-Y recorder. The optimum temperature was 35°C. Under the optimum conditions described above, the calibration curve was linear in the concentration range up to 0.1 μ M. The detection limit was 2 n M.