455 — Effect of pH on the electroactivity of horse heart cytochrome c

The effect of pH on the electroactivity of horse heart cytochrome c has been studied by differential pulse polarography, differential pulse voltammetry at the gold electrode, cyclic voltammetry and spectro-photometry; the gold electrode has been activated by 4.4′-bipyridine. The experimental results have been interpreted on the basis of the existence of a pH-dependent equilibrium between two electroactive cytochrome c forms; in agreement with the Lambeth et al.'s scheme the existence of transient species can be postulated.From spectrophotometric and differential pulse voltammetric experiments the pK_a value of 8.1±0.1 has been calculated for the equilibrium between the neutral and alkaline cytochrome c forms: state III ? state IV+H⁺.     

Effect of methyl viologen on the electrochemical behavior of denatured cytochrome c at mercury and solid electrodes

Denatured cytochrome c has been studied in 9.5 M urea medium in the presence of methyl viologen, using differential pulse polarography (DPP) and voltammetry (DPV), alternating current polarography (ACP) and cyclic voltammetry (CV). The DPP and CV curves exhibit an additional peak, which is assigned to the catalytic reduction of denatured cytochrome c.     

Polarographic analysis of corticosteroids: Part 6. Mechanism of polarographic electroreduction of some Δ4-3-ketosteroids and Δ1,4-3-ketosteroids

Prednisolone and dexamethasone give waves in d.c. and normal pulse polarography and peaks in differential pulse polarography which correspond to a one-electron uptake. The transfer of the first electron is reversible and at pH < 7 (i₁) is preceded or at pH >7 (i₃) is followed by a proton transfer. Protonation occurs on the carbonyl group and pinacol is formed. At pH >7, wave i₃ is followed by wave i₄ which involves transfer of another proton and electron on the carbonyl group and yields the unsaturated alcohol. At pH <3, the radical formed in the first electron uptake is further protonated and is reduced in wave (i₁₊₂). In tetramethylammonium hydroxide solution, reduction of the side chains on C-17 occurs at more negative potentials. Reduction of testosterone and hydrocortisone follows a similar mechanism (A)—(G), but the second electron uptake in wave i₄ at pH >7 is not observed.     

The kinetics of reduction of palladium (II) complexes with β-alanine at dropping mercury electrode and the complexes’ stability constants

By using dc and ac polarography, the kinetics of electroreduction of the palladium (II) complexes with β-alanine at a dropping mercury electrode was studied in solutions with the palladium (II) concentration from 2 × 10^?5 to 2 × 10^?4 M and variable β-alanine and sodium perchlorate concentrations (pH 6–12). One polarographic wave was observed in solutions with pH 9 and 10 at the β-alanine overall concentration of c _βala = 1 × 10^?3 to 5 × 10^?2 M; two waves, at lower pH or higher c _βala. It was concluded on the formation of different forms of palladium (II) complexes in the studied solutions; the complexes contained two to four β-alanine coordinated anions. Using the limiting diffusion currents for the two waves at pH 9–11 and c _βala = 0.1 and 0.5 M, the stepwise stability constant for the Pd(βala) ₄ ^2? complex was calculated. Using two ac peaks observed at pH 7–8 and c _βala = 1 × 10^?2 to 0.1 M, the stepwise stability constant for the Pd(βala) ₃ ^? . was calculated. The perchlorate ions adsorbed at the dropping mercury electrode, as well as βala^? anions at their higher concentrations, hamper the electroreduction of the palladium (II) complexes with β-alanine.     

A study of the polarographic reduction of methaqualone

Methaqualone [2-methyl-3-o-tolyl-4(3H)-quinazolinone] is reduced at pH 1.5–5 in a single two-electron step. At pH 1.5–3, wave i₁ appears and is gradually replaced by wave i₂ which predomiantes in the solution between pH 4.5 and 6.5. At pH > 5, the height of wave i₂ decreases in the shape of a dissociaton curve with increasing pH. For both waves, the diprotonated form of methaqualone (I_a) is reduced to 1,2,-dihydromethaqualone as the final product. In wave i₁, the monoprotonated form I_b is further protonated at the electrode surface before it accepts the first electron; in wave i₂, the free base form I accepts two protons at the electrode surface before the first electron uptake. Polarographic curves are complicated by the presence of three waves of catalytic hydrogen evolution. Wave i_{1, cata} appears at pH < 5, waves i_2,cata at ?1.5 V and i_3,cata at ?1.7 V at pH > 5. Citrate buffers pH 1.5–3 or Britton-Robinson buffers at pH 2.6–3.6 are most suitable for quantitative work with eitehr d.c. polarography or differential pulse polarography.     

Thermal lens determination of cytochrome c and its NO complex

Spectrophotometric and thermal lens measurements showed that cw laser beam (450–530 nm, up to 100 mW) does not affect the absorption band of cytochrome c. Therefore, thermal lensing is used for determining cytochrome c (III) (c _min = 1 × 10^?7 mol/L at λ = 488.0 nm; c _min = 3 × 10^?8 mol/L at λ = 514.5 nm) and its active form, cytochrome c (II) (c _min = 1 × 10^?8 mol/L at λ = 514.5 nm). The enhancement of the sensitivity of determination of these species as compared with conventional spectrophotometry is more than two orders of magnitude. The optimal conditions for the formation of the NO complex of cytochrome c for its photometric determination were selected: the molar ratio of dodecyl sulfate (a modifying agent) to cytochrome c is 1: 30 at a working wavelength of 560 nm. When exposed to laser radiation, the nitrosyl complex of cytochrome c dissociates to form cytochrome c (III). The decomposition of this complex can be monitored by thermal lensing (514.5 nm) down to a level of 1 × 10^?7 mol/L.     

On the electroreduction of 4-chloro-2,6-diisopropylamino-s-triazine (propazine) on mercury electrodes

This paper presents a polarographic (dc and differential pulse (DP)) study of the reduction of the s-triazine derivative propazine (4-chloro-2,6-diisopropylamino-s-triazine). The study is performed in acidic media (from solutions 2.25 M in H₂SO₄ to pH 5) because no signals were obtained above pH 5 (even at pH values of 11–12). In the pH range 2–4 the polarograms decreased until they vanished. In DP polarography, two main reduction peaks were observed, accompanied by a pre-peak at less negative potentials, and a post-peak at more negative potentials, due to the adsorption of propazine on the electrode. The main peaks corresponded to two-electron reduction processes. At pH below the protonation pK of the triazine ring (about 1.7), the results showed that, in a first stage, propazine suffers a cleavage of the Cl atom via a CEC process (electron transfer placed between two chemical reactions) to yield a dechlorinated intermediate, which is reduced through an irreversible two-electron process, the rate-determining step (r.d.s.) being the second electron transfer. At pH above the pK, a protonation of the triazine ring precedes the reduction process, this reaction being also responsible for the observed decrease in limiting current.     

Polyparaxylylene-CdS nanocomposite films: Optical spectra, photoluminescence, and surface topography

For polyparaxylylene-CdS nanocomposite films prepared by solid-phase cryochemical synthesis, the dependences of the optical absorption spectra, photoluminescence intensity, and surface topography on the CdS concentration were examined. It was revealed that the most significant changes, such as the shift of the position of the exciton absorption band by ～1 eV and the increase of the surface roughness (the magnitude of variation of heights h _max ? h _min along a surface profile) occur within a CdS concentration range of from ～5 to ～8 vol %. The average size of the nanoparticles was determined from the position of the exciton absorption maximum: 2R _np ? ? 3 nm at c ≤ 5 vol % and 2R _np ≥ 5?7 nm at c ≥ 10 vol %. The formation of 3-nm particles at low CdS concentrations was confirmed by the existence of exciton photoluminescence with maximum at 370 nm. Exciton luminescence was not observed at c ≥ 10 vol %. By contrast, at all c values, photoluminescence with maxima at ～520 and ～570 nm, which is usually assigned to interstitial S and Cd atoms, correspondingly, and a the weak emission of unknown nature with a maximum at 545 nm were observed.     

tert-Butylthiacalix[4]arene monolayers as a biomimetic model for the oxidation of antioxidants with cytochrome c

The possibility of designing a model of sensor to antioxidants based on thiacalix[4]arene monolayers with immobilized cytochrome c was shown. The molecular surface area S ₀ of thiacalix[4]arene in the monolayers and the surface pressure coefficient ?dπ/dS (elasticity) reflect changes in the redox state of cytochrome c in the presence of dihydroquercetin and ascorbic acid in the aqueous subphase. The absorption spectra in the visible and UV ranges of solutions of the subphase and transferred thiacalix[4]arene monolayers with immobilized cytochrome c confirm the oxidation of the antioxidants to quinones and formation of the reduced form of cytochrome c.     

EC(EE) processes in the reduction of some 2-methylthio-4,6-di(alkylamino)-1,3,5-triazines on mercury electrodes

Polarographic (direct current, dc, and differential pulse, DP) studies of the electroreduction of the s-triazine derivatives ametryn (2-methylthio-4-ethylamino-6-isopropylamino)-1,3,5-triazine), dimethametryn (2-methylthio-4-ethylamino-6-(1,2-dimethylpropyl) amino-1,3,5-triazine) and simetryn (2-methylthio-4,6-di(ethylamino)-1,3,5-triazine) were made in the acidity range from 2.25 M H₂SO₄ to pH 6.5. Above this last pH value no signals were obtained. In DP polarography, two main reduction peaks were observed, accompanied by a pre-peak due to the adsorption of the herbicides on the electrode. The main peaks corresponded to two-electron irreversible reduction processes, at pH values higher than the protonation pK of the triazine ring (ca. 4). In this pH range, the protonation of the triazine ring preceding the reduction process is responsible for decrease in limiting current. At pH<pK the herbicides suffer a cleavage of the –SCH₃ group via two different intermediates related by a chemical reaction, whose extension depends on the herbicide.     

Electrochemical study and polarographic determination of ranitidine

In the polarographic reduction of ranitidine, an H₂-antagonist of histamine, three waves are observed; their half-wave potentials and limiting currents depend strongly on the pH of the solution. The first and second waves are due to reduction of teh protonated, CHNO₂H⁺, and unprotonated, CHNO₂, nitroethene group of ranitidine, respectively; the origin of the third wave is unknown. The characteristics of the second and third waves are studied in acetic acid/acetate buffer at pH 5.5; the first wave does not appear at this pH. The second wave (E_{$\text{1}\text{2}$} = ?0.90 V, vs. Ag/AgCl) is useful for determining ranitidine in the range 2.4–4.9 × 10^?4 M by direct current polarography and in the range 2.5 × 10^?7?2.05 × 10^?5 M by differential pulse polarography.     

Pulse polarographic determinations of nitrosamines: Part I. Determination of N-nitrosodiethanolamines in grinding fluids

Differential pulse polarography is assessed as amethod for the determination od N-nitrosodiethanolamine (NDEA) in aqueous media. Optimum conditions with respect to pH and supporting electrolyte are found with simple mineral acid solutions (H₂SO₄) at pH 1–2. Detection limits in this medium are of the order 5 × 10^-5 M NDEA. the procedure is applied to the direct determination of NDEA in commercially available grinding fluids which contain, on manufacture, precursors for nitrosamine formation. High concentrations of NDEA were found (75 μM) with an estimated accuracy better than 92% and a precision of ±8%.     

The determination of the ionisation constants of polymaleic acid and the stability constants of its complexes with copper

The ionisation constants of unfractionated polymaleic acid, H₃A, a synthetic analogue of fulvic acid, and the stability constants of its copper complexes were determined by differential pulse polarography. The ionisation constants are: pK₁ 5.16, pK₂ 7.04 and pK₃ 9.82 at 20° C and ionic strength 0.12 M. The stability constant of the CuHA complex, which is the predominant species at pH 5–9, lies in the range log (CuHA) = 7.15–8.7; the mean value is 8.17.     

Determination of pseudouridine at submicromolar concentrations by cathodic stripping voltammetry at a mercury electrode

Pseudouridine (5-ribosyluracil), uridine (N,1-ribosyluracil), deoxyuridine (N,1-deoxyribosyluracil) and uracil are investigated by means of d.c. polarography and by differential and normal pulse polarography. Pseudouridine, which is known to be a cancer marker, yields anodic polarographic currents in the pH range 7–11, whereas uridine and deoxyuridine are inactive under the same conditions. The polarographic response of pseudouridine obtained is due to the formation of a sparingly soluble mercury compound. Pseudouridine can be determined by differential pulse polarography in the concentration range 2–6 × 10^?6 M and by differential-pulse cathodic stripping voltammetry at concentrations two orders of magnitude lower. Small excesses of uridine, deoxyuridine or proteins do not interfere with the determination.     

Electrochemistry and biosensing activity of cytochrome c immobilized on a mesoporous interface assembled from carbon nanospheres

We report on an amperometric biosensor for hydrogen peroxide. It is obtained via layer-by-layer assembly of ordered mesoporous carbon nanospheres and poly(diallyldimethylammonium) on the surface of an indium tin oxide (ITO) glass electrode and subsequent adsorption of cytochrome c. UV–vis absorption spectroscopy was applied to characterize the process of forming the assembled layers. Cyclic voltammetry revealed a direct and quasi-reversible electron transfer between cytochrome c and the surface of the modified ITO electrode. The surface-controlled electron transfer has an apparent heterogeneous electron-transfer rate constant (k _s ) of 5.9?±?0.2?s^?1 in case of the 5-layer electrode. The biosensor displays good electrocatalytic response to the reduction of H₂O₂, and the amperometric signal increase steadily with the concentration of H₂O₂ in the range from 5?μM to 1.5?mM. The detection limit is 1?μM at pH 7.4. The apparent Michaelis-Menten constant (K _m ) of the sensor is 0.53?mM. We assume that the observation of a direct electron transfer of cytochrome c on mesoporous carbon nanospheres may form the basis for a feasible approach for durable and reliable detection of H₂O₂.

The determination of some 2-thiobarbiturates by cathodic stripping voltammetry

Pulse polarography and cyclic voltammetry are employed in studies of the electrochemical behaviour of 5-ethyl-5'-(l-methylbutyl)-2-thiobarbituric acid (I), l-methyl-5-ethyl-5'-(l-methylpropyl)-2-thiobarbituric acid (II) and l,3-dimethyl-5-ethyl-5'-p-chlorophenyl)-2-thiobarbituric acid (III) in the pH range 4–12. All three compounds show anodic and cathodic waves or peaks in this pH range. Compounds (I) and (II) are oxidized at mercury indicator electrodes to produce mercury salts which can adsorb thereon and are thus amenable to cathodic stripping voltammetric analysis (c.s.v.) down to concentrations of the order of 10^-6 M, which is superior to the sensitivities obtained by differential pulse polarography (d.p.p.) based on a reduction peak. Compound (III) oxidizes to produce sulphur which is subsequently plated as HgS. Again the sensitivity of the c.s.v. method is of the order of lO^-6 M and analytically superior to d.p.p. The optimum pH for the three determinations is 8. The determination of (II) in the presence of its oxygenated analogue and metabolite, phemitone, and the effect of chloride ions are reported.     

Determination of Methimazole and Carbimazole Using Polarography and Voltammetry

Abstract

Anodic waves of methimazole (I) (1-methylimidazole-2-thiol) and carbimazole (II) (1-ethoxycarbonyl-3-methyl-2-thio-4-imidazoline) on mercury electrodes correspond to mercury salt formation. Both compounds form in the thiono form a soluble complex at pH < 6, compound (I) at higher pH-values a slightly soluble salt of the thiol form. Electrode processes involving the thiol form are complicated by adsorption. Oxidation at solid electrodes occurs only at potentials more than 0.5 V more positive. For compound (I) spectrophotometry indicated pK_a=12.0 ± 0.2. By d.c. polarography in 0.1 M H₂SO₄ containing 10% ethanol the determination of both compounds is possible between 4 × 10^? and 1 × 10^?3 M, by differential pulse polarography between 1 × 10^? and 1 × 10^?4 M, by differential pulse voltammetry at HMDE between 5 × l0^?7 and 6 × 10^? M.     

A study of the electrochemical reduction of bilirubin and its derivatives

The electrochemical behaviour of bilirubin on mercury electrode in aqueous solution at physiological pH has been investigated by differential pulse polarography. The reduction of bilirubin is marked by two polarographic peaks at about −1.30 and −1.42V versus s.c.e. which however depend on the nature of the substituents in the biladiene skeleton.     

Direct electrochemistry of bacterial cytochrome c551 at surface-modified gold electrodes

The direct electrochemistry of the single heme cytochrome c₅₅₁ from the bacterium Pseudomonas aeruginosa has been investigated at gold electrodes surface-modified through chemisorption of polyfunctional organic molecules. The results have been compared and contrasted with those obtained under the same conditions for the eukaryotic cytochrome c from horse heart. Both cytochromes give a quasi-reversible electrode reaction at pH 6.0 at a modified interface presenting only 4-pyridyl groups to the solution suggesting the occurrence, in both cases, of a hydrogen bonding interaction from lysine side-chains on the protein to pyridyl-nitrogens on the electrode surface. However, in contrast, gold electrodes modified by Pyridine-n-AldehydeThioSemicarbazones (n = 2, 3, 4) give electrochemistry which is strongly isomer-dependent in the case of horse heart cytochrome c but completely isomer-independent in the case of cytochrome c₅₅₁. It is suggested that interaction of the eukaryotic protein with surfaces is dominated by its lysine residues only, but that interaction of the bacterial cytochrome is through hydrogen bonding from the surface to both lysines and carboxylate groups of aspartate residues. This is supported by observation of the loss of cytochrome c₅₅₁ electrochemistry at 4-pyridyl-only modified gold at pH 9.0 compared with the good, quasi-reversible electrochemistry maintained under the same conditions at PATS-4 modified gold. It is concluded that, while the two cytochromes show many similarities with respect to their structures and functions, they have quite different interfacial electron transfer reactions, particularly at PATS-modified electrodes. This may correlate with the known large differences between the two proteins in net electrostatic charge and surface charge distribution.     

A spectroelectrochemical study of carboxymethylated cytochrome-c

Direct electrochemical reduction of dicarboxymethylated cytochrome c [(Cm)-cyt c] has been carried out to investigate the effect of the displacement of methionine 80 (Met 80) from the sixth coordination position on the redox potential of heme iron. Differential pulse and cyclic voltammetry were employed in the present study. At gold microelectrodes or gold-coated RVC electrodes, the reduction process occurs in the absence of mediators. The rate of heterogeneous charge transfer for (Cm)-cyt c appears to be greater than that operative in native cytochrome c. A value of −0.218 versus s.h.e. at neutral pH was determined for the formal standard potential (U′_0,7) from spectroelectrochemical measurements.The large change in U′₀ of the modified cytochrome c with respect to the native protein (ΔU ⋍0.5 V) is in agreement with rearrangement of the tertiary structure induced by rupture of the Met 80-heme iron bond, which leads to a high degree of heme exposure to the solvent. In addition, the results support the important role of Met 80 in assuring stability, through its bond with the heme iron, to the close crevice structure.