Maria Rosa Moncelli

Trends in the adsorption behaviour of n-aliphatic alcohols on mercury from aqueous 0.5 M Na2SO4

Abstract The electrosorption behaviour of n-aliphatic alcohols from n-butanol to n-octanol at the mercury/water interface was determined from capacitive charge measurements by using a computerized chronocoulometric technique. The surface area A occupied by one adsorbed alcohol molecule at maximum coverage was found to increase by about 0.06 nm2 per each CH2 group, thus suggesting strongly that the alcohols are adsorbed on mercury from aqueous solutions of 0.5 M Na2SO4 in a flat orientation up to maximum coverage. This interpretation is further supported by the approximate constancy of the differential capacity at maximum coverage for all alcohols investigated, and by the gradual increase in the coefficient with an increase in chain length, where ΔGoads is the standard Gibbs energy of adsorption and σm is the charge density of maximum adsorption. This adsorption behaviour differs from that from aqueous 0.1 M NaF as reported by B.B. Damaskin, A.A. Survila and L.E. Rybalka (Elektrokhimiya, 3 (1967)146); in fact, the latter behaviour suggests a vertical orientation of the hydrocarbon chains at maximum coverage.

Pre-steady State Electrogenic Events of Ca2+/H+ Exchange and Transport by the Ca2+-ATPase

Native or recombinant SERCA (sarco(endo)plasmic reticulum Ca2+ ATPase) was adsorbed on a solid supported membrane and then activated with Ca2+ and ATP concentration jumps through rapid solution exchange. The resulting electrogenic events were recorded as electrical currents flowing along the external circuit. Current transients were observed following Ca2+ jumps in the absence of ATP and following ATP jumps in the presence of Ca2+. The related charge movements are attributed to Ca2+ reaching its binding sites in the ground state of the enzyme (E1) and to its vectorial release from the enzyme phosphorylated by ATP (E2P). The Ca2+ concentration and pH dependence as well as the time frames of the observed current transients are consistent with equilibrium and pre-steady state biochemical measurements of sequential steps within a single enzymatic cycle. Numerical integration of the current transients recorded at various pH values reveal partial charge compensation by H+ in exchange for Ca2+ at acidic (but not at alkaline) pH. Most interestingly, charge movements induced by Ca2+ and ATP vary over different pH ranges, as the protonation probability of residues involved in Ca2+/H+ exchange is lower in the E1 than in the E2P state. Our single cycle measurements demonstrate that this difference contributes directly to the reduction of Ca2+ affinity produced by ATP utilization and results in the countertransport of two Ca2+ and two H+ within each ATPase cycle at pH 7.0. The effects of site-directed mutations indicate that Glu-771 and Asp-800, within the Ca2+ binding domain, are involved in the observed Ca2+/H+ exchange.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH2) as well as that of UQH2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH2.+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH. neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ.- resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ.- uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.

A novel model of the hanging mercury drop electrode

A novel version of a home-made hanging mercury drop electrode (HME) prepared in our laboratory is presented. Its main technical features are described, as well as the advantages it offers over the commercial ones. With this version the area of the electrode surface can be measured with great accuracy, and the reproducibility of the area from one mercury drop to another is better than 1 × 10−1 cm2, corresponding to less than 1% of the total amount of the surface area of an average mercury drop. The constancy of the area, for time periods of up to 90 min, and the reproducibility from one drop to another make this version of HME suitable for use in the analyical field in the study of adsorption or electron transfer phenomena when it is necessary to know the exact electrode surface area in order to determine surface density charge or differential capacitance values.

Translocation of Platinum Anticancer Drugs by Human Copper ATPases ATP7A and ATP7B

Cisplatin, carboplatin, and oxaliplatin are widely used anticancer drugs. Their efficacy is strongly reduced by development of cell resistance. Down-regulation of CTR1 and up-regulation of the Cu-ATPases, ATP7A and ATP7B, have been associated to augmented drug resistance. To gain information on translocation of Pt drugs by human Cu-ATPases, we performed electrical measurements on the COS-1 cell microsomal fraction, enriched with recombinant ATP7A, ATP7B, and selected mutants, and adsorbed on a solid supported membrane. The experimental results indicate that Pt drugs activate Cu-ATPases and undergo ATP-dependent translocation in a fashion similar to that of Cu. We then used NMR spectroscopy and ESI-MS to determine the binding mode of these drugs to the first N-terminal metal-binding domain of ATP7A (Mnk1).

The influence of diffusion-controlled adsorption upon interfacial tension measurements

When studying the adsorption of highly surface-active organic substances by the drop time method, measurements of the interfacial tension γ must be extended to bulk surfactant concentrations c* less than 10−4 M. At these low concentrations surfactant adsorption unavoidably occurs under diffusion control, which affects the shape of γ versus log c* curves at constant potential. Thus, as log c* is gradually increased, γ shows an abrupt drop from a non-equilibrium value equal to that in the absence of the surfactant to equilibrium values corresponding to full coverage. An approximate expression of γ as a function of log c* in the absence of diffusion equilibrium is derived on the basis of the diffusion-layer concept, under the assumption that the surfactant is adsorbed according to a Frumkin isotherm. Based on the above theoretical treatment, a method is proposed for determining the adsorption coefficient and the attraction factor of the surfactant from γ versus log c* curves. An application of this method to the adsorption of Triton X-100 from 0.1 M HClO4 is provided.

Parthenolide inhibits nociception and neurogenic vasodilatation in the trigeminovascular system by targeting the TRPA1 channel.

Summary Parthenolide, a major constituent of feverfew, acts as a partial agonist of TRPA1. Parthenolide’s ability to target TRPA1 could explain its therapeutic effects on migraine. Abstract Although feverfew has been used for centuries to treat pain and headaches and is recommended for migraine treatment, the mechanism for its protective action remains unknown. Migraine is triggered by calcitonin gene‐related peptide (CGRP) release from trigeminal neurons. Peptidergic sensory neurons express a series of transient receptor potential (TRP) channels, including the ankyrin 1 (TRPA1) channel. Recent findings have identified agents either inhaled from the environment or produced endogenously that are known to trigger migraine or cluster headache attacks, such as TRPA1 simulants. A major constituent of feverfew, parthenolide, may interact with TRPA1 nucleophilic sites, suggesting that feverfew’s antimigraine effect derives from its ability to target TRPA1. We found that parthenolide stimulates recombinant (transfected cells) or natively expressed (rat/mouse trigeminal neurons) TRPA1, where it, however, behaves as a partial agonist. Furthermore, in rodents, after initial stimulation, parthenolide desensitizes the TRPA1 channel and renders peptidergic TRPA1‐expressing nerve terminals unresponsive to any stimulus. This effect of parthenolide abrogates nociceptive responses evoked by stimulation of peripheral trigeminal endings. TRPA1 targeting and neuronal desensitization by parthenolide inhibits CGRP release from trigeminal neurons and CGRP‐mediated meningeal vasodilatation, evoked by either TRPA1 agonists or other unspecific stimuli. TRPA1 partial agonism, together with desensitization and nociceptor defunctionalization, ultimately resulting in inhibition of CGRP release within the trigeminovascular system, may contribute to the antimigraine effect of parthenolide.

A voltammetric study of monolayers and bilayers self-assembled on metal electrodes

Abstract Three types of aliphatic thiols, n-dodecanethiol, n-hexadecanethiol and n-octadecanethiol, differing in their physical state in self-assembled monolayers at ambient temperature were tested from the point of view of their integrity and usefulness as the sub-layer for the second, adjacent layer of phospholipids. These self-assembled monolayers were formed on mercury and polycrystalline gold electrodes in order to assess their electrochemical behavior as monolayers passivating the electrodes against various redox probes present in aqueous phase, such as hexaamineruthenium(III) chloride and benzoquinone, differing in their heterogeneous rate constants and hydrophobic–hydrophilic character. Subsequently, as such a type of hydrophobic alkanethiol monolayer is frequently used as a part of alkanethiol|phospholipid asymmetric bilayer membranes, chemically bound to the metallic surface, the monolayer was covered with the adjacent phosphatidylcholine monolayer, and the electrochemical behavior of such an asymmetric system was again tested. The results obtained show that the liquid-like monolayers, particularly those formed on mercury, possess better passivating properties (much smaller number of defects) as compared to crystalline thiols, also providing a better driving force for the attachment of the second liquid-crystalline phosphatidylcholine monolayer. An interesting observation was also noted that the outer phospholipid monolayer imposes a larger energy barrier to the penetration of benzoquinone that hexaamineruthenium(III) cations.

On the inhibitory effect of neutral organic surfactants upon simple electrode reactions at high surface coverages

Abstract A treatment of the inhibitory effect of neutral organic surfactants at high surface coverages is provided, based on the following assumptions: o (I) The activated complex for the electrode reaction is specifically adsorbed and is the same independent of whether it is formed by displacement of adsorbed solvent molecules or of adsorbed surfactant molecules. (II) The theory of absolute reaction rates applies. (III) The surfactant is adsorbed under equilibrium conditions. With these assumptions, a statistical mechanical treatment of different models of the adsorbed monolayer leads to a general expression for the ratio (kvS=1/kvS=0) of the rate constant at high surface coverages to the corresponding rate constant in the absence of the surfactant. This expression consists of a factor depending upon the bulk concentration of the surfactant, which is identical to that proposed by Lipkowski and Galus [1], a factor depending upon the standard free energy of adsorption of the surfactant at zero coverage, as well as a factor accounting for the lateral interaction energies of adsorbed solvent molecules, surfactant molecules and reacting particles between themselves. The inhibitory effect of aliphatic alcohols upon the kinetics of Cd2+ and Cu2+ electroreduction is examined in the light of this theoretical expression. It is suggested that the energy spent in overcoming the ion-dipole interactions between the charged activated complex and the neighbouring adsorbed water molecules as we pass from a solvent-covered to a surfactant-covered electrode surface is responsible for a decrease in the (kvS=1/kvS=0) ratio by some orders of magnitude.

Surface charge density measurements on mercury electrodes covered by phospholipid monolayers

Abstract A novel method to measure the surface charge density at a hanging mercury drop electrode (HMDE) coated with a selfassembled phospholipid monolayer mimicking a biological membrane is described. The charge density is obtained by analogical integration of the capacitive current which flows at constant applied potential as a consequence of a slight contraction of the mercury drop. The contraction must be carried out while keeping the neck of the lipidcoated mercury drop in contact with the lipid film spread on the surface of the electrolytic solution. The validity of this procedure is tested by comparison with differential capacity measurements carried out at a lipidcoated HMDE, fully immersed into the solution, under otherwise identical conditions. The potential of this method in the investigation of the properties of selfassembled lipid monolayers, either pure or incorporating lipophilic species, is briefly outlined. The method is applied to the determination of the charge density of tetraphenylphosphonium cations adsorbed in the polar head region of a phosphatidylserine monolayer supported by the HMDE.