The effect of platinum on carbon corrosion behavior in polymer electrolyte fuel cells

To assess the catalytic effect of platinum on the corrosion of the high surface area carbon support, single triangular potential sweeps with various upper and lower limits were applied to fuel cells comprising electrodes having different Pt/C compositions. Carbon loss rates in H₂/N₂ and air/air mode were determined by integration of the resulting CO₂ concentration peaks in the exhaust gas of the positive electrode. Generally, the contribution of platinum catalyzed carbon corrosion to total CO₂ evolution was found to decrease with increasing upper potential limit. Similar carbon loss rates obtained for Pt/C and pure carbon electrodes in case of lower potential limits of 1.0 V indicate that the catalytic activity of platinum is substantially lowered by the formation of a passivating oxide layer on the platinum particles. Changes in corrosion behavior in the potential range below 0.6 V, which cannot be attributed to platinum effects, are suggested to originate from modifications in carbon surface oxide composition. Due to the high oxygen equilibrium potential of approximately 1 V, carbon corrosion in air/air mode is significantly influenced by platinum oxide formation. However, the polarization of the negative electrode and the influence of platinum oxidation on the equilibrium potential results in a passivating effect that is less pronounced than expected from measurements in H₂/N₂ mode.     

Simulation of grafted polymers on nanopatterned surfaces

Structural properties of polymer brushes on nanopatterned surfaces in good solvent have been determined by computer simulations. Scaling relations for the brush height and brush width are proposed. The properties of the central part of the patterned brush remain constant as long as the pattern is wider than a few times the brush height. The results agree qualitatively with recent AFM experiments, but some quantitative differences call for a reassessment of experimental procedures.     

Probing colloid-substratum contact stiffness by acoustic sensing in a liquid phase

In a quartz crystal microbalance, particles adhering to a sensor crystal are perturbed around their equilibrium positions via thickness-shear vibrations at the crystal's fundamental frequency and overtones. The amount of adsorbed molecular mass is measured as a shift in resonance frequency. In inertial loading, frequency shifts are negative and proportional to the adsorbed mass, in contrast with "elastic loading", where particles adhere via small contact points. Elastic loading in air yields positive frequency shifts according to a coupled resonance model. We explore here the novel application of a coupled resonance model for colloidal particle adhesion in a liquid phase theoretically and demonstrate its applicability experimentally. Particles with different radii and in the absence and presence of ligand-receptor binding showed evidence of coupled resonance. By plotting the frequency shifts versus the quartz crystal microbalance with dissipation overtone number, frequencies of zero-crossing could be inferred, indicative of adhesive bond stiffness. As a novelty of the model, it points to a circular relation between bandwidth versus frequency shift, with radii indicative of bond stiffness. The model indicates that bond stiffness for bare silica particles adhering on a crystal surface is determined by attractive Lifshitz-van der Waals and ionic-strength-dependent, repulsive electrostatic forces. In the presence of ligand-receptor interactions, softer interfaces develop that yield stiffer bonds due to increased contact areas. In analogy with molecular vibrations, the radii of adhering particles strongly affect the resonance frequencies, while bond stiffness depends on environmental parameters to a larger degree than for molecular adsorption.     

Electropolymerization of ethylene dioxythiophene (EDOT) in micellar aqueous solutions studied by electrochemical quartz crystal microbalance and surface plasmon resonance

Electropolymerization of 3,4-ethylene dioxythiophene (EDOT) on gold electrodes was studied in situ by means of an electrochemical quartz crystal microbalance (EQCM) and by surface plasmon resonance spectroscopy (SPR) using aqueous micellar monomer solutions. Electrodeposition from micellar solutions performed by cyclic voltammetry (CV) displayed an enhanced rate as compared to surfactant concentrations lower than the critical micelle concentration (cmc) or in surfactant-free monomer solutions. EQCM data indicate the occurrence of additional film growth during the reductive scan to large negative potentials which does not occur in electropolymerization experiments using potentiostatic pulses or cyclovoltammetric cycles with less negative potential scans. Our results further suggest that in the case of the anionic surfactant the oxidation potential of the monomer is lowered even at surfactant concentrations below the cmc. AFM investigations show a slightly enhanced roughness of the films obtained from micellar monomer solutions.     

Hydrophobic core substitutions in calbindin D9k: effects on Ca2+ binding and dissociation

Hydrophobic core residues have a marked influence on the Ca2+-binding properties of calbindin D9k, even though there are no direct contacts between these residues and the bound Ca2+ ions. Eleven different mutants with substitutions in the hydrophobic core were produced, and their equilibrium Ca2+-binding constants measured from Ca2+ titrations in the presence of chromophoric chelators. The Ca2+-dissociation rate constants were estimated from Ca2+ titrations followed by 1H NMR1 and were measured more accurately using stopped-flow fluorescence. The parameters were measured at four KCl concentrations to assess the salt dependence of the perturbations. The high similarity between the NMR spectra of mutants and wild-type calbindin D9k suggests that the structure is largely unperturbed by the substitutions. More detailed NMR investigations of the mutant in which Val61 is substituted by Ala showed that the mutation causes only very minimal perturbations in the immediate vicinity of residue 61. Substitutions of alanines or glycines for bulky residues in the center of the core were found to have significant effects on both Ca2+ affinity and dissociation rates. These substitutions caused a reduction in affinity and an increase in off-rate. Small effects, both increases and decreases, were observed for substitutions involving residues far from the Ca2+ sites and toward the outer part of the hydrophobic core. The mutant with the substitution Phe66 --> Trp behaved differently from all other mutants, and displayed a 25-fold increase in overall affinity of binding two Ca2+ ions and a 6-fold reduction in calcium dissociation rate. A strong correlation (R = 0.94) was found between the observed Ca2+-dissociation rates and affinities, as well as between the salt dependence of the off-rate and the distance to the nearest Ca2+-coordinating atom. There was also a strong correlation (R = 0.95) between the Ca2+ affinity and stability of the Ca2+ state and a correlation (R = 0. 69) between the Ca2+ affinity and stability of the apo state, as calculated from the results in the present and preceding paper in this issue [Julenius, K., Thulin, E., Linse, S., and Finn, B. E. (1998) Biochemistry 37, 8915-8925]. The change in salt dependencies of koff and cooperativity were most pronounced for residues completely buried in the core of the protein (solvent accessible surface area approximately 0). Altogether, the results suggest that the hydrophobic core residues promote Ca2+ binding both by contributing to the preformation of the Ca2+ sites in the apo state and by preferentially stabilizing the Ca2+-bound state.     

On the Aggregation of Apolipoprotein A-I

In vivo, apolipoprotein A-I (ApoA-I) is commonly found together with lipids in so-called lipoprotein particles. The protein has also been associated with several diseases—such as atherosclerosis and amyloidosis—where insoluble aggregates containing ApoA-I are deposited in various organs or arteries. The deposited ApoA-I has been found in the form of amyloid fibrils, suggesting that amyloid formation may be involved in the development of these diseases. In the present study we investigated ApoA-I aggregation into amyloid fibrils and other aggregate morphologies. We studied the aggregation of wildtype ApoA-I as well as a disease-associated mutant, ApoA-I K107Δ, under different solution conditions. The aggregation was followed using thioflavin T fluorescence intensity. For selected samples the aggregates formed were characterized in terms of size, secondary structure content, and morphology using circular dichroism spectroscopy, dynamic light scattering, atomic force microscopy and cryo transmission electron microscopy. We find that ApoA-I may form globular protein-only condensates, in which the α-helical conformation of the protein is retained. The protein in its unmodified form appears resistant to amyloid formation; however, the conversion into amyloid fibrils rich in β-sheet is facilitated by oxidation or mutation. In particular, the K107Δ mutant shows higher amyloid formation propensity, and the end state appears to be a co-existence of β-sheet rich amyloid fibrils and α-helix-rich condensates.