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Biomedical subjects

Andrew A Gewirth

Publications and source records attributed to Andrew A Gewirth.

At least 19 recordsLinked to original sources
PubMed2006

In situ EC-STM studies of MPS, SPS, and chloride on Cu100: structural studies of accelerators for dual damascene electrodeposition.

Electrochemical, differential capacitance, and in situ electrochemical scanning tunneling microscopy (EC-STM) methods are used to examine the interaction of bis(3-sulfopropyl)-disulfide (SPS) and mercaptopropylsulfonic acid (MPS) with Cu(100) surfaces both in the absence and presence of chloride. Both electrochemical and differential capacitance results are weakly perturbed by the addition of either MPS or SPS in the potential region between -0.2 and -0.5 V versus Ag/AgCl relative to the additive-free case. EC-STM images obtained from solutions of MPS alone exhibit a c(2 x 2) adlattice whereas those from SPS alone yield only the (1 x 1) structure. In the presence of Cl-, both adsorbates evince only a c(2 x 2) adlattice on the Cu(100) surface. The desorption potential of these structure is identical to that found with Cl- alone. These results show that neither MPS nor SPS adsorbs strongly on Cu(100) in the presence of Cl-.

Journal Article↗
PubMed2006

Poly-L-lysine-induced morphology changes in mixed anionic/zwitterionic and neat zwitterionic-supported phospholipid bilayers.

Poly-L-lysine-induced morphological changes in liquid phase supported bilayers consisting of mixed anionic/zwitterionic and neat zwitterionic headgroup phospholipids were studied with atomic force microscopy and epifluorescence microscopy. Results obtained from these studies indicate that poly-L-lysine can induce domains, defects, and aggregate structures on both mixed bilayers and strictly zwitterionic bilayers. The structures formed on liquid phase supported bilayers were observed to be immobile from a timescale of 50 ms to several minutes. We propose that poly-L-lysine of sufficient length interacts with the mica substrate and phospholipids to create the stationary structures noted.

Gels↗
PubMed2006

Mechanism of oxygen electroreduction on gold surfaces in basic media.

The mechanism of the electroreduction of oxygen on Au surfaces in basic media is examined using surface-enhanced Raman scattering (SERS) measurements and density functional theory (DFT) calculations. The spectroscopy reveals superoxide species as a reduction intermediate throughout the oxygen electroreduction, while no peroxide is detected. The spectroscopy also shows the presence of superoxide after the addition of hydrogen peroxide. The calculations show no effect of OH addition to the Au(100) surface with regard to O-O length. These results suggest that the four-electron reduction of O(2) on Au(100) in base arises from a disproportionation mechanism which is enhanced on Au(100) relative to the other two low Miller index faces of Au.

Electrochemistry↗
PubMed2006

Electrostatic stitching in gel-phase supported phospholipid bilayers.

We show that mixing zwitterionic lipids with up to 20% mole % cationic lipids produces gel-phase supported lipid bilayers that are morphologically free of defects detectable using noncontact mode atomic force microscopy (AFM). This contrasts with the observation of massive defects when anionic lipid was added, and also when no charged lipid was added. Infrared measurements of headgroup orientation in the presence of cationic lipid show that the mean headgroup orientation changes only minimally when temperature is lowered from the fluid phase to the gel phase. This is consistent with a tentative explanation, based on simple electrostatic arguments, in which cationic lipids "stitch" the bilayers together. On the functional side, this study demonstrates a simple method by which to minimize defects in gel-supported phospholipid bilayers.

Gels↗
PubMed2005

Potential dependent organization of water at the electrified metal-liquid interface.

In situ infrared visible sum frequency generation spectroscopy (SFG) is used to examine the structure of water at the Ag-water interface in NaF and KF electrolyte solutions. Water is observed in environments associated with both the electrode surface and the diffuse double layer. Peaks are observed that are correlated with low-order water, water interacting with electrolyte ions, specifically adsorbed water to the electrode surface, and hydronium. Spectra obtained from a thiol-modified Ag surface enabled discrimination between surface-bound water and that in the double layer. The water organization is dependent on applied potential, with the observed intensities for specifically adsorbed and ion solvating water diminishing near the pzc.

Journal Article↗
PubMed2005

Potential-dependent adsorption and orientation of a small zwitterion: p-aminobenzoic acid on Ag(111).

We report on the potential-dependent behavior of the zwitterionic molecule p-aminobenzoic acid (PABA) at a Ag(111) electrode surface. Infrared-visible sum frequency generation spectroscopy (SFG) in tandem with electrochemical capacitance and CV measurements are used to study the effects of applied potential on the adsorption and orientation of PABA. Changes in the SFG signal indicate that PABA changes orientation in response to the charge on the electrode surface, orienting one way positive of the potential of zero charge (pzc) and oppositely negative of the pzc. At positive potentials, a phase change is observed associated with the formation of a condensed layer. PABA is observed to remain on the surface at all potentials examined. These results show that the orientation of small molecules with large dipoles, like zwitterions, can be controlled by applied potential.

4-Aminobenzoic Acid↗
PubMed2005

Electroreduction activity of hydrogen peroxide on Pt and Au electrodes.

Hydrogen peroxide electroreduction on both catalytically active Pt and inactive Au surfaces are studied by using both surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculations. SERS measurements on Pt show the presence of Pt-OH at negative potentials, which suggests that hydroxide is formed as an intermediate during the electroreduction process. Additionally, the O-O stretch mode of H(2)O(2) is observed on Pt, which shifts to lower energy as potential is swept negatively, indicating that the O-O bond is elongated. For comparison, there is no variation in the energy of the same O-O mode on Au surfaces, and there is no observation of Au-OH. DFT calculations show that H(2)O(2) adsorption on Pt(110) results in the dissociation of O-O bond and the formation of Pt-OH bond. On Au, O-O bond elongation is calculated to occur only on the (110) face. However, the magnitude of the elongation is much smaller than that found on Pt(110).

Journal Article↗
PubMed2005

Single molecule adhesion measurements reveal two homophilic neural cell adhesion molecule bonds with mechanically distinct properties.

Neural cell adhesion molecule (NCAM) is a cell surface adhesion glycoprotein that plays an important role in the development and stability of nervous tissue. The homophilic binding mechanism of NCAM is still a subject of debate on account of findings that appear to support different mechanisms. This paper describes single molecule force measurements with both full-length NCAM and NCAM mutants that lack different immunoglobulin (Ig) domains. By systematically applying an external, time-dependent force to the bond, we obtained parameters that describe the energy landscape of NCAM-NCAM bonds. Histograms of the rupture forces between the full-length NCAM extracellular domains revealed two binding events, one rupturing at higher forces than the other. These bond rupture data show that the two bonds have the same dissociation rates. Despite the energetic and kinetic similarities, the bond strengths differ significantly, and are mechanically distinct. Measurements with NCAM domain deletion mutants mapped the weaker bond to the Ig1-2 segment, and the stronger bond to the Ig3 domain. Finally, the quantitative agreement between the fragment adhesion and the strengths of both NCAM bonds shows that the domain deletions considered in this study do not alter the intrinsic strengths of either of the two bonds.

Amino Acid Sequence↗
PubMed2005

AFM force measurements between SAM-modified tip and SAM-modified substrate in alkaline solution.

The reversible desorption and adsorption of ethanethiol (ET) and hexadecane thiol (HDT) self-assembled monolayers (SAMs) on gold substrates are addressed with potential-dependent AFM force measurements where both tip and substrate potentials are controlled independently. For HDT-modified tip and substrate, the potential dependence of the force curve corresponds to the observed voltammetric features. The adhesion interaction between HDT-modified tip and substrate exhibits a large adhesion, whereas the adhesion is reduced to one-quarter of its original value after HDT on the substrate is removed. The presence of both attractive features on the approach curve and large adhesion on retraction after thiol desorption are ascribed to micelle formation from the desorbed, insoluble, thiols above the Au surface. For the ET-modified tip and substrate, the force curve evinces time-dependent recovery after the thiol adsorption peak which arises from the finite time of diffusion of the desorbed thiol back to the substrate. However, the force curves exhibit little potential dependence when the ET-desorbed tip is interacted with ET-modified substrate.

Alkalies↗
PubMed2005

Electrocatalysis of peroxide reduction by Au-stabilized, Fe-containing poly(vinylpyridine) films.

We show that poly(vinylpyridine) (PVP) coated glassy carbon surfaces containing Fe(CN)6(3-) exhibit catalytic activity toward electroreduction of H2O2. While Fe(CN)6(3-) is catalytically inactive in solution phase, it exhibits catalytic activity upon incorporation into the PVP film, because film incorporation leads to an open coordination site in the otherwise inert Fe(CN)6(3-) molecule. However, this catalytic activity is quickly lost during H2O2 electroreduction due to leaching of the Fe species from the PVP film. The Fe catalyst in the PVP film could be stabilized by 1 order of magnitude in time by electrodeposition of small Au particles. Characterization of the film using scanning electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy shows that covalent attachment between the Au particle and the Fe-based catalyst is a likely mechanism for catalyst stabilization.

Journal Article↗
PubMed2005

Oxygen electroreduction through a superoxide intermediate on bi-modified Au surfaces.

The mechanism of the electroreduction of oxygen on bare and Bi-submonolayer-modified Au(111) surfaces is examined using surface enhanced Raman scattering (SERS) measurements along with detailed density functional theory (DFT) calculations. The spectroscopy reveals the presence of superoxide-level species at potentials where oxygen is reduced. These species are not present in solutions absent either oxygen or Bi at these potentials. The spectroscopy also reveals the presence of Bi-OH species which are associated with peroxide reduction. Detailed calculations show oxygen associates much more strongly with Bi in the (2 x 2) configuration on Au(111) relative to the bare Au surface. Additionally, the O-O bond is elongated following O2 association, which follows as a consequence of Bi-O bond formation and partial oxidation of the Bi adatom. These results show for the first time that the four-electron electroreduction of oxygen electroreduction occurs via a series pathway on the Bi-modified surface in acid solution.

Journal Article↗
PubMed2005

Single-molecule measurements of the impact of lipid phase behavior on anchor strengths.

This work describes atomic force microscopy studies of the physical parameters determining the strength of lipid anchorage in bilayers as a function of the phase state of the lipid matrix. These investigations used biotinylated lipids and streptavidin-derivatized tips to quantify the lipid pullout force from different lipid matrices. Analysis of the data using both dynamic force spectroscopy and full microscopic models show that the anchorage strength is greater in gel-phase relative to fluid-phase lipids. Additional model parameter estimates provide further insights into the hidden energy barriers that determine the mechanical integrity of lipid anchors in biological membranes.

Animals↗
PubMed2004

Direct visualization of asymmetric behavior in supported lipid bilayers at the gel-fluid phase transition.

We utilize in situ, temperature-dependent atomic force microscopy to examine the gel-fluid phase transition behavior in supported phospholipid bilayers constructed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1,2-dipentadecanoyl-sn-glycero-3-phosphocholine, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. The primary gel-fluid phase transition at T(m) occurs through development of anisotropic cracks in the gel phase, which develop into the fluid phase. At approximately 5 degrees C above T(m), atomic force microscopy studies reveal the presence of a secondary phase transition in all three bilayers studied. The secondary phase transition occurs as a consequence of decoupling between the two leaflets of the bilayer due to enhanced stabilization of the lower leaflet with either the support or the water entrained between the support and the bilayer. Addition of the transmembrane protein gramicidin A or construction of a highly defected gel phase results in elimination of this decoupling and removal of the secondary phase transition.

Gels↗
PubMed2004

Modification of a supported lipid bilayer by polyelectrolyte adsorption.

Addition of a weak polyelectrolyte, poly(methacrylic acid) (PMA), to a supported phospholipid bilayer made from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) depresses the melting temperature and alters the morphology of the bilayer in the gel phase. Ellipsometry measurements show that PMA adsorption lowers the phase transition temperature by 2.4 degrees C. Atomic force microscopy (AFM) showed no visible contrast in the fluid phase (above the melting temperature) but a rich morphology in the gel phase. In the gel phase, adsorption leads to formation of significantly less mobile phospholipid islands and other defects. One consequence of this lower mobility is a decrease in the implied cooperativity number of the phase transition, N, when polymer is added. Additionally, AFM images of the gel-phase bilayer show a highly defected structure that anneals significantly more slowly than in the absence of adsorbed polymer. Tentatively, we suggest that PMA preferentially decorates island and defect edges of the DMPC bilayer.

Adsorption↗
PubMed2004

Organic nanoparticles whose size and rigidity are finely tuned by cross-linking the end groups of dendrimers.

Dendrimers with molecular weights ranging from ca. 2700 to 11 000 and from 16 to 64 homoallyl ether end groups were cross-linked using the Grubbs ring-closing metathesis reaction. A combination of SEC, MALDI-TOF-MS, and AFM were used to characterize the cross-linked nanoparticles. The data suggest a significant decrease in volume with cross-linking and a concomitant increase in rigidity, both of which can be controlled independently with a fair degree of precision.

Cross-Linking Reagents↗
PubMed2004

Infrared-visible sum frequency generation investigation of Cu corrosion inhibition with benzotriazole.

Infrared-visible sum frequency generation spectroscopy is used to investigate the corrosion inhibitor benzotriazole (BTAH) adsorbed on Cu(100) and Cu(111) in acidic solution. Potential-dependent in situ spectra indicate that the adsorbed molecule is the benzotriazole anion (BTA-) at all potentials investigated. The Cu(100) surface is shown to form an ordered adlayer at all potentials probed, while the Cu(111) face is shown to be disordered at negative potentials, but to order with applied positive potential. The ordered adlayer is shown to consist of the BTA- in two configurations, one coordinated to the surface and Cu+ ions in solution and the other coordinated only to the surface. The BTA- coordinated to Cu+ is shown to be more stable with respect to Cl- addition than BTA- coordinated to only the surface. This study demonstrates the viability of using sum frequency generation to study corrosion inhibition in situ.

Journal Article↗
PubMed2003

Potential-dependent reorientation of thiocyanate on Au electrodes.

Thiocyanate (SCN) adsorption on an Au electrode is examined using surface-enhanced Raman scattering (SERS) measurements, along with detailed density functional theory (DFT) calculations. Both the calculation and the spectroscopic measurements show that three different geometries are adopted by SCN adsorption in the potential region studied (0.0 V <or= E <or= 1.2 V vs NHE). At low potential both N-bound and S-bound forms are found, at intermediate potentials around the potential of zero charge (E(pzc)) the S-bound form dominates, and at high potentials SCN associates with the Au surface via a bridging geometry. The Stark slope observed for the C-N stretch in the N- and S-bound forms is positive, while the bridge form exhibits a negative Stark slope. DFT calculations show that the potential-dependent reorientation of SCN arises from overlap of specific SCN-based orbitals with the Fermi level of the Au surface; as the Fermi energy changes, different orbital overlaps are found. The calculations also provide an explanation for the different signs of the Stark slope observed for different geometries. For the end-on adsorption, the lowest unoccupied molecular orbital (LUMO) associated with the C-N bond is antibonding, while the corresponding LUMO in the bridge form is bonding.

Journal Article↗