Gediminas Niaura

Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH− ions at copper electrode ☆

The surface of a polycrystalline roughened Cu electrode in 1 M NaOH solution, has been studied in situ using surface-enhanced Raman spectroscopy (SERS). Cu 2 O, adsorbed OH - ions, and water molecules have been detected as the electrode potential was varied from open circuit value to - 1.20 V versus SHE. The vibrational spectrum of Cu 2 O consisted of three main peaks located at 150, 528, and 623 cm -1 . It was found that the intense and narrow feature at 150 cm -1 is highly characteristic, and could be used for SER monitoring of Cu 2 O. Two different states of adsorbed OH - ions, giving Cu-OH vibrations around 450-470 cm -1 and 540-580 cm -1 , have been detected. The distinct nature of the bands has been shown by opposite isotopic frequency shifts changing the solvent from H 2 O to D 2 O. The frequency of the first band decreased by 12 cm -1 , while the frequency of the second band increased by ∼ 35 cm -1 in D 2 O solutions. These differences have been explained in terms of distinct surface ligation and the formation of strong hydrogen bonds between water molecules and the second type of adsorbed OH - ion. Water molecules were observed at the interface at an applied potential - 1.20 V.

Size-dependent neurotoxicity of β-amyloid oligomers

The link between the size of soluble amyloid beta (Abeta) oligomers and their toxicity to rat cerebellar granule cells (CGC) was investigated. Variation in conditions during in vitro oligomerization of Abeta(1-42) resulted in peptide assemblies with different particle size as measured by atomic force microscopy and confirmed by dynamic light scattering and fluorescence correlation spectroscopy. Small oligomers of Abeta(1-42) with a mean particle z-height of 1-2 nm exhibited propensity to bind to phospholipid vesicles and they were the most toxic species that induced rapid neuronal necrosis at submicromolar concentrations whereas the bigger aggregates (z-height above 4-5 nm) did not bind vesicles and did not cause detectable neuronal death. A similar neurotoxic pattern was also observed in primary cultures of cortex neurons whereas Abeta(1-42) oligomers, monomers and fibrils were non-toxic to glial cells in CGC cultures or macrophage J774 cells. However, both oligomeric forms of Abeta(1-42) induced reduction of neuronal cell densities in the CGC cultures.

Structure and properties of tethered bilayer lipid membranes with unsaturated anchor molecules.

The self-assembled monolayers (SAMs) of new lipidic anchor molecule HC18 [Z-20-(Z-octadec-9-enyloxy)-3,6,9,12,15,18,22-heptaoxatetracont-31-ene-1-thiol] and mixed HC18/β-mercaptoethanol (βME) SAMs were studied by spectroscopic ellipsometry, contact angle measurements, reflection-absorption infrared spectroscopy, and electrochemical impedance spectroscopy (EIS) and were evaluated in tethered bilayer lipid membranes (tBLMs). Our data indicate that HC18, containing a double bond in the alkyl segments, forms highly disordered SAMs up to anchor/βME molar fraction ratios of 80/20 and result in tBLMs that exhibit higher lipid diffusion coefficients relative to those of previous anchor compounds with saturated alkyl chains, as determined by fluorescence correlation spectroscopy. EIS data shows the HC18 tBLMs, completed by rapid solvent exchange or vesicle fusion, form more easily than with saturated lipidic anchors, exhibit excellent electrical insulating properties indicating low defect densities, and readily incorporate the pore-forming toxin α-hemolysin. Neutron reflectivity measurements on HC18 tBLMs confirm the formation of complete tBLMs, even at low tether compositions and high ionic lipid compositions. Our data indicate that HC18 results in tBLMs with improved physical properties for the incorporation of integral membrane proteins (IMPs) and that 80% HC18 tBLMs appear to be optimal for practical applications such as biosensors where high electrical insulation and IMP/peptide reconstitution are imperative.

Adsorption of l-histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

Abstract The adsorption of l -histidine on a copper electrode from H 2 O- and D 2 O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO 2− 4 and Cl − ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO 2− 4 and Cl − ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH 2 group and the neutral imidazole ring. The Cl − ions cause the protonation and detachment of the α-NH 2 group from the surface and the formation of the ion pair NH + 3 … Cl − can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO − group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH 2 group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.

Hydration of Lysozyme Studied by Raman Spectroscopy

Hydration plays a fundamental role in maintaining the three-dimensional structure and function of proteins. In this study, Raman spectroscopy was used to probe the hydration induced structural changes at various sites of lysozyme under isothermal conditions in the range of water contents from 0 to 44 wt %. Raman hydration curves were constructed from detailed analysis of marker bands. Transition inflection points (w(m)) and onsets determined from the hydration curves have shown that structural changes start at 7-10 and end at about 35 wt % water. The onset of structural changes coincides with the onset of the broad glass transition earlier observed in this system. The increase of α-helix content starts at very low concentrations of water with w(m) = 12 wt %. Monitoring the development of importance for enzymatic action hydrophobic clusters has revealed wm = 15 wt % and completion of the process at 25 wt %. The parameters of 621 cm(-1) (Phe) and 1448 cm(-1) (CH2 bending) modes were found to be sensitive to hydration, suggesting changes in organization of water molecules near the protein surface. The native structure of lysozyme was achieved at 35 wt % water where its content is high enough for filling the space between lysozyme molecules.

Potential-dependent studies on the interaction between phenylalanine-substituted bombesin fragments and roughened Ag, Au, and Cu electrode surfaces.

In this work, we report systematic surface-enhanced Raman spectroscopy (SERS) and generalized two-dimensional correlation analysis (G2DCA) studies of the structures of five specifically modified phenylalanine-substituted C-terminal bombesin 6-14 fragments (BN(6-14)). The fragments studied have all been tested as chemotherapeutic agents in cancer therapy, and they form amino acid sequences in bombesin: cyclo[d-Phe(6),His(7),Leu(14)]BN(6-14), [D-Phe(6),Leu-NHEt(13),des-Met(14)]BN(6-14), [D-Phe(6),Leu(13)-((R))-p-Cl-Phe(14)]BN(6-14), [D-Phe(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), and [D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14). We adsorbed these fragments onto roughened Ag, Au, and Cu electrode surfaces, using a potential range from -1.200 to 0.400 V, at physiological pH. We compared the adsorption mechanism of each fragment on these substrates, as well any changes observed with varying electrode potential, to determine the relationship between adsorption strength and geometry of each of the peptides wherever it was possible. For example, we showed that none of these fragments directly interact with the Ag, Au, and Cu surfaces via residues of Phe (phenylalanine) and Trp(8) (L-tryptophane at position 8 of the BN amino acid sequence) or by an amide bond, due to a very small shift in wavenumber of their characteristic vibrations. Specific interactions were recognized from the broadening, wavenumber shift, and increase in intensity of the W18 Trp(8) mode near 759 cm(-1) and decrease in nu(12) vibration frequency of the Phe residue. In general, more intense SERS bands were observed due to the Phe ring, compared with the Trp(8) ring, which suggested a preferential adsorption of phenylalanine over tryptophane. For [D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), the data also suggest some interaction of a D-Tyr(6) residue (D-tyrosine at position 6). Finally, only slight rearrangements of these moieties on the substrates are observed with changes in electrode potential.

Potential-dependent characterization of bombesin adsorbed states on roughened Ag, Au, and Cu electrode surfaces at physiological pH.

This paper reports the direct surface-enhanced Raman spectroscopic (SERS) and generalized two-dimensional correlation analysis observations of the different orientations of the neurotransmitter bombesin (BN) chemisorbed on electrochemically roughened Ag, Au, and Cu electrode surfaces at different applied electrode potentials and at physiological pH. The presence of the indole ring of Trp(8) and the amide bond between Gln(7) and Trp(8) of BN on these surfaces generates a specific SERS profile of BN adsorbed on the roughened Ag and Au electrodes that is affected by the electrode potential. Furthermore, for BN on Au, slight changes are observed in the band enhancement in comparison to what is observed for this neurotransmitter immobilized on Ag. In addition, there are larger changes in the spectra triggered by the substitution of Ag with Au electrodes and Ag with Cu electrodes than by substitution of Au with Cu electrodes.

Oxygen electroreduction catalysed by laccase wired to gold nanoparticles via the trinuclear copper cluster

Specific wiring of biocatalysts par excellence, viz. redox enzymes, to an electrode can be exploited in the fabrication of high-performance bioelectronic devices. Here we report oxygen electroreduction catalysed by Didymocrea sp. J6 laccase wired to gold nanoparticles via the trinuclear copper cluster. Bypassing the intramolecular electron transfer, which under certain conditions is the rate-limiting step of oxygen bioelectroreduction, has resulted in the fabrication of a high current density biocathode based on high-redox-potential laccase, which is able to operate in electrolytes with a broad pH range in the presence of high fluoride concentrations.

Potential Induced Changes in Neuromedin B Adsorption on Ag, Au, and Cu Electrodes Monitored by Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS), electrochemistry, and generalized two-dimensional correlation analysis (G2DCA) methods were used to define neuromedin B (NMB) ordered superstructures on Ag, Au, and Cu electrode surfaces at different applied electrode potentials in an aqueous solution at physiological pH. The orientation of NMB and the adsorption mechanism were determined based on the analysis of enhancement, broadness, and shift in wavenumber of particular bands, which allow drawing some conclusions about NMB geometry and changes in this geometry upon change of the electrode type and applied electrode potential. The presented data demonstrated that NMB deposited onto the Ag, Au, and Cu electrode surfaces showed bands due to vibrations of the moieties that were in contact/close proximity to the electrode surfaces and thus were located on the same side of the polypeptide backbone. These included the Phe(9) and Trp(4) rings, the sulfur atom of Met(10), and the -CCN- and -C═O units of Asn(2). However, some subtle variations in the arrangement of these fragments upon changes in the applied electrode potential were distinguished. The Amide-III vibrations exhibited an electrochemical Stark effect (potential dependent frequencies) with Stark tuning slope sensitive to the electrode material. Potential-difference spectrum revealed that the imidazole ring of His(8) was bonded to the Cu electrode surface at relatively positive potentials.

Synthesis of new SAM-forming ferrocene derivatives and their interfacial properties on gold

Abstract We report the synthesis of ferrocene-terminated (C12–C13)-alkylthiols containing an ester group removed from the ferrocene ring by 3-4 carbon atoms. The basic redox properties and some structural features of the self-assembled monolayers formed by these compounds on gold were studied by cyclic voltammetry and surface-enhanced Raman spectroscopy. The monolayers of 9-mercaptanonyl-4′-ferrocenyl butanoate ( 4c ) and -5′-ferrocenyl pentanoate ( 4d ) demonstrated exclusive stability.