Chiral vibrational structures of proteins at interfaces probed by sum frequency generation spectroscopy

We review the recent development of chiral sum frequency generation (SFG) spectroscopy and its applications to study chiral vibrational structures at interfaces. This review summarizes observations of chiral SFG signals from various molecular systems and describes the molecular origins of chiral SFG response. It focuses on the chiral vibrational structures of proteins and presents the chiral SFG spectra of proteins at interfaces in the C-H stretch, amide I, and N-H stretch regions. In particular, a combination of chiral amide I and N-H stretches of the peptide backbone provides highly characteristic vibrational signatures, unique to various secondary structures, which demonstrate the capacity of chiral SFG spectroscopy to distinguish protein secondary structures at interfaces. On the basis of these recent developments, we further discuss the advantages of chiral SFG spectroscopy and its potential application in various fields of science and technology. We conclude that chiral SFG spectroscopy can be a new approach to probe chiral vibrational structures of protein at interfaces, providing structural and dynamic information to study in situ and in real time protein structures and dynamics at interfaces.     

Understanding surfaces and buried interfaces of polymer materials at the molecular level using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies on polymer surfaces/interfaces using sum frequency generation (SFG) vibrational spectroscopy. SFG theory, technique, and some experimental details have been presented. The review is focused on the SFG studies on buried interfaces involving polymer materials, such as polymer–water interfaces and polymer–polymer interfaces. Molecular interactions between polymer surfaces and adhesion promoters as well as biological molecules such as proteins and peptides have also been elucidated using SFG. This review demonstrates that SFG is a powerful technique to characterize molecular level structural information of complicated polymer surfaces and interfaces in situ. Copyright © 2006 Society of Chemical Industry     

In situ scanning probe spectroscopy at nanoscale solid/liquid interfaces

Electrochemistry provides unique features for the preparation of low-dimensional structures, but in situ spectroscopy with atomic/molecular resolution at such structures is at present not well established yet. This paper shows that in situ scanning probe spectroscopy at solid/liquid interfaces can be utilized to study electronic properties at nanoscale, if appropriate conditions are applied. Tunneling spectroscopy provides information about tunneling barrier heights and electronic states in the tunneling gap, as shown on Au(1 1 1) substrates, contact spectroscopy allows for transport measurements at single nanostructures, as shown at Au/n-Si(1 1 1) nanodiodes. The influence of the electrolytic environment on spectroscopic investigations is not a principal limitation, but offers additional degrees of freedom, which allow, for example, spectroscopic studies of potential dependent surface phenomena at solid/liquid interfaces.     

Formation of 2D supramolecular architectures at electrochemical solid/liquid interfaces

The controlled formation of supramolecular architectures on chloride pre-covered Cu(1 0 0) has been studied by means of in situ scanning tunneling microscopy (STM) in an electrochemical environment. On top of the c(2 × 2)-Cl layer, ordered arrays of supramolecular cavitand structures could be obtained either by a surface assisted assembly of monomer building-blocks (1,1′-dibenzyl-4,4′-bipyridinium molecules) or by a direct adsorption of supramolecular assemblies (metallo-supramolecular squares) from the solution phase. Besides the omnipresent van-der-Waals-like interactions additional electrostatic interactions between the anionic chloride layer and the positively charged (metallo)-organic molecules are supposed to have strong impact on the 2D phase behavior in both cases.The obtained supramolecular entities with their cavities oriented towards the solution phase can be regarded as potential host assemblies for the specific inclusion of guest molecules.     

Monitoring surfaces on the molecular level during catalytic reactions at high pressure by sum frequency generation vibrational spectroscopy and scanning tunneling microscopy

Sum frequency generation (SFG), using non-linear laser optics, detects vibrational spectra of submonolayer amounts of adsorbates with excellent energy and time resolution. Scanning tunneling spectroscopy (STM) is sensitive to the atomic surface structure; readily imaging defects, steps and kinks as well as stationary adsorbed species. Both of these techniques can be used during reactions at high pressures and temperatures to obtain molecular information in situ. We report studies of propylene hydrogenation over Pt(111) crystal surfaces at atmospheric pressures and 300 K using SFG and STM. Four surface species (2-propyl, -bonded propylene, di -bonded propylene, and propylidyne) were identified; the first two being implicated as reaction intermediates. The platinum surface structure remains unchanged during the reaction, consistent with the structure insensitive nature of olefin hydrogénation. Propylene decomposition induced substantial surface reconstruction.     

Reaction kinetics and in situ sum frequency generation surface vibrational spectroscopy studies of cycloalkene hydrogenation/dehydrogenation on Pt(111): Substituent effects and CO poisoning

The influence of substituent effects and CO poisoning were examined during the hydrogenation/dehydrogenation of cycloalkenes (cyclohexene and 1- and 4-methylcyclohexene) on a Pt(111) single crystal. Reaction rates for both hydrogenation and dehydrogenation decreased when a methyl group was added to the cycloalkene ring. The location of a methyl group relative to the CC double bond was influential in the overall kinetics for both reaction pathways. All cycloalkenes demonstrated “bend-over” Arrhenius behavior, after which rates for hydrogenation and dehydrogenation decreased with increasing temperature (inverse Arrhenius behavior). This is explained in terms of a change in surface coverage of the reactive cycloalkene. The potential importance of hydrogen effects is discussed. Introduction of CO in the Torr pressure range (0.015 Torr) led to a decrease in turnover frequency and increase in apparent activation energy for both the hydrogenation and dehydrogenation of all cycloalkenes. Sum frequency generation (SFG) surface vibrational spectroscopy revealed that upon adsorption, the three cycloalkenes form a surface species with similar molecular structure. SFG results under reaction conditions in the presence of CO demonstrated that the cycloalkene coverage is low on a CO-saturated surface. Substituted cyclohexenes were more sensitive than cyclohexene to the presence of adsorbed CO, with larger increases in the apparent activation energy, especially in the case of dehydrogenation. A qualitative explanation for the changes in activity with temperature and the increase in apparent activation energy for cycloalkene hydrogenation/dehydrogenation in the presence of CO is presented from a thermodynamic and kinetic perspective.     

Enzymes at solid surfaces: Nature of the interfaces and physico-chemical processes

Understanding the behavior of enzymes at solid surfaces requires coping with the complexity of the interfaces. Examples are given to illustrate key considerations: (i) influence of the adventitious contamination; the amount of glucose oxidase adsorbed on stainless steel in conditions producing an electrochemical effect is very low compared to the amount of organic contaminants; (ii) structure (multilayered, random) of an adsorbed phase containing an enzyme and other organic constituents (self-assembled monolayer, adventitious contaminants); (iii) competition between proteins for adsorption and influence of the adsorption procedure (simultaneous or sequential) and (iv) reality vs. expectation in enzyme (glucose oxidase) immobilization by tentative covalent grafting.The study of enzyme adsorption in several model systems (catalase on surface-modified carbon blacks, beta-glucosidase on clay at different pHs, beta-glucosidase on sand and surface-modified sand) showed that a strong adsorption, whether by electrostatic or hydrophobic interactions, provokes an extensive deactivation. After adsorption, which is a quick process, the enzyme activity may continue to decrease both in the adsorbed phase and in the solution owing to dynamic processes which involve the adsorbed enzyme and exchanges between the adsorbed phase and the liquid phase.     

固/液界面纳米气泡形成及稳定性研究进展

固/液界面上形成界面纳米气泡（SNBs）,广泛存在于电催化、流体输送、矿物浮选等领域中,并影响各个过程的效率,因此明确其形成及稳定机理对过程调控具有重要意义。首先从实验观察和模拟计算两个角度,对纳米气泡的研究方法进行探讨,综述了不同气体类型、固体界面性质、液相添加剂下纳米气泡的形成规律。由于目前纳米气泡形成后的稳定性尚不十分明确,主要总结了现阶段广为接受的接触线钉扎稳定机制,并分析了该领域的研究现状。此外,考虑到离子液体作为重要的化工溶剂,概述了该体系中微纳气泡的相关研究。最后简要对未来工作进行了展望,以期为离子液体体系中纳米气泡的研究提供新思路。     

Modelling bentonite–water interactions at high solid/liquid ratios: swelling and diffuse double layer effects

Previous experimental studies on bentonite–water interactions have demonstrated the importance of ion exchange and surface complexation reactions occurring at the clay surface as well as dissolution/precipitation reactions of mineral impurities. Based on these findings, thermodynamic models have been widely used to derive porewater compositions for the compacted bentonite used as backfill material for nuclear repositories. Conventional models typically neglect phenomena important in compacted clays such as anion exclusion induced by swelling of the expandable clay fraction and by the formation of electrical double layers on charged edge surfaces. In this study, we evaluate such phenomena by applying a refined diffuse double layer (DDL) approach to model porewater composition in a compacted bentonite backfill surrounded by argillaceous host rock, as foreseen for the Swiss high-level waste repository. Model calculations also include the effect of water incorporation in the structural interlayers.The results indicate that the conventional model and the refined DDL model without distinction between interlayer and external water only differ slightly. The main buffering reactions include ion exchange of Ca for Na, calcite and gypsum dissolution and deprotonation of surface hydroxyl groups. On the other hand, the calculation accounting for the distinction of external and interlayer water indicates significant anion exclusion effects on the external water composition. Most notably, this leads to an increased salinity and drop in pH.From a performance assessment perspective, however, the differences induced by the inclusion of swelling and diffuse double layer effects are not very significant relative to uncertainties related to system variables, such as the pCO₂ of the host rock. Finally, it should be emphasised that significant uncertainties related to the thermodynamic properties of water in compacted clays, e.g., dielectric constant, are still unresolved and deserve further investigations.     

Conformation and thermodynamics of adsorbed macromolecules at the liquid/solid interface

Calorimetric measurements and results of the adsorption of poly(ethylene glycols) on Aerosil out of solution are reported. With the aid of conformation models for the adsorbed macromolecules and by comparing with the results on low molecular weight model substances, the measured values and their dependence on coverage, molecular weight and solvent are discussed. The resulting structure of the polymer layer, characterized by the adsorbed amount, the number of adhesive segments, the adsorption enthalpy, the thickness and the concentration of the adsorbed layer is verified by i.r. spectroscopic and ellipsometric data.     

New mechanism of gas transport at the interface solid/liquid

It was recently shown that an abnormally fast transport of CO molecules takes place at the electrode/electrolyte interface of Pt and PtRu electrodes in H₂SO₄ and HClO₄ solutions. In the present paper, this phenomenon is tested for other gases, such as hydrogen and oxygen. The fast transport is also observed at the solid/electrolyte solution interface of other electrode materials and at the glass/electrolyte interface. Several experiments are shown, demonstrating that mass transfer takes place at a velocity, which is more than one order of magnitude higher than expected for usual diffusion conditions.Assuming radial mass transfer at the interface of a Pt disc, the activation energy, E_a = 23 kJ mol⁻¹, was calculated from Arrhenius plots. The same value was measured in H₂SO₄ and HClO₄ as supporting electrolytes. The mass transport parameter, Y, at 298 K was 4.8 × 10⁻³ cm² s⁻¹ and 2.9 × 10⁻³ cm² s⁻¹ in 0.5 M H₂SO₄ and 1 M HClO₄ respectively.     

Activated carbon surface heterogeneity seen by parallel probing by inverse liquid chromatography at the solid/liquid interface and by gas adsorption analysis at the solid/gas interface

The energetic surface heterogeneity of four different activated carbons was assessed by the parallel probing at the solid/liquid and solid/gas interfaces. At the solid/liquid interface a method of inverse liquid chromatography, frontal analysis by characteristic point was applied using phenylalanine in water solution as a probe molecule. At the solid/gas interface, argon was used as a probe in the low pressure quasi equilibrium volumetry method. The treatment of the adsorption isotherms by the derivative isotherm summation procedure revealed similar adsorption energy distributions for both argon and phenylalanine. Such an agreement between both methods suggests that, at the solid/liquid interface and on the solid/gas interface, the adsorption was mainly controlled by geometric parameters and no specific interaction was observed and physisorbed water did not played a significant role in adsorption process on three from four studied carbons.     

Electrochemical impedance spectroscopy (EIS) studies of the corrosion of pure Fe and Cr at 600 °C under solid NaCl deposit in water vapor

The corrosion behavior of pure Fe and pure Cr at 600 °C under a deposit of solid NaCl, with and without the presence of water vapor, was studied by using electrochemical impedance spectroscopy (EIS) and mass gain measurements. The mass gain of both metals sharply increased when water vapor was introduced into the system. In EIS measurement, only one capacitive loop obtained on the pure Fe and Cr coated with solid NaCl and gave the information of oxide layer on them. For the oxide in air, there is a good relationship between the R_ox and the reaction rate for both pure Fe and Cr with different oxide time. The lower the R_ox is, the higher the reaction rate is. Although no good relationship can be set up between the R_ox and the reaction rate when water vapor presented, its trend with oxide time for both metals is generally in accordance with that of the corrosion rate measured by the mass gain curves. The electrochemical technique is an effective method for studying corrosion performance at high temperature.