Keith R. McCrea

SFG-surface vibrational spectroscopy studies of structure sensitivity and insensitivity in catalytic reactions: cyclohexene dehydrogenation and ethylene hydrogenation on Pt (1 1 1) and Pt (1 0 0) crystal surfaces

Abstract The effect of the surface structure of Pt (1xa01xa01) and Pt (1xa00xa00) has been investigated for cyclohexene hydrogenation and dehydrogenation, and ethylene hydrogenation by using sum frequency generation. Cyclohexene dehydrogenation is a structure sensitive reaction, and the rate was found to proceed more rapidly on the Pt (1xa00xa00) crystal surface than on the Pt (1xa01xa01) crystal surface. On Pt (1xa00xa00), the major reaction intermediate during cyclohexene dehydrogenation was 1,3-cyclohexadiene, whereas on Pt (1xa01xa01), both 1,3- and 1,4-cyclohexadiene were present. Both 1,3- and 1,4-cyclohexadiene can dehydrogenate to form benzene, although the reaction proceeds more rapidly through the 1,3-cyclohexadiene intermediate. Because of this, the structure sensitivity of cyclohexene dehydrogenation is explained by noting that there are both a fast and slow reaction pathway for Pt (1xa01xa01), whereas there is only a fast reaction pathway on Pt (1xa00xa00). Ethylene hydrogenation is a structure insensitive reaction. Both ethylidyne and di-σ-bonded ethylene are present in both Pt (1xa01xa01) and Pt (1xa00xa00) under reaction conditions, although the ratio of the concentrations of the two species are different. The rate of the reaction was found to be 11±1 and 12±1 molecules per site per second for Pt (1xa01xa01) and Pt (1xa00xa00), respectively. Since the reaction rate is essentially the same on the two surfaces, while the concentration of ethylidyne and di-σ-bonded ethylene are different, these species must not be the active species which turnover under catalytic ethylene hydrogenation. The most likely species which turnover are π-bonded ethylene and ethyl, and their concentrations are near the detection limit of SFG.

Assembly and structure of α-helical peptide films on hydrophobic fluorocarbon surfaces

The structure, orientation, and formation of amphiphilic α-helix model peptide films on fluorocarbon surfaces has been monitored with sum frequency generation (SFG) vibrational spectroscopy, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, and x-ray photoelectron spectroscopy (XPS). The α-helix peptide is a 14-mer of hydrophilic lysine and hydrophobic leucine residues with a hydrophobic periodicity of 3.5. This periodicity yields a rigid amphiphilic peptide with leucine and lysine side chains located on opposite sides. XPS composition analysis confirms the formation of a peptide film that covers about 75% of the surface. NEXAFS data are consistent with chemically intact adsorption of the peptides. A weak linear dichroism of the amide π is likely due to the broad distribution of amide bond orientations inherent to the α-helical secondary structure. SFG spectra exhibit strong peaks near 2865 and 2935 cm−1 related to aligned leucine side chains interacting with the hydrophobic surface. Water modes near 3200 and 3400 cm−1 indicate ordering of water molecules in the adsorbed-peptide fluorocarbon surface interfacial region. Amide I peaks observed near 1655 cm−1 confirm that the secondary structure is preserved in the adsorbed peptide. A kinetic study of the film formation process using XPS and SFG showed rapid adsorption of the peptides followed by a longer assembly process. Peptide SFG spectra taken at the air-buffer interface showed features related to well-ordered peptide films. Moving samples through the buffer surface led to the transfer of ordered peptide films onto the substrates.

Sum frequency generation: Surface vibrational spectroscopy studies of catalytic reactions on metal single-crystal surfaces

Abstract Sum frequency generation (SFG) has been used for molecular-level investigations of adsorbates under both ultrahigh vacuum and high-pressure catalytic reaction conditions to characterize several important catalytic reactions on platinum single crystals. These reactions include ethylene hydrogenation, propylene hydrogenation and dehydrogenation, cyclohexene hydrogenation and dehydrogenation, and carbon monoxide oxidation. From SFG spectra, important catalytic reaction intermediates were determined, and possible reaction spectator species were also identified. For ethylene and propylene hydrogenation, important reaction intermediates were determined to be π-bonded ethylene and propylene, respectively. During cyclohexene hydrogenation, 1,3-cyclohexadiene was observed to be the important reaction intermediate on both Pt(111) and Pt(100). By comparing SFGspectra characterizing cyclohexene dehydrogenation on Pt(111) and on Pt(100), it was determined that 1,3-cyclohexadiene is also an important reaction intermediate. Another intermediate, 1,4-cyclohexadiene, was also observed under dehydrogenation conditions on Pt(111) but not on Pt(100). This species was determined to be a spectator species. During carbon monoxide oxidation, incommensurate CO species were determined to be important reaction intermediates.

Multi-technique Characterization of Adsorbed Peptide and Protein Orientation: LK310 and Protein G B1

The ability to orient biologically active proteins on surfaces is a major challenge in the design, construction, and successful deployment of many medical technologies. As methods to orient biomolecules are developed, it is also essential to develop techniques that can an accurately determine the orientation and structure of these materials. In this study, two model protein and peptide systems are presented to highlight the strengths of three surface analysis techniques for characterizing protein films: time-of-flight secondary ion mass spectrometry (ToF-SIMS), sum-frequency generation (SFG) vibrational spectroscopy, and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. First, the orientation of Protein G B1, a rigid 6 kDa domain covalently attached to a maleimide-functionalized self-assembled monolayer, was examined using ToF-SIMS. Although the thickness of the Protein G layer was similar to the ToF-SIMS sampling depth, orientation of Protein G was successfully determined by analyzing the C2H5S+ intensity, a secondary ion derived from a methionine residue located at one end of the protein. Next, the secondary structure of a 13-mer leucine-lysine peptide (LK310) adsorbed onto hydrophilic quartz and hydrophobic fluorocarbon surfaces was examined. SFG spectra indicated that the peptides lysine side chains were ordered on the quartz surface, while the peptides leucine side chains were ordered on the fluorocarbon surface. NEXAFS results provided complementary information about the structure of the LK310 film and the orientations of amide bonds within the LK310 peptide.

Directions in peptide interfacial science

The evolution of biological surface science can be credited to the development of traditional surface-chemistry tools and techniques to investigate molecular and atomic-scale bonding, structure, conformation, physical properties (e.g., chemical, electronic, mechanical), and dynamics of adsorbates at various interfaces:1 Both classical measurements of surface behavior and features (i.e., adsorption isotherms, surface areas, roughness, thickness, and topography) and modern spectroscopic-based techniques that provide information on elemental composition, oxidation state, depth profiling, and distribution of chemical species have shown applicability to the study of biomolecular interactions.1 However, experiments that probe with electrons, atoms or ions require ultrahigh vacuum (UHV) or reduced pressures at the interface, and are thus intrinsically limited with regards to interfacial explorations in an aqueous environment, i.e., the study of at biomolecules the solid/water interface.1

Removal of Carbapenem-Resistant Enterobacteriaceae (CRE) from blood by heparin-functional hemoperfusion media.

Bloodstream infections due to Carbapenem-Resistant Enterobacteriaceae (CRE) are becoming more frequent and are associated with a high mortality. At present, combination antimicrobial therapy yields the best outcomes, but treatment options are limited. Many bacteria utilize heparan sulfate to bind to human cells. We studied the ability of a biomimetic device composed of polyethylene beads with endpoint-attached heparin to bind both sensitive and (CRE) E. coli and Klebsiella pneumoniae from spiked blood samples. Greater than 90% of susceptible, E. coli, CRE E. coli and CRE Klebsiella were removed by the beads. Future studies in human bacteremia with this technology are planned.

High-Pressure CO Dissociation and CO Oxidation Studies on Platinum Single Crystal Surfaces Using Sum Frequency Generation Surface Vibrational Spectroscopy

Using sum frequency generation surface vibrational spectroscopy, platinum single crystal surfaces were investigated at high-pressures and high-temperatures under pure CO or CO and O2 environments. In 40 Ton of CO, the molecule dissociates on the (111), (557) and (100) surfaces of platinum single crystals at 673 K, 548 K, and 500 K, respectively, indicating CO dissociation is structure sensitive. The Pt(111) surface must be heated to a temperature where the surface is roughened creating step and kink sites, which are known to dissociate CO. The stepped Pt(557) surface does not need to be heated as high as Pt(111) to dissociate CO since there are step sites already available on the surface. The outer most surface atoms of Pt(100) are mobile compared to the low energy (111) surface and so the surface can roughen at a much lower temperature than observed on Pt(111). Under 40 Ton of CO and 100 Torr of O2, CO oxidation ignition temperature of 620 K, 640 K and 500 K were observed for Pt(111), Pt(557) and Pt(100), respectively, indicating ignition is also structure sensitive. The ignition temperatures for Pt(111) and Pt(557) are similar because the higher concentration of surface atoms on the (111) terraces, common to both surfaces, are more dominant during oxidation than the step sites. Since both CO dissociation and CO oxidation ignition are structure sensitive and follow the same trend of decreasing temperatures for the two processes, it is likely that CO dissociation is important for the onset of ignition.