Amine Terminated SAMs: Investigating Why Oxygen is Present in these Films

Self-assembled monolayers (SAMs) on gold prepared from amine-terminated alkanethiols have long been employed as model positively charged surfaces. Yet in previous studies significant amounts of unexpected oxygen containing species are always detected in amine terminated SAMs. Thus, the goal of this investigation was to determine the source of these oxygen species and minimize their presence in the SAM. The surface composition, structure, and order of amine-terminated SAMs on Au were characterized by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), sum frequency generation (SFG) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS determined compositions of amine-terminated SAMs in the current study exhibited oxygen concentrations of 2.4 ± 0.4 atomic %, a substantially lower amount of oxygen than reported in previously published studies. High-resolution XPS results from the S(2p), C(1s) and N(1s) regions did not detect any oxidized species. Angle-resolved XPS indicated that the small amount of oxygen detected was located at or near the amine head group. Small amounts of oxidized nitrogen, carbon and sulfur secondary ions, as well as ions attributed to water, were detected in the ToF-SIMS data due to the higher sensitivity of ToF-SIMS. The lack of N-O, S-O, and C-O stretches in the SFG spectra are consistent with the XPS and ToF-SIMS results and together show that oxidation of the amine-terminated thiols alone can only account for, at most, a small fraction of the oxygen detected by XPS. Both the SFG and angle-dependent NEXAFS indicated the presence of gauche defects in the amine SAMs. However, the SFG spectral features near 2865 cm(-1), assigned to the stretch of the methylene group next to the terminal amine unit, demonstrate the SAM is reasonably ordered. The SFG results also show another broad feature near 3200 cm(-1) related to hydrogen-bonded water. From this multi-technique investigation it is clear that the majority of the oxygen detected within these amine-terminated SAMs arises from the presence of oxygen containing adsorbates such as tightly bound water.

Formation of lysozyme oligomers at model cell membranes monitored with sum frequency generation spectroscopy

A growing number of studies suggest that the formation of toxic oligomers, precursors of amyloid fibrils, is initiated at the cell membrane and not in the cytosolic compartments of the cell. Studies of membrane-induced protein oligomerization are challenging due to the difficulties of probing small numbers of proteins present at membrane surfaces. Here, we employ surface-sensitive vibrational sum frequency generation (VSFG) to investigate the secondary structure of lysozyme at the surface of lipid monolayers. We investigate lysozyme aggregation at negatively charged 1,2-dipalmitoyl-sn-glycero-3-(phospho-rac-1-glycerol) (DPPG) lipid monolayers under different pH conditions. The changes in the molecular vibrations of lipids, proteins, and water as a function of pH and surface pressure allow us to simultaneously monitor details of the conformation state of lysozyme, the organization of lipids, and the state of lipid-bound water. At pH = 6 lysozyme induces significant disordering of the lipid layer, and it exists in two states: a monomeric state with a predominantly α-helix content and an oligomeric (za-mer) state. At pH ≤ 3, all membrane-bound lysozyme self-associates into oligomers characterized by an antiparallel β-sheet structure. This is different from the situation in bulk solution, for which circular dichroism (CD) shows that the protein maintains an α-helix conformation, under both neutral and acidic pH conditions. The transition from monomers to oligomers is also associated with a decreased hydration of the lipid monolayer resulting in an increase of the lipid acyl chains ordering. The results indicate that oligomerization requires cooperative action between lysozyme incorporated into the lipid membrane and peripherally adsorbed lysozyme and is associated with the membrane dehydration and lipid reorganization. Membrane-bound oligomers with antiparallel β-sheet structure are found to destabilize lipid membranes.

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 peptide's lysine side chains were ordered on the quartz surface, while the peptide's 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.

The structure of insulin at the air/water interface: monomers or dimers?

The hydrophobic character of the air/water interface affects the oligomeric composition of insulin. By using interface-specific vibrational sum frequency spectroscopy and calculations of insulin monomer and dimer second-order nonlinear susceptibilities χ((2)), we show that insulin monomers segregate to the air/water interface.

Interaction of Amyloid-β-(1-42) Peptide and Its Aggregates with Lipid/Water Interfaces Probed by Vibrational Sum-Frequency Generation Spectroscopy

In this study, we use surface-sensitive vibrational sum-frequency generation (VSFG) spectroscopy to investigate the interaction between model lipid monolayers and Aβ(1-42) in its monomeric and aggregated states. Combining VSFG with atomic force microscopy (AFM) and thioflavin T (ThT) fluorescence measurements, we found that only small aggregates with probably a β-hairpin-like structure adsorbed to the zwitterionic lipid monolayer (DOPC). In contrast, larger aggregates with an extended β-sheet structure adsorbed to a negatively charged lipid monolayer (DOPG). The adsorption of small, initially formed aggregates strongly destabilized both monolayers, but only the DOPC monolayer was completely disrupted. We showed that the intensity of the amide-II' band in achiral (SSP) and chiral (SPP) polarization combinations increased in time when Aβ(1-42) aggregates accumulated at the DOPG monolayer. Nevertheless, almost no adsorption of preformed mature fibrils to DOPG monolayers was detected. By performing spectral VSFG calculations, we revealed a clear correlation between the amide-II' signal and the degree of amyloid aggregates (e.g., oligomers or (proto)fibrils) of various Aβ(1-42) structures. The calculations showed that only structures with a significant amyloid β-sheet content have a strong amide-II' intensity, in line with previous Raman studies. The combination of the presented results substantiates the amide-II(') band as a legitimate amyloid marker.

Probing Albumin Adsorption onto Calcium Phosphates by XPS and ToF-SIMS

In this study the binding and assembly of bovine serum albumin (BSA) onto three different calcium phosphate phases (hydroxyapatite, dibasic calcium phosphate dihydrate, and β-tricalcium phosphate) was investigated using a combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS was used to record adsorption isotherms and to quantify the amount of BSA adsorbed onto the different CaP surfaces. On all three surfaces a monolayer of adsorbed BSA was formed. ToF-SIMS was then used to investigate how the structure of BSA changes upon surface binding. ToF-SIMS data from BSA films on the three CaP surfaces showed intensity differences of secondary ions originating from both hydrophobic and hydrophilic amino acids. For a more quantitative examination of structural changes, we developed a ratio comparing the sum of intensities of secondary ions from hydrophobic and hydrophilic residues. A small, but statistically significant, increase in the value of this ratio (7%) was observed between a BSA film on hydroxyapatite versus dibasic calcium phosphate dihydrate. From this ratio we can make some initial hypotheses about what specific changes in BSA structure relate to these differences observed in the ToF-SIMS data.

Cloning and functional expression of the D-beta-hydroxybutyrate dehydrogenase gene of Rhodobacter sp. DSMZ 12077

Nucleotide sequence and biochemical analysis of D-beta-hydroxybutyrate dehydrogenase (EC 1.1.1.30), isolated from Rhodobacter sp., indicate functional oligomers composed of subunits of 257 amino acids with a calculated M(r) of 26,800 and a pI of 5.90. Compared to mammalian short-chain alcohol dehydrogenases, the bacterial enzyme lacks a C-terminal lipid anchor domain and was found to be highly active upon expression in Escherichia coli even without lipid supplement. The recombinant enzyme could be highly enriched using a single chromatography step and was shown to be stable over a broad range of pH and temperature.

Effect of exercise on upper respiratory tract infection in sedentary subjects

OBJECTIVE

To determine if exercise training affects the severity and duration of a naturally acquired upper respiratory tract infection (URTI) in sedentary subjects.

METHODS

Subjects were sedentary volunteers (two or fewer days a week of exercise for less than 30 minutes a day for the previous three months), 18-29 years of age, with a naturally acquired URTI (three to four days of onset). All subjects were screened-for example, asthma, hay fever-by a doctor and were afebrile. Volunteers were alternately assigned to an exercise (EX) group (four men, seven women) or a non-exercise (NEX) group (three men, eight women). Subjects in the EX group completed 30 minutes of supervised exercise at 70% of target heart rate range for five days of a seven day period. For the initial screening, and every 12 hours, all subjects completed a 13 item symptom severity checklist and a physical activity log. Cold symptom scores were obtained until the subjects were asymptomatic. Significance was set at p</=0.05.

RESULTS

There were no significant differences between EX and NEX group mean symptom scores for the morning and evening reporting periods. There were also no differences between the groups for the mean number of days from the baseline symptom score to when the subjects were asymptomatic. There were no differences between physical activity levels, other than what was assigned in the EX group.

CONCLUSION

Moderate exercise in sedentary subjects with naturally acquired URTI probably does not alter the overall severity and duration of the illness. Previously sedentary people who have acquired a URTI and have just initiated an exercise programme may continue to exercise.

Gramicidin-D-stimulated influx of monovalent cations into plant roots

Gramicidin D and nigericin were found to stimulate K(+) influx into oat roots. Valinomycin and nonactin had little effect on K(+) influx. The region of the root most sensitive to gramicidin was the elongation zone. Monocot roots were more sensitive to gramicidin than dicot roots. At 0.2 mM KCl, gramicidin stimulated K(+) influx by 4- to 8fold over a 30-min absorption period. Although a gramicidin response is detectable within one minute, maximum stimulation occurred after a slight (approximately 2-min) lag period. The gramicidin effect was much greater at 0.2 mM KCl than at 20 mM KCl. Respiratory inhibitors reduced the gramicidin-stimulated K(+) influx by 50-80%. The results are discussed in terms of possible mechanisms of action of the various ionophores on ion transport in roots.

Membrane Structure of Aquaporin Observed with Combined Experimental and Theoretical Sum Frequency Generation Spectroscopy

High-resolution structural information on membrane proteins is essential for understanding cell biology and for the structure-based design of new medical drugs and drug delivery strategies. X-ray diffraction (XRD) can provide angstrom-level information about the structure of membrane proteins, yet for XRD experiments, proteins are removed from their native membrane environment, chemically stabilized, and crystallized, all of which can compromise the conformation. Here, we describe how a combination of surface-sensitive vibrational spectroscopy and molecular dynamics simulations can account for the native membrane environment. We observe the structure of a glycerol facilitator channel (GlpF), an aquaporin membrane channel finely tuned to selectively transport water and glycerol molecules across the membrane barrier. We find subtle but significant differences between the XRD structure and the inferred in situ structure of GlpF.