Methyl Termination Research Articles

Dissociation characteristics of α,ω-dihydride poly(dimethylsiloxane) ammonium adducts generated by electrospray ionization

Dissociation behavior of ammonium adducts of α,ω-dihydride poly(dimethylsiloxane) (H-PDMS) produced by electrospray ionization was studied in collisional activation conditions. In contrast to MS/MS spectra of synthetic polymers usually displaying fragment series on the whole m/z range, ammonium adducts of H-PDMS were shown to always generate the same product ions regardless of the size of the precursor ion. Based on accurate mass measurement and ab initio calculation, mechanisms could be proposed to account for this MS/MS pattern. A primary reaction consisting of release of ammonia and hydrogen from activated oligomer adducts would be rapidly followed by elimination of a three dimethylsiloxane-containing cyclic species to generate the three main product ions at m/z 207, 281 and 355. Further dissociation of these three species would explain additional signals in MS/MS spectra. Although a similar behavior was previously observed for PDMS bearing methyl terminations (CH3-PDMS), strong variations in product ion relative abundance were measured as the size of oligomeric precursor increased in the case of H-PDMS. Perfect similarity of MS/MS spectra is shown to occur once hydride end-groups are no longer involved in primary dissociation reactions. Measuring the degree of similarity between MS/MS spectra of consecutive oligomers within PDMS distribution indicates a much higher reactivity of hydride terminations towards the adducted ammonium cation, as compared to methyl end-groups.

Air Stable, Efficient Hybrid Photovoltaic Devices Based on Poly(3-hexylthiophene) and Silicon Nanostructures

Efficient, stable hybrid photovoltaic (PV) devices based on poly(3-hexylthiophene) (P3HT) and silicon nanowire arrays (SiNWs) are reported. A two-step, chlorination/methylation procedure is used to convert Si−H bonds into Si−C ones to reduce the velocity of charge recombination at the silicon surface as well as achieve a favorable alignment of band-edge energies. In addition, Pt nanodots (PtNDs) are deposited onto the surface of the SiNWs to further tune the band-edge alignment and passivate nonmethylated silicon sites. Methylated silicon surfaces modified with PtNDs possess a favorable internal electric field in accord with expectations based on the electron affinity (∼3.7 eV) and net positive surface dipole measured on such surfaces by ultraviolet photoemission spectroscopy. This attests to the degree of chemical control that can be exerted over the internal electric field in such systems by surface functionalization. In concert with methyl termination and decoration with PtNDs, hybrid PV devices based ...

Transgenic conversion of omega-6 into omega-3 fatty acids in a mouse model of Parkinson's disease

We have recently identified a neuroprotective role for omega-3 polyunsaturated fatty acids (n-3 PUFAs) in a toxin-induced mouse model of Parkinson's disease (PD). Combined with epidemiological data, these observations suggest that low n-3 PUFA intake is a modifiable environmental risk factor for PD. In order to strengthen these preclinical findings as prerequisite to clinical trials, we further investigated the neuroprotective role of n-3 PUFAs in Fat-1 mice, a transgenic model expressing an n-3 fatty acid desaturase converting n-6 PUFAs into n-3 PUFAs. Here, we report that the expression of the fat-1 transgene increased cortical n-3:n-6 PUFA ratio (+28%), but to a lesser extent than dietary supplementation (92%). Such a limited endogenous production of n-3 PUFAs in the Fat-1 mouse was insufficient to confer neuroprotection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine neurotoxicity as assessed by dopamine levels, tyrosine hydroxylase (TH)-positive neurons and fibers, as well as nigral Nurr1 and dopamine transporter (DAT) mRNA expression. Nevertheless, higher cortical docosahexaenoic acid (DHA) concentrations were positively correlated with markers of nigral dopaminergic neurons such as the number of TH-positive cells, in addition to Nurr1 and DAT mRNA levels. These associations are consistent with the protective role of DHA in a mouse model of PD. Taken together, these data suggest that dietary intake of a preformed DHA supplement is more effective in reaching the brain and achieving neuroprotection in an animal model of PD.

Characterizing stability of “click” modified glass surfaces to common microfabrication conditions and aqueous electrolyte solutions

Microfluidic and nanofluidic systems are dominated by fluid-wall interactions due to enormous surface-area-to-volume ratios in these devices. Therefore, strategies to control wall properties in a reliable and repeatable manner can be important for device operation. Chemical modification of surfaces provides one such method. However, the stability of the surface adhered layers under fabrication and likely device operating conditions have not been evaluated in depth. This paper presents the stability analysis of three surface layers used in the 'click' chemistry methodology for surface modification. The three surface layers have bromo, amine, and methyl termination on glass surfaces. All three surface groups are exposed to various wet and dry conditions including acid, base, solvent, electrolyte buffer solutions, oxidative plasmas, UV light, and thermal processing conditions. Contact angle measurements, X-ray photoelectron spectroscopy, and atomic force microscopy were used to quantify the stability of the adhered surface layers. The data show that the brominated surface was stable to most test conditions, while both the amine and methyl surface layers were stable to a narrower set of test conditions.

PH-triggered changes in the supramolecular self-assembly of Cu(II) malonate complexes

Two different hydrate phases, namely, {[Cu(mal)2](picH)2·2H2O}n (1) and {[Cu(mal)2](picH)2·5H2O}n (2) [malH2 = malonic acid; picH = protonated 2-amino-4-picoline] could be synthesized from purely aqueous media simply by mixing the reactants in stoichiometric ratio but with a slight alteration in the acidity of the reaction media. When the solid-state structure of 1 is compared with the previously reported complex 2, interesting changes in the supramolecular self-assembly are noticed. We found that the observed pH-triggered change is exclusive in the ternary system stated and not for Ni(II), Co(II) or Mn(II). We also noticed the presence of a –CH3⋯Ow interaction between the methyl terminals of protonated picoline molecules and the oxygen atoms of lattice water molecules in 1.

High-resolution photoemission studies of the interfacial reactivity and interfacial energetics of Au and Cu Schottky barriers on methyl-terminated Si(1 1 1) surfaces

The Schottky junction formation by the stepwise evaporation of gold and copper, respectively, onto methyl-terminated silicon, CH 3–Si(1 1 1), was investigated by synchrotron X-ray photoelectron spectroscopy. During the junction formation process, interface reactions occurred as revealed by the appearance of chemically shifted Si 2p components. Upon deposition of Au, the formation of about one monolayer of gold silicide, SiAu 3, with a Si 2p chemical shift of +0.75(2) eV, was observed. The SiAu 3 floated on top of the growing gold layer. Similarly, for the deposition of Cu, the methyl termination layer was partially disrupted, as indicated by the appearance of a −0.28(2) eV chemically shifted Si 2p component attributable to an interfacial copper silicide phase, SiCu 3. Hence, the termination of the Si(1 1 1) surface by methyl groups did not completely prevent interfacial reactions, but did reduce the amount interfacial reaction products as compared to bare Si(1 1 1)-(7 × 7) surfaces. Electron Schottky barrier heights of 0.78(8) eV (Au) and 0.61(8) eV (Cu) were measured. Within the experimental uncertainty the observed Schottky barriers were identical to those ones obtained on non-passivated, (7 × 7)-reconstructed Si(1 1 1) surfaces. Thus, the modification of the electronic properties of the silicon–metal contact requires the complete absence of interfacial reactions.

Control of the Stability, Electron-Transfer Kinetics, and pH-Dependent Energetics of Si/H2O Interfaces through Methyl Termination of Si(111) Surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.

Dielectric properties of self-assembled layers of octadecylamine on mica in dry and humid environments

Atomic force microscopy operating in noncontact electrostatic force mode was used to study the interaction of water with films of alkylamines and alkylsilanes on mica. The films efficiently block water adsorption except in exposed mica areas, where it strongly modifies the mobility of surface ions. We also studied the molecular orientation of octadecylamine molecules forming monolayers and multilayer islands. In monolayer films the molecules bind to mica through the amino group, producing a positive contact potential relative to mica (dipole pointing up). In multilayer films the methyl and amino group terminations are exposed in alternating layers that give rise to alternating values of the contact potential. These findings correlate with low and high friction forces measured in the methyl termination and amino terminations.

Chemical and electronic characterization of methyl-terminated Si(111) surfaces by high-resolution synchrotron photoelectron spectroscopy

The chemical state, electronic properties, and geometric structure of methyl-terminated Si(111) surfaces prepared using a two-step chlorination/alkylation process were investigated using high-resolution synchrotron photoelectron spectroscopy and low-energy electron diffraction methods. The electron diffraction data indicated that the methylated Si surfaces maintained a (1×1) structure, where the dangling bonds of the silicon surface atoms were terminated by methyl groups. The surfaces were stable to annealing at 720 K. The high degree of ordering was reflected in a well-resolved vibrational fine structure of the carbon 1s photoelectron emission, with the fine structure arising from the excitation of C-H stretching vibrations having hnu=0.38±0.01 eV. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of 0.30±0.01 eV relative to bulk Si. Electronically, the Si surface was close to the flat-band condition. The methyl termination produced a surface dipole of –0.4 eV. Surface states related to piCH3 and sigmaSi-C bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si(111) surfaces can thus be achieved by methyl termination using the two-step chlorination/alkylation process.

Inter- and Intramolecular Temperature-Dependent Vibrational Perturbations of Alkanethiol Self-Assembled Monolayers

The infrared spectra of octanethiol, dodecanethiol, and hexadecanethiol (C8, C12, and C16, respectively) monolayers adsorbed on Au(111) textured surfaces have been explored in the 25−300 K regime. The C−H stretching modes typically shift by several wavenumbers to lower frequencies and the intensities increase by as much as 75% as the temperature is decreased, providing evidence of, among other effects, the coupling of these stretching modes with lower energy vibrational modes. In contrast, for all temperatures below 300 K, the positions of the C−H bands of fully hydrogenated C16 shift by several wavenumbers to higher frequencies as the hydrogenated adsorbates are increasingly diluted in a matrix of fully deuterated C16, showing that all bands are subject to intermolecular couplings. The analysis of the behavior of the C−H stretching bands suggests that the temperature dependence of the vibrational frequencies associated with the methylene stretching modes is principally due to intermolecular couplings, whereas the temperature dependence of the vibrations associated with the methyl terminations is largely due to intramolecular couplings. It is suggested that the highly constrained geometry of the isotopically diluted monolayer may provide an environment that is less sensitive to intramolecular couplings with low-frequency modes than that of urea−clathrate-isolated species.

Preparation and Characterization of Self-Assembled Multilayers of Octadecylamine on Mica from Ethanol Solutions

Films of octadecylamine molecules on mica have been prepared from ethanol solutions and characterized by atomic force microscopy. Multilayer films are formed exposing terraces with alternating frictional and wear properties, which are assigned to methyl and amino group terminations. Micellar formation in the ethanol solution and subsequent deposition onto the mica is the proposed growth mechanism. By addition of acid, the ratio between protonated and unprotonated molecules was changed. The influence of this parameter on the structure of the terraces is also reported.

ＮＭＲで観測する高分子電解質中のイオンの運動と拡散

PEO-based polymer electrolytes doped with LiN (SO2CF3)2 were studied by multinuclear NMR, where the motions and diffusion of the polymer, lithium ion and anion were independently measured by 1H, 7Li and 19F NMR, respectively. Since the temperature dependences of the relaxation times, T1, of the 1H and 7Li NMR showed minima, the correlation times of the polymer segmental and lithium hopping motions were obtained. The diffusion phenomena were observed by the pulsed field-gradient spin-echo (PEG) NMR method. Without doping the salt, the branched polymer chains having the methyl terminals diffuse free and the self-diffusion coefficients obtained were time dependent (anomalous diffusion). After doping the lithium salt, the polymer chains showed restricted diffusion. The anion self-diffusion coefficients can be determined and the values are also time dependent. The lithium ion clearly showed the restricted diffusion and the barrier height and the size of the sphere are fluctuating. The lithium ions are located near the PEO chains and can make the cross-linking between the chains which forms a transient sphere of the restricted diffusion of lithium ions and polymer terminals.

Conformational Analysis of Amphiphilic Molecules Incorporated in Hexagonal, Lamellar, and Reversed Hexagonal Aggregates of a Ternary System of Sodium Octanoate, 1-Decanol, and Water by the Rotational Isomeric State Scheme Combined with the Maximum Entropy Method

Conformational and orientational characteristics of amphiphiles incorporated in the hexagonal, lamellar, and reversed hexagonal aggregates formed by a ternary lyotropic system of sodium octanoate, 1-decanol, and water have been investigated. Deuterium NMR quadrupolar splittings observed from the randomly and selectively deuterated compounds were simulated by a combined use of the rotational isomeric state scheme and the maximum entropy method. For the individual mesophases, the bond conformations and orientational order parameters of octanoate and 1-decanol were evaluated and related to the aggregate structures. In the lamellar phase, in particular, the 1-decanol chain was found to exhibit peculiar conformations; the C−C bond nearest the OH group is fixed in the gauche state, the adjacent C−C bond also shows a gauche preference, and the second C−C bond from the methyl terminal has a high gauche probability. It follows that the chain length of 1-decanol is almost equal to that of octanoate in the all-trans conformation. Ab initio molecular orbital calculations at the MP2/6-31+G*//HF/6-31G* level for 1-butanol have revealed the inherent conformational preference of the head portion of the alcohol chains; the first C−C bond from the headgroup prefers the gauche state to the trans state by 0.27 kcal mol-1, and the g± g± states in the first and second C−C bonds from the OH group have a comparatively small free energy of 0.42 kcal mol-1 relative to the tt state.

Brillouin scattering and dielectric relaxation in PPG of different chain lengths and end groups

We have investigated the structural relaxational behavior of poly(propylene glycol), PPG, for polymer chains of increasing number of monomer units, n(=3, 7, 70), and for different end groups (OH and CH3) using Brillouin scattering and dielectric relaxation. In the case of the OH capped systems the relaxation dynamics are rather insensitive to chain length variations, whereas large effects are demonstrated when changing to CH3 end groups. For chains with n≲20, there is a radical decrease in the sound velocity and the average relaxation time, when the hydroxyl terminations are replaced by methyl terminations. By using the nonpolar CH3 endcapping the linking of adjacent chains, as is the case in the hydrogen bonded OH capped systems, is avoided.

Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams

We have conducted a study of electron-stimulated reactions in butanethiol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorbed onto Au/mica substrates, using in situ infrared spectroscopy to quantify the processes; the electron dose dependence of the depletion of various C–H stretching modes has permitted the determination of the first dissociation cross sections for electron stimulated reactions in self-assembled organic monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0–15 eV regime is shown to proceed principally via dissociative electron attachment, thus confirming previous work that directly measured H2 desorption yields during irradiation. The dissociation probabilities exhibit a well-resolved maximum at 10 eV, with a full-width at half-maximum of ∼4 eV. Unlike previous studies, our spectroscopic investigation shows that the dehydrogenation is not uniformly distributed throughout the organic film, but is strongly localized near the methyl terminations of the film. The dissociation cross sections at this interface increase rapidly with increasing chain length. We have shown that these increases are not due to the interaction of the dissociative anionic state with the film via charge-induced dipole forces, nor are they due to interactions with the metal substrate via charge-image charge forces. Our results are consistent with a dipole-image dipole quenching model, whereby the excited state lifetimes are reduced from an estimated ∼26 fs (for a gas-phase electron-alkane collision) to ∼2–10 fs, depending on the chain length. These distance-dependent lifetimes cause the dissociation yields for short-chain systems to be significantly lower than long-chain systems, and it is predicted that the electron-induced dissociation cross sections for alkanethiol monolayers should show much stronger isotopic dependencies than found with the gas-phase alkane species.

Ether Lipids from Thermophilic Archaea

The lipids of the archaebacteria grown at a normal temperature is fundamentally the archaeol in which sn-2 and sn-3 of the glycerol are ether linked with the phytanol of C20, but those of the bacteria grown at high temperature are mainly the caldarchaeol with the structure in which the archaeol of two molecules confronts each hydrocarbon and methyl terminals of hydrocarbon chains are linked.The thermophilic archaea can be separated into two types, i.e., those living in a strong acidic condition and others living around neutral condition, and also, the former consists of tetraether-type lipids without exception. In the bacteria growing in a neutral condition, however, not only the tetraether-type lipids, but also some of those being composed of the diether-type lipids as their main lipids have been detected.Main lipids of the archaea belonging to family Sulfolobaceae possess the calditocaldarchaeol as a core lipid, which formed by substituting the glycerol of the one side of the caldarchaeol with the calditol and it seems highly probable that such characteristic structure is contributed to the acid resistance and the thermostability of these archaebacteria.In addition, it became clear that the polar heads containing phosphorus in lipids of the thermophilic archaea consist of the negative electric charge components such as inositol phosphate.

Mechanism of the γ → α and γ1 → α1 Reversible Solid-State Phase Transitions of Erucic Acid

Two reversible solid-state phase transitions of erucic acid (cis-13-docosenoic acid), the γ → α and γ1 → α1 phase transitions, have been followed by vibrational spectroscopy (IR and Raman) and X-ray diffraction. The crystal structure analyses of the higher temperature phases, α and α1, have been performed. The symmetry of crystal lattices remains unchanged during the both transitions, i.e., the γ and α phases belong to a monoclinic system (P21/a) and the γ1 and α1 phases belong to a triclinic one (P1). At the γ → α transition point, the subcell structure of the methyl-terminal chains is reconstructed, accompanying a large conformational change of cis-olefin groups. On the γ1 → α1 transition, the inclination manner of hydrocarbon chains changes, while there are no significant changes in the subcell structure and the conformation of cis-olefin groups. The conformational disorder at methyl terminals increases continuously for the γ1 → α1 transition, contrary to an abrupt occurrence at the γ → α transition. ...

Molecularly Resolved Surface Superstructures of Self-Assembled Butanethiol Monolayers on Gold

Self-assembled monolayers of butanethiol adsorbed onto highly uniform Au/mica films have been studied using in-air scanning tunneling microscopy (STM) and infrared spectroscopy. STM images taken of fully equilibrated films show molecularly resolved features that are believed to correspond to the methyl terminations of the organic surface. Large domains of organized adsorbates and a variant of the previously reported p×√3 superlattice structure are observed on these surfaces; using this nomenclature, the basic repeat unit of the structure reported herein corresponds to 7×√3. Lateral variations in the elevations of striped structures are tentatively assigned to thiolates bound into fcc sites of the Au(111) substrate. We have also observed molecular structure in regions of the superlattice that were structureless in UHV-STM studies. These observations suggest that the filling factor of the fully equilibrated surfaces is highly sensitive to the manner of sample preparation and storage. Large-scale desorption of thiolates does not appear to be a necessary step in the formation of highly organized films of these short-chain systems when equilibrated in thiol/methanol solutions, and the sample processing history may have significant effects on the ultimate surface topology.

ダイヤモンド表面の水素吸着

Surface vibration of CVD-grown diamond surface is measured by HR-EELS. Hydrogen is adsorbed on the sp3-hybridized carbon. On the (001) 2×1 surface, one C-H stretching vibration appears consistently with the dieter-chain model. On the (111) 1×1 surface, two C-H stretching modes appear, which suggests methyl termination model. But the phonon dispersion in the low energy region is well explained by the monohydride termination model. Uniformity of the surface should be checked to discuss this inconsistency. Hydrogen exchange with the gas-phase and the hydrogen desorption/adsorption processes are also investigated.

Transmembrane diffusion of fluorescent phospholipids in human erythrocytes

The outside-inside passage and transmembrane equilibrium distribution of several amphiphilic fluorescent phospholipids were examined in human erythrocytes. The results were compared with previous kinetic data obtained with spin-labeled phospholipids and with the equilibrium distribution of endogenous lipids in erythrocytes. When a nitro benzoxadiazole (NBD) was at the terminal position of a 6 carbon β-chain, the outside-inside diffusion of the fluorescent phosphatidylserine (PS) analogue was slower, and the plateau lower than with long chain radioactive PS or spin-labeled PS. The corresponding phosphatidylethanolamine (PE) did not flip nor did the phosphatidylcholine (PC) analogue. With a NBD at the 12th carbon of a 18C α-chain, the amino-derivatives behaved more like endogenous PS and PE, i.e. they accumulated rapidly on the inner monolayer; however, the phosphatidylcholine analogue reached a plateau corresponding to 50% inside within 2 h at 37°C, indicative of an abnormal rapid diffusion. In the latter case, changing the β-chain from four to eight carbons had no influence on this rapid diffusion. We conclude that when the NBD is close to the glycerol moiety, it diminishes the affinity of the aminophospholipids for the aminophospholipid translocase. When it is close to the methyl terminal of an acyl chain, there is an acceleration of the spontaneous flip-flop. Presumably the polarity of the NBD is responsible for an unconventional orientation of the flexible acyl chain, thereby causing the transmembrane destabilization of the phospholipid. Overall these results illustrate the respective roles of spontaneous diffusion and translocase activity on transmembrane equilibrium distribution of phospholipids. They also show that NBD derivatives should be used cautiously as indicators of endogenous phospholipids.