Terminal CH3 Groups Research Articles

Transferable Anisotropic United-Atom Force Field Based on the Mie Potential for Phase Equilibrium Calculations: n-Alkanes and n-Olefins.

A new transferable force field parametrization for n-alkanes and n-olefins is proposed in this work. A united-atom approach is taken, where hydrogen atoms are lumped with neighboring atoms to single interaction sites. A comprehensive study is conducted for alkanes, optimizing van der Waals force field parameters in 6 dimensions. A Mie n-6 potential is considered for the van der Waals interaction, where for n-alkanes we simultaneously optimize the energy parameters ϵCH3 and ϵCH2 as well as the size parameters σCH3 and σCH2 of the CH3(sp(3)) and CH2(sp(3)) groups. Further, the repulsive exponent n of the Mie n-6 potential is varied. Moreover, we investigate the bond length toward the terminal CH3 group as a degree of freedom. According to the AUA (anisotropic united-atom) force field, the bond length between the terminal CH3 group and the neighboring interaction site should be increased by Δl compared with the carbon-carbon distance in order to better account for the hydrogen atoms. The parameter Δl is considered as a degree of freedom. The intramolecular force field parametrization is taken from existing force fields. A single objective function for the optimization is defined as squared relative deviations in vapor pressure and in liquid density of propane, n-butane, n-hexane, and n-octane. A similar study is also done for olefins, where the objective function includes 1-butene, 1-hexene, 1-octene, cis-2-pentene, and trans-2-pentene. Molecular simulations are performed in the grand canonical ensemble with transition-matrix sampling where the phase equilibrium properties are obtained with the histogram reweighting technique. The 6-dimensional optimization of strongly correlated parameters is possible, because the analytic PC-SAFT equation of state is used to locally correlate simulation results. The procedure is iterative but leads to very efficient convergence. An implementation is proposed, where the converged result is not affected (disturbed) by the analytic equation of state. The resulting transferable anisotropic Mie-potential (TAMie) force field shows average relative deviations in vapor pressure of 1.1% and in liquid density of 0.9% for alkanes, and 2% and 1.5% for olefins, respectively, in a wide range of (reduced) temperature, Tr = 0.55-0.97. For substances that were not members of the objective function, the TAMie force field enables predictions of phase equilibrium properties with good accuracy.

From Hydrophobic to Hydrophilic: The Structure and Density of the Hexadecane Droplet/Alkanol/Water Interface

The molecular structure of the hexadecane droplet/alkanol/water interface has been investigated using vibrational sum frequency scattering and second harmonic scattering. This combination of methods allows us to investigate the interfacial alkyl chain conformation of the oil and several alkanols, ranging from 1-pentanol to 1-dodecanol, the orientational distribution of the methyl groups, the surface density of the alkanols, as well as the orientational alignment of water. For the hexadecane/1-dodecanol/water interface, dodecanol alkyl chains form a fluid layer with a wide distribution of tilt angles of the terminal CH3 groups. Indistinguishable spectra are recorded for the alkanol alkyl chains of 1-pentanol, 1-hexanol, 1-octanol, and 1-dodecanol, and alkanols with chain length longer than 6 C atoms all form films with similar densities. In contrast, the alkyl chains of the oil phase are relatively more distorted with respect to the pure oil/water interface for alkanols with shorter chain lengths. The proj...

Molecular dynamics simulation of spin-lattice NMR relaxation in poly-L-lysine dendrimers: manifestation of the semiflexibility effect.

NMR relaxation experiments are widely used to investigate the local orientation mobility in dendrimers. In particular, the NMR method allows one to measure the spin-lattice relaxation rate, 1/T1, which is connected with the orientational autocorrelation function (ACF) of NMR active groups. We calculate the temperature (Θ) and frequency (ω) dependences of the spin-lattice NMR relaxation rates for segments and NMR active CH2 groups in poly-L-lysine (PLL) dendrimers in water, on the basis of full-atomic molecular dynamics simulations. It is shown that the position of the maximum of 1/T1(ω) depends on the location of the segments inside the dendrimer. This dependence of the maximum is explained by the restricted flexibility of the dendrimer. Such behavior has been predicted recently by the analytical theory based on the semiflexible viscoelastic model. The simulated temperature dependences of 1/T1 for terminal and inner groups in PLL dendrimers of n = 2 and n = 4 generations dissolved in water are in good agreement with the NMR experimental data, which have been obtained for these systems previously by us. It is shown that in the case of PLL dendrimers, the traditional procedure of the interpretation of NMR experimental data - when smaller values of 1/T1 correspond to higher orientation mobility - is applicable to the whole accessible frequency interval only for the terminal groups. For the inner groups, this procedure is valid only at low frequencies.

Calcium phosphate nanoparticles functionalized with a dimethacrylate monomer

The synthesis of calcium phosphate nanoparticles may include modifying agents to tailor particle size, reduce agglomeration and add specific functionalities. This study describes the synthesis of dicalcium phosphate dihydrate (DCPD) nanoparticles functionalized with triethylene glycol dimethacrylate (TEGDMA), added to one of the reacting solutions, with the purpose of reducing agglomeration and improving the compatibility with vinyl-based resin matrices. The nanoparticles were characterized by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), elemental analysis, thermogravimetric analysis (TGA), transmission electronic microscopy (TEM), dynamic light scattering (DLS), and surface area (BET). As controls, proprietary DCPD nanoparticles and nanoparticles synthesized without the addition of TEGDMA (“bare”) were subjected to the same analytical methods. XRD revealed a similar crystalline structure of the synthesized materials in comparison to the proprietary nanoparticles. The presence of a TEGDMA layer was confirmed by elemental analysis and TGA, corresponding to a mass fraction of 8.5%. FTIR analysis of the functionalized nanoparticles revealed the suppression of some absorbance bands found in the neat TEGDMA. A chemisorption mechanism between TEGDMA and the surface of primary particles by ion–dipole interaction involving TEGDMA oxyethylene, and also an interaction mechanism between the particle surface and terminal-CH3 groups are proposed. Functionalized nanoparticles showed 3 to 11 times higher surface area than the controls, in agreement with DLS data, indicating lower agglomeration.

Disruption of heme-peptide covalent cross-linking in mammalian peroxidases by hypochlorous acid

Myeloperoxidase (MPO), lactoperoxidase (LPO) and eosinophil peroxidase (EPO) play a central role in oxidative damage in inflammatory disorders by utilizing hydrogen peroxide and halides/pseudo halides to generate the corresponding hypohalous acid. The catalytic sites of these enzymes contain a covalently modified heme group, which is tethered to the polypeptide chain at two ester linkages via the methyl group (MPO, EPO and LPO) and one sulfonium bond via the vinyl group (MPO only). Covalent cross-linking of the catalytic site heme to the polypeptide chain in peroxidases is thought to play a protective role, since it renders the heme moiety less susceptible to the oxidants generated by these enzymes. Mass-spectrometric analysis revealed the following possible pathways by which hypochlorous acid (HOCl) disrupts the heme-protein cross-linking: (1) the methyl-ester bond is cleaved to form an alcohol; (2) the alcohol group undergoes an oxygen elimination reaction via the formation of an aldehyde intermediate or undergoes a demethylation reaction to lose the terminal CH2 group; and (3) the oxidative cleavage of the vinyl-sulfonium linkage. Once the heme moiety is released it undergoes cleavage at the carbon-methyne bridge either along the δ–β or a α–γ axis to form different pyrrole derivatives. These results indicate that covalent cross-linking is not enough to protect the enzymes from HOCl mediated heme destruction and free iron release. Thus, the interactions of mammalian peroxidases with HOCl modulates their activity and sets a stage for initiation of the Fenton reaction, further perpetuating oxidative damage at sites of inflammation.

Delicate Balance of Hydrogen Bonding Forces ind-Threoninol

The seven most stable conformers of D-threoninol (2(S)-amino-1,3(S)-butanediol), a template used for the synthesis of artificial nucleic acids, have been identified and characterized from their pure rotational transitions in the gas phase using chirped-pulse Fourier transform microwave spectroscopy. D-Threoninol is a close analogue of glycerol, differing by substitution of an NH2 group for OH on the C(β) carbon and by the presence of a terminal CH3 group that breaks the symmetry of the carbon framework. Of the seven observed structures, two are H-bonded cycles containing three H-bonds that differ in the direction of the H-bonds in the cycle. The other five are H-bonded chains containing OH···NH···OH H-bonds with different directions along the carbon framework and different dihedral angles along the chain. The two structural types (cycles and chains of H-bonds) are in surprisingly close energetic proximity. Comparison of the rotational constants with the calculated structures at the MP2/6-311++G(d,p) level of theory reveals systematic changes in the H-bond distances that reflect NH2 as a better H-bond acceptor and poorer donor, shrinking the H-bond distances by ∼0.2 Å in the former case and lengthening them by a corresponding amount in the latter. Thus revealed is the subtle effect of asymmetric substitution on the energy landscape of a simple molecule, likely to be important in living systems.

The Cyclization of Alkyl Side Chains of Naphthalenes: The GC/Potential Energies/FTIR Approach

Gas chromatographic measurements of the retention times of alkyl naphthalenes on packed columns with polar and non-polar stationary phases have proven that the logarithm of the relative retention time increases bi-linearly (not linearly) with the number of carbon atoms in a molecule. This is caused by a strong inclination of alkyl side chains toward intramolecular cyclization. A FTIR spectral analysis has shown that longer alkyl side chains of alkyl naphthalenes are cyclized through an interaction between the terminal CH3 group and the aromatic ring. Conventional aromatic-aliphatic molecules thus become new molecules with quasi-alicyclic rings. This, however, alters the effect of non-covalent van der Waals attractive forces both inside and outside the molecules, which is reflected in an exponential increase of the retention times of alkyl naphthalenes with a side chain longer than propyl and in the bi-linearity of the logarithmic dependence of the relative retention times on the number of carbons in the molecule.

Thiol-based molecular overlayers adsorbed on C60: Role of the end-group and charge state on the stability of the complexes

We present pseudo-potential density functional theory calculations dedicated to analyze the stability and electronic properties of thiol-based molecular overlayers adsorbed on C60. We consider short molecules having a S atom as a headgroup, alkyl chains containing one to three C atoms, and a CH3 species as a terminal group. The thiol molecules are bonded to the carbon surface (through the S atom) with adsorption energies that vary in the range of ~1-2 eV and with S-C bond lengths of ~1.8 Å. For neutral C60(SCH3)n complexes, low energy atomic configurations are obtained when the thiol groups are distributed on the surface forming small molecular domains (e.g., pairs, trimers, or tetramer configurations of neighboring thiol molecules). In contrast, less stable random distributions are defined by orientationally disordered overlayers with highly distorted underlying carbon networks. The inclusion of London dispersion interaction slightly affects the structure of the molecular coating but increases the adsorption energies by values as large as 0.3 eV. Interestingly, the relative stability of the previous adsorbed phases differ from the one obtained when considering single sulfur adsorption on C60, a result that reveals the crucial role played by the terminal CH3 groups on the structure of the molecular coating. The positive (negative) charging of the [C60(SCH3)n](±q) complexes, with q as large as 8e, changes the geometrical structure and the chemical nature of the ligand shell inducing lateral molecular displacements, S-S bonding between neighboring thiols, as well as the partial degradation of the molecular coating. Finally, we consider the stability of two-component mixed overlayers formed by the coadsorption of CH3-, OH-, and NH2-terminated alkanethiols of the same length. In agreement with the results found on Au surfaces, we obtain lowest energy atomic configurations when molecular domains of a single component are stabilized on C60, a result that could be of fundamental importance in biomedical applications.

Binuclear methylene and difluoromethylene iron carbonyls: A density functional theory study

The binuclear iron carbonyl derivatives Fe2(CH2)(CO)8 and Fe2(CF2)(CO)8 are known compounds that have been synthesized and characterized structurally by X-ray crystallography. Their low-lying structures have now been investigated using density functional theory. The C2v isomers of Fe2(CH2)(CO)8 with triply and singly bridged Fe–Fe bonds are found to have closely spaced energies. These observations are consistent with the experimental observation of an equilibrium between the Fe2(CH2)(CO)8 isomer with two bridging CO groups and the isomer with all terminal CO groups. The Fe2(CH2)(CO)8 system appears to be analogous to the well-documented Co2(CO)8 system, where an equilibrium between doubly bridged and unbridged isomers is observed experimentally under ambient conditions. In addition to these two low energy Fe2(CH2)(CO)8 structures, a higher energy structure is found in which one of the CO groups has inserted into an Fe–CH2 bond to give a bridging ketene (CH2CO) ligand. The lowest-lying structure for the corresponding fluorinated derivative Fe2(CF2)(CO)8 is the C2v isomer with two symmetrical bridging CO groups as well as a bridging CF2 group. The corresponding Fe2(CF2)(CO)8 isomer with all terminal CO groups, also of C2v symmetry, is also a low-energy structure. In addition to these two low-energy Fe2(CF2)(CO)8 structures with a bridging CF2 group, two higher energy structures are found with terminal CF2 groups. The higher energies of ∼15kcal/mol above the lowest energy structure for these terminal CF2 structures suggest that CF2 groups prefer to be bridging rather than terminal ligands in binuclear metal derivatives. No corresponding Fe2(CH2)(CO)8 structures with terminal CH2 groups were found at accessible energies relative to the lowest energy structure.

Electronic structures and molecular structures of polyynes

AbstractRecently, many interesting materials made from carbon atoms have been discovered such as carbon nanotubes, fullerenes, and graphenes. In this article, we studied electronic structures and molecular structures of polyynes with sp2 carbons on both ends. Polyynes calculated were H2CCnCH2 with various number of n. These molecules have two π systems which are perpendicular to each other and these π systems have their tendency to make double bond nature by Peierls instability but for different alternating bonds. In other words, these two π systems are competing to form the double bond formations. The calculations were performed at Hartree–Fock level geometry optimization with 6‐31G, 6‐311G, and 6‐31G** basis sets. For CH2(C)2mCH2, we have found that there are two metastable structures, one of which is the bond‐alternating structure and another is nearly equi‐bond structure. The former has small highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap while the latter has relatively large gap although it is smaller in comparison with the value of polyyne with sp carbon atoms on both ends: H(C)2mH. The small highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap in bond‐alternating structure is due to the fact that HOMO and MO's near to HOMO have relatively localized electronic distribution in both end regions, and thus, the weak interaction between the both regions leads to the small HOMO–LUMO gap. The stability is larger for the former especially in long polyynes. For CH2(C)2m+1CH2, we have found only one stable structure, that is, equi‐bond structure. This type of polyyne oligomers has two terminal CH2 groups which are perpendicular with each other. It is found that the longer the length of oligomer is, the smaller the HOMO–LUMO gap is as is usually expected. By summarizing these results, we proposed a design for a single‐chain electronically conductive polymers. © 2012 Wiley Periodicals, Inc.

First spectroscopic study on the structural features of dissolved organic matter isolated from rainwater in different seasons

The complexity of rainwater dissolved organic matter (DOM) and the large percentage considered uncharacterized has made it difficult to determine the role of rainwater DOM in regional and global carbon budgets. Recent studies have concentrated on determining the structural characteristics of the bulk DOM in rainwater, but a comparison between the structural characteristics of rainwater DOM from different seasons is lacking. In this work, DOM was extracted from rainwater collected in different seasons by a procedure based on adsorption onto DAX-8 resin and a comparison between the spectroscopic characteristics of extracted DOM was performed using UV–visible, excitation–emission matrix (EEM) fluorescence and 1H NMR spectroscopies. Similar structural characteristics were observed for extracted DOM from the different seasons: high content of aliphatic structures, hydroxy and alkoxy groups, carbonyl groups and unsaturated carbon atoms, and low content of aromatic structures when compared with aliphatic structures. The obtained results suggest a model of chemical structures for the extracted DOM from rainwater, as consisting mainly of aliphatic chains, with COOH, –CH2OH, –COCH3, or –CH3 terminal groups, and with only a minor aromatic component. Moreover, this study suggests that the DOM extracted from rainwater has higher aliphatic character and lower aromatic content than aquatic humic substances. Thus, the chemical characteristics of aquatic humic substances may not be good models for DOM extracted from rainwater.

Selective growth and photoelectrochemical properties of Bi2S3 thin films on functionalized self-assembled monolayers

A convenient method for preparing highly crystalline Bi2S3 thin films by combining chemical bath deposition with self-assembled monolayers (SAMs) is described. The surface morphology, structure and composition of the resultant Bi2S3 thin films were characterized by XPS, XRD and FESEM, respectively. The optical and photoelectrochemical properties of as-prepared Bi2S3 thin films with different reaction times were investigated by UV-vis absorption and photocurrent action spectroscopy. It was demonstrated that these properties were dependent on the film thickness which increased with the deposition time. According to the different deposition mechanisms on the NH2 and CH3 terminal groups, patterned Bi2S3 microarrays with different feature sizes (50, 130, 250 μm) were successfully deposited on –NH2-terminated SAMs regions of –NH2/–CH3 patterned SAMs surfaces. Clearly, with a decrease in feature size, the absorption intensity and photocurrent density of the patterned Bi2S3 thin film increased, which was due to the increased Bi2S3 surface area as well as the increased optical path length within the patterned Bi2S3 thin film, resulting from multiple reflection of incident light. The photocurrent density of the patterned thin film for the 50 μm-size feature exhibited an almost 9 times larger photocurrent density than a similar thin film without patterning. This work indicates that patterned Bi2S3 thin films are attractive systems for surface tailoring and also provide a novel approach to effectively control the photoelectrochemical properties of nanostructured Bi2S3 thin films with promising applications in microsystem devices for solar energy conversion.

Optical investigations of intermolecular electronic interactions and “free” charge carriers in quasi-two-dimensional organic conductors and superconductors of the group κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x

The reflectance spectra and optical conductivity spectra of quasi-two-dimensional organic conductor crystals of the group of compounds κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x with x = 0, 0.40, 0.73, 0.85, and 0.90 in the spectral region from 6 meV to 0.74 eV at temperatures T = 90-20 K have been quantitatively analyzed in the framework of a combined semiempirical model including the cluster (tetramer) theory for strongly correlated electrons coupling with intramolecular vibrations and the Drude theory of free electrons with the aim of studying intermolecular electronic interactions and their influence on the formation of the ground state in these crystals. It has been established that the parameters characterizing the charge transfer between molecules in dimers and tetramers and the vibronic coupling constants are almost identical for the compounds under investigation. A large value of the effective Coulomb repulsion (U/t = 1.3–1.5) indicates that strong electron correlations occur in the compounds both with a metal-insulator phase transition (x = 0, 0.40) and with a metal-superconductor phase transition (x = 0.85, 0.90). The conclusion has been drawn that, at x = 0, electron correlations favor an antiferromagnetic spin ordering and a metal-insulator phase transition, whereas for superconductors (x = 0.85, 0.90), the metal-insulator phase transition is hindered as a result of the structural disorder due to the difference in orientations of the terminal CH2 groups of the BEDT-TTF molecule. It has been shown that quasi-free charge carriers interact with electrons localized on the clusters and do not interact with intramolecular vibrations. It has been noted that the oscillator strength of the observed electronic transitions in the initial metallic band (N eff = 0.38–0.31 per dimer) is considerably less than the corresponding value for free (noninteracting) charge carriers (N = 1). This suggests that the Coulomb correlations and vibronic coupling play a decisive role in the kinetic phenomena observed in the molecular conductors and superconductors under investigation.

Fourier transform infrared spectroscopy investigation of chemical bonding in low-k a-SiC:H thin films

Fourier Transform Infrared (FTIR) Spectroscopy has long been utilized as an analytical technique for qualitatively determining the presence of various different chemical bonds in gasses, liquids, solids, and on surfaces. Most recently, FTIR has been proven to be extremely useful for understanding the different types of bonding present in low dielectric constant “low-k” organosilicate materials. These low-k materials are predominantly utilized in the nanoelectronics industry as the interlayer dielectric material in advanced Cu interconnect structures. In this article, we utilize FTIR to perform a detailed analysis of the changes in chemical bonding that occur in Plasma Enhanced Chemically Vapor Deposited (PECVD) low-k a-SiC:H thin films. PECVD low-k a-SiC:H materials are equally important in advanced Cu interconnects and are utilized as both etch stop and Cu diffusion barrier layers. We specifically investigate the changes that occur in low-k a-SiC:H films as the dielectric constant and mass density of these films are decreased from >7 to <3 and from 2.5 to 1g/cm3 respectively. We show that decreases in mass density and dielectric constant are accompanied by both an increase in terminal SiHx and CHx bonding and a decrease in SiC network bonding. At densities of 1.85g/cm3, the concentration of terminal SiHx bonding peaks and subsequent hydrogen incorporation are achieved predominantly via terminal CH3 groups. Low-k a-SiC:H films with k<3.5 and density <1.3g/cm3 can be achieved via incorporating larger organic phenyl groups but result in non-stoichiometric carbon rich films. Electron beam curing of these lower density a-SiC:H films results in volatilization of the phenyl groups leaving behind nanoporous regions and production of some CCC chain linkages in the network.

Hydride Alkenylcarbyne Osmium Complexes versus Cyclopentadienyl Type Half-Sandwich Ruthenium Derivatives

The dihydride complex OsH2Cl2(PiPr3)2 (1) reacts with 2-methyl-1-hexen-3-yne and 2,4-dimethyl-1,3-pentadiene to give the hydride alkenylcarbyne derivatives OsHCl2{≡CC(Me)═CHR}(PiPr3)2 (R = nPr (2), iPr (3)), which have been characterized by X-ray diffraction analysis. DFT calculations (B3PW91) suggest that the enyne is initially hydrogenated to afford a conjugated diene. The latter evolves into the hydride alkenylcarbyne derivative by means of two hydrogen migrations. The first migration is a 1,4-hydrogen shift within the diene (from the terminal CH2 group to the internal double bond) which takes place through the metal center. The second migration is a 1,2-hydrogen shift from the terminal CH2 group to the osmium atom. In contrast to the case for 1, the ruthenium counterpart RuH2Cl2(PiPr3)2 (16) reacts with 2-methyl-1-hexen-3-yne to give a complex mixture of compounds, from which the derivatives Ru(η5-C5HR1R2R3R4)Cl(PiPr3) (17; R1 = C(CH3)═CH2, R2 = Et, R3 = nPr, R4 = Me) and RuCl2{═C(Et)CH═CMe2}(PiPr3)2 (18, traces) are isolated. Both 17 and 18 have been also characterized by X-ray diffraction analysis. DFT calculations (B3PW91) on the formation of 17 suggest that in the ruthenium case the hydrogenation of the enyne leads to an alkenylcarbene intermediate, which reacts with a second enyne molecule to afford the tetrasubstituted cyclopentadienyl group.

Confirmation of Changes in Human Meibum Lipid Infrared Spectra with Age Using Principal Component Analysis

Purpose: Changes in the phase transition temperatures and conformation of human meibum lipid with age and meibomian gland dysfunction (MGD) have been quantified. Less than 1% of the infrared spectral range was used in the previous studies to demonstrate differences. In this study, the remaining 99% of the spectral frequencies were analyzed to gain insight into changes that occur in meibum with age.Methods: Infrared spectra of meibum from 27 normal donors were acquired. Principal component analysis (PCA) was used to quantify the variance between the spectra.Results: PCA was applied to a set of training spectra of human meibum to predict the age of meibum donors. The plot of predicted age versus actual age was linear, p < 0.001 with a slope of 1.00 and r = 0.909. This indicates that changes in constituents of the meibum spectra (eigenvectors) were due to age-related compositional differences. Eigenvector 1 accounted for 92% of the variance observed among all of the meibum spectra. The spectral features of the two major eigenvectors indicate that with increasing age, the meibum contains more wax, double bonds and terminal CH3 groups, and is less ordered. The environment of the carbonyl band becomes less polar with increasing age. These results are similar to those obtained for human sebum.Conclusions: PCA is an excellent chemometric algorithm that may be used to characterize MGD and age-related changes in human meibum. The eigenvectors that define the variations in the spectra provide clues to the compositional changes that occur in meibum with age.

Investigation of the Effects of Surface Chemistry and Solution Concentration on the Conformation of Adsorbed Proteins Using an Improved Circular Dichroism Method

In this paper we present the development of methods using circular dichroism spectropolarimetry with a custom-designed cuvette to increase the signal-to-noise ratio for the measurement of the secondary structure of adsorbed proteins, thus providing enhanced sensitivity and reproducibility. These methods were then applied to investigate how surface chemistry and solution concentration influence both the amount of adsorbed proteins and their secondary structure. Human fibrinogen and albumin were adsorbed onto alkanethiol self-assembled monolayers (SAMs) on gold with CH3, OCH2-CF3, NH2, COOH, and OH terminal groups from both dilute (0.1 mg/mL) and moderately concentrated (1.0 mg/mL) solutions. An increase in surface hydrophobicity was found to cause an increase in both the amount of the protein adsorbed and the degree of structural change that was caused by the adsorption process, while an increase in solution concentration caused an increase in the amount of protein adsorbed but a decrease in the degree of conformational change, with these effects being more pronounced on the more hydrophobic surfaces. The combined use of these two parameters (i.e., surface chemistry and solution concentration) thus provides ameans of independently varying the degree of structural change following adsorption from the amount of adsorbed protein. Further studies are underway to examine which of these factors most strongly influences platelet response, with the overall goal of developing a better understanding of the fundamental factors governing the hemocompatibility of biomaterial surfaces.

Localization vs Conduction: Anionic Excitations in Alkanethiol Self-Assembled Monolayers

Low-energy (6-11 eV) electron injection into Xe-coated self-assembled alkanethiol monolayers (SAMs) is reported. At most energies, the presence of the Xe film has negligible effect on the incident electrons, which penetrate the overlayer and induce significant C--H bond rupture at the terminal methyl sites and the subsurface methylene sites of the organic substrates. However, irradiation at 7.7 +/- 0.2 eV can lead to resonant electronic excitations of the Xe adsorbates to create transient anionic states in the Xe overlayer. Transfer of anionic excitations from the Xe overlayer to the SAM initially prepares excited anionic states at the terminal CH3 groups and leads to highly selective dissociations at the methyl sites, with negligible conduction along the alkane chain which would lead to subsurface C-H bond rupture at the methylene sites. These results demonstrate that the mobility of electronically excited charged states along the alkanethiol chains is significantly less than that of simple excess electrons and that highly site-selective chemical modifications can be induced by low-energy electrons in these highly homogeneous organic films.

Tuning morphology and magnetic properties of sputtered permalloy by organic underlayers

We report the effect of polarity of self-assembled monolayers on magnetic properties and morphology of a deposited magnetic material. Sputtering of permalloy (Ni79Fe21) on a patterned structure of self-assembled monolayers (SAMs), with −COOH and −CH3 terminal groups, results in the formation of a film on −COOH regions and clusters on −CH3 regions. Further investi-gations reveal that the cluster formation gives rise to superparamagnetism, while the film shows usual ferromagnetic behavior. The observed contrast in morphology and magnetism of permalloy is attributed to different growth mechanisms arising from the difference in reactive nature of the terminal functional groups of the SAMs.

Room-temperature Chemical Vapor Deposition of Aluminum and Aluminum Oxides on Alkanethiolate Self-Assembled Monolayers

We have investigated the reaction of trimethylaluminum (TMA) with −CH3, −OH, and −COOH terminated self-assembled monolayers (SAMs) adsorbed on Au, using time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. TMA is a well-known atomic layer deposition precursor that is employed commercially to deposit compound semiconductors, alumina, and nitrides. We demonstrate that TMA can be employed to deposit both alumina and aluminum on SAMs at room temperature. TMA reacts with −OH and −COOH terminated SAMs to form a surface-bound dimethyl aluminum complex but does not react with −CH3 terminal groups. If deposition is performed in a nitrogen-purged glovebox, an alumina film is grown on −CH3, −OH and −COOH terminated SAMs. The alumina film can be removed from −CH3 terminated SAMs by rinsing with organic solvents. However if the base pressure of the deposition chamber is below 10-8 Torr, a metallic Al overlayer is selectively deposited on −OH and −COOH terminated SAMs, and no reaction is...