Methyl Stretch Research Articles

Interaction of nitrofurantoin with lipid langmuir monolayers as cellular membrane models distinguished with tensiometry and infrared spectroscopy

Knowing how a drug interacts with cell membranes is important to understand and predict its effects at the molecular level. Therefore, we aimed to study the interaction of nitrofurantoin (NFT), a compound with potential antibiotic and antitumor properties, with lipidic biological interfaces using Langmuir monolayers. We employed the phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS), which were spread on the surface of water to form Langmuir films, to investigate the membrane-drug interactions. The interaction of the drug with the lipid monolayers was evaluated by using surface pressure-area isotherms, surface pressure-time kinetic curves, Brewster angle microscopy (BAM), and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). Nitrofurantoin shifted the isotherms to lower DPPC molecular areas, indicating monolayer condensation, and to higher DPPS molecular areas, indicating monolayer expansion. Meanwhile, BAM images showed the appearance of interfacial aggregates for DPPS, but not for DPPC, in the presence of NFT. PM-IRRAS spectra showed that bands related to methylene stretches changed their relative intensities and maximum position related to their asymmetric and symmetric modes for both lipids. This suggested an alteration of the monolayer packing degree and the conformational order of the lipid alkyl chains, which were related to an increase in configurational order for DPPS, but disorder for DPPC. In conclusion, NFT caused distinctive changes in the thermodynamic, morphological, and structural properties of DPPC and DPPS monolayers, which may be associated with its bioactivity in cellular membranes and other lipidic interfaces of pharmaceutical interest.

Vibrational Energy Redistribution between CH Stretching Modes in Alkyl Chain Monolayers Revealed by Time-Resolved Two-Color Pump-Probe Sum Frequency Spectroscopy.

The vibrational dynamics of the various CH stretching modes in a fatty acid Langmuir-Blodgett film was studied using a resonant narrowband infrared (IR) laser pulse for pumping and a broadband femtosecond IR visible pulse pair for detection in a sum frequency spectroscopy setup. The resulting two-dimensional spectra indicate that pumping either the antisymmetric methyl or methylene stretch results in the transfer of energy to the other modes on a time scale faster than 2 ps. This rapid process is followed by energy redistribution to other modes, presumably the bending and internal rotational modes, with a time constant of approximately 85 ps. The formation of gauche defects is not observed within the first 250 ps. The whole spectrum recovers on a time scale of several nanoseconds, indicating dissipation of the excitation energy into the substrate.

The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency Generation Spectroscopy.

The plasmonic amplification of nonlinear vibrational sum frequency spectroscopy (SFG) at the surfaces of gold nanoparticles is systematically investigated by tuning the incident visible wavelength. The SFG spectra of dodecanethiol-coated gold nanoparticles chemically deposited on silicon are recorded for 20 visible wavelengths. The vibrational intensities of thiol methyl stretches extracted from the experimental measurements vary with the visible color of the SFG process and show amplification by coupling to plasmon excitation. Because the enhancement is maximal in the orange-red region rather than in the green, as expected from the dipolar model for surface plasmon resonances, it is attributed mostly to hotspots created in particle multimers, in spite of their low surface densities. A simple model accounting for the longitudinal surface plasmons of multimers allows the recovery of the experimental spectral dispersion.

Orientation of Methylguanidinium Ions at the Water-Air Interface.

We use heterodyne-detected vibrational sum-frequency generation (HD-VSFG) to determine the orientation of the molecular plane of methylguanidinium ions at the surface of aqueous solutions. We measure the VSFG response of the symmetric and antisymmetric methyl stretch vibrations of the methylguanidinium ion with different polarization combinations. We find that for at least 50% of the methylguanidinium ions the molecular plane is at an angle >20° with respect to the surface plane. Hence, for only a minor fraction of the ions does the molecular plane have an orientation (near-)parallel to the surface plane, in contrast to the predictions of recent molecular dynamics simulation studies.

Self-assembly of alkanethiolates directs sulfur bonding with GaAs(100)

Molecules that contain linear alkane chains self-assemble on a variety of surfaces changing the degree of wetting, lubricity, and reactivity. We report on the reoxidation of GaAs(100) in air after adsorbing five alkanethiols (CnH2n+1-SH where n=3, 6, 12, 18, 20) and one alkanedithiol (HS-(CH2)8-SH) deposited from the liquid phase. The alignment of the alkane chains forms a self-assembled layer, however, air diffuses readily through the carbon layer and reaches the surface. The impact of alignment is to improve the bonding of sulfur with the surface atoms which reduces the oxidation rate based on fitting the data to a reaction-diffusion model. The layer thickness and molecular density scale linearly with the number of carbon atoms in the alkane chain. The thickness of the alkanethiolate (RS−) layer grows by 0.87±0.06Å for each C atom in the chain and the surface density by 0.13±0.03molecule pernm2 per C atom up to a coverage of 5.0molecules/nm2 for n=20 or 0.8 monolayer. The surface coverage increases with length because interactions between methylene (CH2) groups in neighboring chains reduce the tilt angle of the molecules with the surface normal. The tight packing yields areas per alkanethiolate as low as 20Å2 for n=20. The amount of C in the layer divided by the chain length is approximately constant up to n=12 but increases sharply by a factor of 2–4× for n=18 and 20 based on the C 1s X-ray photoelectron spectroscopy (XPS) peak. Fourier transform infrared (FTIR) spectroscopy shows that the asymmetric methylene stretch shifts continuously to lower wavenumber and the relative peak area increases sharply with the length of the alkane chain. Fitting the data to a reaction-diffusion model shows that for times less than 30min the surface oxide coverage does not depend on the thickness of the self-assembled layer nor the diffusivity of oxygen through the layer. Instead increasing the layer thickness makes more S available for bonding with the predominately As termination and reduces the rate coefficient for reaction of oxygen with the GaAs surface.

Self-assembly of a 1-eicosanethiolate layer on InSb(100)

1-Eicosanethiolate molecules form relatively weak bonds with the surface of InSb(100) limiting the order of the self-assembled monolayer despite the long length of the alkyl chain. Heating to only 225°C in vacuum completely desorbed the eicosanethiolate layer from the surface based on X-ray photoelectron spectroscopy. Even after deposition times as long as 20h in ethanol, the asymmetric methylene stretch was at 2925cm−1 in the attenuated total reflection Fourier transform infrared spectrum, which is indicative of alkane chains that are incompletely ordered. Atomic force microscopy images combined with ellipsometry showed that the eicosanethiolate layer conformed to the rough InSb(100) starting surface (2.3±0.2nm RMS). The reoxidation kinetics in air of InSb(100) and InSb(111)B covered with eicosanethiolate layers was the same despite the lower surface roughness of the latter (0.64±0.14nm). The bond that the S head group makes with the substrate is the primary factor that determines the cohesiveness of the molecules on the surface. Although interactions between the alkane chains in the layer are sufficient to form a self-assembled layer, the fluidity of the molecules in the layer compromised the chemical passivation of the surface resulting in reoxidation in air after 20min.

Orientational Time Correlation Functions for Vibrational Sum-Frequency Generation. 3. Methanol

Molecular dynamics simulations have been used to study the orientational dynamics of methanol at the liquid/vapor and liquid/silica interfaces. Orientational time-correlation functions for the symmetric and asymmetric methyl stretches of methanol have been calculated to assess the role of reorientation in the vibrational sum-frequency generation (VSFG) spectroscopy of these modes at the two interfaces. We find that internal methyl rotation plays a significant role in suppressing the intensity of the asymmetric methyl stretches at the liquid/vapor interface. The broad orientational distribution of the methyl groups and the properties of the Raman spectra of the asymmetric stretches are also major contributors to the low intensity of these modes in VSFG spectra at this interface. We find that after an initial rapid, inertial decay, there is little coupling among the orientational degrees of freedom of the methyl group, which suggests that time-dependent VSFG spectroscopy may be used to probe interfacial ori...

Quantifying Surfactant Alkyl Chain Orientation and Conformational Order from Sum Frequency Generation Spectra of CH Modes at the Surfactant-Water Interface.

We combine second-order nonlinear vibrational spectroscopy and quantum-chemical calculations to quantify the molecular tilt angle and the structural variation of a decanoic acid surfactant monolayer on water. We demonstrate that there is a remarkable degree of delocalization of the vibrational modes along the backbone of the amphiphilic molecule. A simulation-based on modeled sum frequency generation (SFG) spectra offers quantitative insights into the disorder of surfactant monolayers at the water-air interface. It is shown that an average of one gauche defect in the alkyl chain suffices to give rise to the methylene stretch intensity similar in magnitude to the methyl stretch.

Structure Sensitivity in Pt Nanoparticle Catalysts for Hydrogenation of 1,3-Butadiene:In SituStudy of Reaction Intermediates Using SFG Vibrational Spectroscopy

The product selectivity during 1,3-butadiene hydrogenation on monodisperse, colloidally synthesized, Pt nanoparticles was studied under reaction conditions with kinetic measurements and in situ sum frequency generation (SFG) vibrational spectroscopy. SFG was performed with the capping ligands intact in order to maintain nanoparticle size by reduced sintering. Four products are formed at 75 °C: 1-butene, cis-2-butene, trans-2-butene, and n-butane. Ensembles of Pt nanoparticles with average diameters of 0.9 and 1.8 nm exhibit a ∼30% and ∼20% increase in the full hydrogenation products, respectively, as compared to Pt nanoparticles with average diameters of 4.6 and 6.7 nm. Methyl and methylene vibrational stretches of reaction intermediates observed under working conditions using SFG were used to correlate the stable reaction intermediates with the product distribution. Kinetic and SFG results correlate with previous DFT predictions for two parallel reaction pathways of 1,3-butadiene hydrogenation. Hydrogenation of 1,3-butadiene can initiate with H-addition at internal or terminal carbons leading to the formation of 1-buten-4-yl radical (metallocycle) and 2-buten-1-yl radical intermediates, respectively. Small (0.9 and 1.8 nm) nanoparticles exhibited vibrational resonances originating from both intermediates, while the large (4.6 and 6.7 nm) particles exhibited vibrational resonances originating predominately from the 2-buten-1-yl radical. This suggests each reaction pathway competes for partial and full hydrogenation and the nanoparticle size affects the kinetic preference for the two pathways. The reaction pathway through the metallocycle intermediate on the small nanoparticles is likely due to the presence of low-coordinated sites.

Vibrational Sum Frequency Spectroscopic Investigation of the Azimuthal Anisotropy and Rotational Dynamics of Methyl-Terminated Silicon(111) Surfaces

Polarization-selected vibrational sum frequency generation spectroscopy (SFG) has been used to investigate the molecular orientation of methyl groups on CH3-terminated Si(111) surfaces. The symmetric and asymmetric C–H stretch modes of the surface-bound methyl group were observed by SFG. Both methyl stretches showed a pronounced azimuthal anisotropy of the 3-fold symmetry in registry with the signal from the Si(111) substrate, indicating that the propeller-like rotation of the methyl groups was hindered at room temperature. The difference in the SFG line widths for the CH3 asymmetric stretch that was observed for different polarization combinations (SPS and PPP for SFG, visible, and IR) indicated that the rotation proceeded on a 1–2 ps time scale, as compared to the ∼100 fs rotational dephasing of a free methyl rotor at room temperature.

Improved assignment of vibrational modes in sum-frequency spectra in the C[sbnd]H stretch region for surface-bound C18 alkylsilanes

Previous assignments of the CH stretching modes for surface-bound alkylsilanes in vibrationally resonant sum-frequency generation (VR-SFG) spectra have differed, leading to uncertainty in how to interpret vibrational spectra of these systems. In particular, the assignment of Fermi resonances, including which modes are coupled, has been unclear. To aid in these assignments, partially deuterated alkylsilanes were synthesized and characterized by FT-IR and VR-SFG. Density functional theory (DFT) calculations complement the spectroscopic investigation. Based on these results, we identify multiple contributions to the VR-SFG feature at ∼2950cm−1; this is primarily a Fermi resonance between the symmetric methyl stretch and symmetric methyl bending modes at 2945cm−1 with a high-frequency shoulder at ∼2960cm−1, assigned to the methyl antisymmetric stretch, and a small contribution from the antisymmetric stretch of the ω CH2 group at ∼2930cm−1. The feature at 2880cm−1 is assigned as the symmetric methyl stretch. Improved mode assignments will aid the interpretation of vibrational spectra with an aim toward developing a better understanding of the molecular basis of retention in liquid chromatography.

Thermal-Oxidation Degradation of Microwave Drying Nature Rubber

The effects of drying methods on thermal-oxidation degradation and ageing behavior of nature rubber (NR) was investigated using Fourier transform infrared/Attenuated total reflection (FTIR-ATR) and thermogravimetric analysis. Examining the samples of thermal-oxidation ageing for 24h by FTIR-ATR spectroscopy allowed appearance of obvious carboxyl absorption of hot-air drying NR at 1723cm-1, decay of methylene stretch and C=C plane stretch during thermal-oxidation ageing process, to be monitored. TG and DTG were used to determine thermal-oxidation degradation temperature in air atmosphere and active energy using Coats-Redfern equation. Showed from the results, the degradation temperature and active energy of microwave drying NR was higher than hot-air drying, thermal-oxidation resistance was more excellent by FTIR analysis, indicating a higher thermal-oxidation stability of microwave drying NR.

Structure of Mixed Phosphatidylethanolamine and Cholesterol Monolayers in a Supported Hybrid Bilayer Membrane Studied by Sum Frequency Generation Vibrational Spectroscopy

The structure of hybrid bilayer membranes (HBMs) containing either a pure cholesterol or mixed cholesterol/dipalmitoylphosphatidylethanolamine (DPPE) proximal layer adsorbed onto an octadecanethiol (ODT) self-assembled monolayer (SAM) on a gold substrate have been investigated by sum frequency generation (SFG) spectroscopy. The HBMs were formed by the adsorption of either a pure cholesterol or mixed DPPE/cholesterol monolayer from the air/water interface of a Langmuir-Blodgett trough at surface pressures of 1, 20, or 40 mN·m(-1). SFG spectra were also recorded of HBMs where cholesterol was replaced by cholesterol-d(7), in which the terminal isopropyl group of the alkyl chain of cholesterol was isotopically labeled. In order to isolate the contribution to the SFG spectra from the cholesterol in the mixed cholesterol/phospholipid films, DPPE-d was used, in which the alkyl chains of the phospholipid were deuterated. The infrared spectra of solvent-cast cholesterol and cholesterol-d(7) films were recorded to aid with assignment of the SFG spectra of the HBMs. Features corresponding to methyl, methylene, and methine stretches of cholesterol were identified in the SFG spectra. Information on the polar orientation of SFG-active groups was obtained from the phases of the spectral features. The structure of the HBMs showed little dependence on the surface pressure at which they were formed. SFG spectra of HBMs with a mixed cholesterol/DPPE proximal layer were very similar to the spectra of HBMs with a pure cholesterol proximal layer, although the features in the spectra were more intense than anticipated for a film with half the number of cholesterol molecules, indicating that DPPE did have some effect on the orientation of cholesterol molecules in the film.

Inelastic Tunneling Spectroscopy of Alkanethiol Molecules: High-Resolution Spectroscopy and Theoretical Simulations

We investigate inelastic electron tunneling spectroscopy (IETS) for alkanethiol self-assembled monolayers (SAM) with a scanning tunneling microscope and compare it to first-principles calculations. Using a combination of partial deuteration of the molecule and high-resolution measurements, we identify and differentiate between methyl (CH3) and methylene (CH2) groups and their symmetric and asymmetric C-H stretch modes. The calculations agree quantitatively with the measured IETS in producing the weight of the symmetric and asymmetric C-H stretch modes while the methylene stretch mode is largely underestimated. We further show that inelastic intermolecular scattering is important in the SAM by plotting the theoretical current densities.

Chemical, electrochemical, and structural stability of low-density self-assembled monolayers.

The stability of low-density self-assembled monolayers of mercaptohexadecanoic acid on gold is studied under a variety of storage conditions--air at room temperature, argon at room temperature and 4 degrees C, and ethanol at room temperature. The structural monotony of the low-density monolayers was assessed by monitoring the alkyl chains of LDSAMs by grazing-angle Fourier transform infrared spectroscopy as a function of time. Independently of the storage conditions, both symmetric and asymmetric methylene stretches at 2923 and 2852 cm-1 decreased after 4 weeks to 2919 and 2849 cm-1, respectively. These data suggest an increased ordering of the alkyl chains that is distinctly different from that of conventional high-density monolayers of mercaptohexadecanoic acid included as a reference in this study. As a further extension of this observation, the electrochemical barrier properties of the low-density monolayers were assessed by electrochemical impedance spectroscopy and did not change significantly for any of the storage conditions over a period of 4 weeks. Moreover, X-ray photoelectron spectroscopy was used to assess the chemical changes in the low-density monolayers over time. The chemical composition was essentially unaltered for all storage conditions. Specifically, oxidation of the sulfur headgroup, a common cause of monolayer degradation, was excluded for all test conditions on the basis of XPS analysis. This study confirms excellent storage stability for low-density monolayers under commonly used storage conditions and bridges an important technological gap between these systems and conventional high-density systems.

Mode-dependent enhancement and intramolecular dynamics via vibrationally mediated photodissociation

Vibrationally mediated photodissociation has been shown to control bond cleavage in molecules and probe their dynamics on the ground and excited potential energy surfaces. The application of this method to two seven-atom molecules illustrates surprising vibrational energy localization in methylamine (CH3NH2), a molecule with a torsional degree of freedom and unique information regarding intramolecular vibrational energy redistribution for the C–H methyl stretches of propyne (CH3C ≡ C–H).

Efficient Inhibition of Class A and Class D β-Lactamases by Michaelis Complexes

A 6-alkylidiene penam sulfone, SA-1-204, is an efficient inhibitor of both SHV-1 and OXA-1 beta-lactamases with K(I) = 42 +/- 4 nm and 1.0 +/- 0.1 microm, respectively. To gain insight into the reaction chemistry of SA-1-204, the reactions between this inhibitor and SHV-1 and OXA-1 were studied by Raman spectroscopy in single crystals and in solution. Raman signatures characteristic of the unreacted beta-lactam ring show that in both phases the inhibitor binds as a noncovalent Michaelis-like complex. This complex is present as the major population for periods of up to an hour. On longer time scales, the Raman data show that beta-lactam ring opening eventually leads to a complex mixture of reaction products. However, the data clearly demonstrate that the key species for inhibition on the time scale of bacterial half-lives is the noncovalent complex preceding acylation.

An infrared spectroscopy study of the hydrogen bonding and water restructuring as a trisiloxane superspreading surfactant adsorbs onto an aqueous–hydrophobic surface

Trisiloxanes are surfactants with a compact trisiloxane group ((CH 3) 3Si–O–Si(CH 3)(C 3H 6– E j )–O–Si(CH 3) 3) as the nonpolar moiety and an oligomeric polyethylene oxide chain, E j ((OCH 2CH 2) j –R) as the polar group. (“R” is the cap of the polar group.) Trisiloxane surfactants with four to eight ethoxylates facilitate the rapid and complete spreading of drops of their aqueous solutions on very hydrophobic surfaces not otherwise wet by water. This superspreading ability, which is not completely understood, is unique to these amphiphiles in that solutions of conventional alkyl chain surfactants do not wet out on very hydrophobic surfaces. In this study, in an effort to understand the origin of superspreading, we use Fourier transform infrared spectroscopy in total attenuated internal reflection mode (FTIR-ATR) to gain a molecular picture of the adsorption of a trisiloxane surfactant onto an aqueous/hydrophobic-solid interface, and for comparison a nonsuperspreading alkyl chain surfactant. We focus on the OH stretch of water molecules to detail the changes in the hydrogen bonding structure of water at the interface, and methyl and methylene stretches to measure the adsorption densities. We compare the trisiloxane superspreader TE 8 ((CH 3) 3Si–O–Si(CH 3)((C 3H 6)(OCH 2CH 2) 8OH)–O–Si(CH 3) 3) with a conventional non-superspreading n-dodecyl polyoxyethylene glycol surfactant with the same ethoxylate chain length C 12E 8 (CH 3(CH 2) 11(OCH 2CH 2) 8OH), at concentrations below and above their critical aggregate concentrations. The spectra of C 12E 8 indicates that the adsorption removes the water molecules in the molecular layers in the vicinity the water/hydrophobic interface which have been shown to have a hydrogen bond coordination greater than those of bulk water. The trisiloxane spectra initially show this same behavior, but for longer times demonstrate the ability to remove water molecules which immediately straddle the surface with a dangling OH bond, and reconfigure them into a tightly hydrogen bonded network. This water restructuring is unobserved in the adsorption of the conventional surfactant, and, as it maximizes hydrogen bonding, is energetically favorable and may provide the enhanced driving force for adsorption and superspreading. Measurements of the adsorption densities provide additional information on how the architectures of the trisiloxane and polyethoxylate molecules determine the adsorption geometry and hydrogen bonding with water. From the measured surface densities at maximum adsorption we infer that the compact hydrophobic trisiloxane groups of the TE 8 molecules pack tightly along the hydrophobic surface where they exclude water molecules. This close packing is unimpeded by the oligomeric ethoxylate chain which is of equal cross-section as the hydrophobe, and extends into the aqueous phase within the hydrophobic footprint. In the case of C 12E 8, the measured densities at maximum packing indicate that the slender alkyl chain, while extending down to the hydrophobic surface, is spaced apart by the larger ethoxylate chain, thereby leaving water molecules immediately adjacent to the hydrophobic surface.

Determining the vibrational pattern via overtone cold spectra: C–H methyl stretches of propyne

Vibrationally mediated photodissociation and photoacoustic (PA) spectroscopy were employed for studying the intramolecular dynamics of propyne initially excited to the first through fourth overtone of methyl C-H stretching modes. Room-temperature PA and jet-cooled action spectra, monitoring the absorption of the parent and the yield of the ensuing H photofragments, respectively, were obtained. The PA spectra exhibit mainly broad features, while the action spectra, due to inhomogeneous structure reduction, expose multiple peaks of recognizable shapes in the differing overtone manifolds. Symmetric rotor simulations of the band contours of the action spectra allowed retrieving of band origins and linewidths. The linewidths of the bands in each manifold enabled estimates for energy redistribution times out of the corresponding states to the bath states, the times ranging from 18+/-6 ps for two quanta of C-H excitation to subpicosecond for five quanta. The data were also analyzed in terms of a normal-mode model and a joint local-/normal-mode model. These models enabled determination of harmonic frequencies, anharmonicities, and interaction parameters reproducing the observed data in all monitored regions and provided spectral assignments. The measured Doppler profiles were well fitted by Gaussians with widths suggesting low average translational energies for the released H photofragments. These low energies and their similarities to those for dissociation of propyne isotopomers preexcited to acetylenic C-H stretches were ascribed to an indirect dissociation process occurring after internal conversion to the ground electronic state and isomerization to allene.

Phase transitions and hindered rotation in dimethylacetylene at high pressures probed by Raman spectroscopy

We present Raman spectroscopy experiments in dimethylacetylene (DMA) using a sapphire anvil cell up to 4 GPa at room temperature. DMA presents phase transitions at 0.2 GPa (liquid to phase I) and 0.9 GPa, which have been characterized by changes in the Raman spectrum of the sample. At pressures above 2.6 GPa several bands split into two components, suggesting an additional phase transition. The Raman spectrum of the sample above 2.6 GPa is identical to that found for the monoclinic phase II (C2/m) at low temperatures, except for an additional splitting of the band assigned to the fourfold degenerated asymmetric methyl stretch. The global analysis of the Raman spectra suggests that the observed splitting is due to the loss of degeneracy of the methyl groups of the DMA molecule in phase II. According to the above interpretation, crystal phase II of DMA extends from 0.9 GPa to pressures close to 4 GPa. Between 0.9 and 2.6 GPa, the methyl groups of the DMA molecules rotate almost freely, but the rotation is hindered on further compression.