Determination of lipid phase transition temperatures in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the lipid monolayer in hybrid bilayer membranes (HBMs) was investigated using vibrational sum frequency spectroscopy (VSFS). In the gel phase, the acyl chains of the lipid molecules assume an ordered, all-trans configuration, whereas in the LC phase, the acyl chains exhibit a significant number of disordered gauche conformers. VSFS has unique sensitivity to the order/disorder transitions in the acyl chains and was used to determine T(m) for a series of saturated phosphatidylcholine lipids on octadecanethiolate self-assembled monolayers (SAMs). The values obtained for T(m) for all lipids studied are significantly higher than for the corresponding lipids in vesicles in solution. Additionally, the transition widths are broader for the lipids in HBMs. The underlying SAM clearly influences the phase behavior of the overlying lipid monolayer.     

月桂胺盐酸晶体相变的红外光谱研究

在290-365K的温度区间内考察了月桂胺盐酸盐晶体红外光谱随温度的变化规律。结果表明:月桂胺盐酸盐在339K发生了固-固结构相变, 该相变的预相变起始点为327K。在327K以下的低温相中, 晶体中分子的碳氢链以高度有序的全反式构象存在, 极性头部的三个N-H-Cl氢键是不等价的。在327-339K的中间过渡相, 碳氢链中出现了旁式构象, 分子链间相互作用减弱。不等价的N-H-Cl氢键的差异减小。在339K以上的高温相中, 分子旁式链构象的增多导致了分子链横向堆积无序性的明显增加, 三个N-H-Cl氢键已变得等价。     

Cholesterol/phospholipid interactions in hybrid bilayer membranes

The interactions between cholesterol and saturated phospholipids in hybrid bilayer membranes (HBMs) were investigated using the interface-sensitive technique of vibrational sum frequency spectroscopy (VSFS). The unique sensitivity of VSFS to order/disorder transitions of the lipid acyl chains was used to determine the main gel to liquid crystal phase transition temperature, Tm, for HBMs of binary cholesterol/phospholipid mixtures on octadecanethiolate self-assembled monolayers. The phase transition temperature and the breadth of the transition were shown to increase with cholesterol content, and the phase boundaries observed in the cholesterol/phospholipid HBMs were comparable to the published phase diagrams of binary cholesterol/phospholipid vesicles. A thermodynamic assessment of the cooperative units of the HBM phase transitions revealed the presence of <10 nm diameter domains that were independent of the cholesterol composition.     

Molecular Orientation and Structure in LB Films of a Liquid Crystal MPUOPM Investigated by UV-Vis and IR Spectroscopy

"?Langmuir monolayers and LB films of 4-((s)-2-methylbutoxy)phenyl-(4'-(10-undecen-1-oyloxy)phenyl) methylenimine (MPUOPM) were investigated by ultraviolet-visible, polarized infrared spectroscopy. ?-A isotherms showed well-defined Langmuir monolayers were formed at an air/water interface for the MPUOPM and their mixture with SA. An inflection point at 13 mN/m appeared on the isotherm, which was due to the transition from the monolayer to multilayer. The polarized IR spectra of LB films of MPUOPM had provided new insight into the molecular orientation and structure. In LB films, the tilted angle between the alkyl chain and the normal line of the substrate was 48ffi, the tilted angle between the dipole moment of C=N and the normal line of the substrate was 51ffi. The alkyl chains assumed a trans-zigzag conformation but it included a few gauche conformers. The C=N groups were almost in one plane in the LB films. "     

Vibrational and conformational analysis of chloromethyldifluorophosphine,ClCH2PF2

The vibrational spectra of chloromethyldifluorophosphine have been recorded in the different physical states. Both gauche and trans conformers are identified in the spectra. Temperature dependent studies and calculated dipole moments (CNDO/2) indicate that the gauche conformer is more stable in the gas-phase while the trans is more stable in the liquid phase. In the solid phase, both conformers are present, although the trans strongly dominates.     

固态磷脂相变行为的红外和拉曼光谱研究

用红外和拉曼光谱方法在２９０～４０５Ｋ的温度范围内考察了无水二肉豆蔻酰磷脂酰乙醇胺的分子构象随温度的变化。结果表明，该磷脂在３６５Ｋ附近的熔化相变过程中，不仅酰链的构象和堆积发生了有序－无序转变，而且极性头部基团和接界区域基团也发生了结构重排；逆相变过程中，烷基链构象重新恢复了有序，但头部基团和甘油骨架构象没有恢复原始有序状态。     

Conformational stability and vibrations of aminopropylsilanol molecule

Density functional theory (DFT), using the B3-LYP/6-31G(d,p) method have been used to investigate the conformation and vibrational spectra of aminopropylsilanetriol (APST) NH2CH2CH2CH2Si(OH)3. The potential function for CCCSi torsion gives rise to two distinct conformers trans and gauche. The predicted energy of the more stable trans conformer is 337 cm-1 less than the energy of gauche conformer. The calculated barriers to the conformation interchange are: 1095, 2845 and 438 cm-1 for the trans to gauche, gauche to gauche and gauche to trans conformers, respectively. For the trans conformer the potential energy curve for the Si(OH)3 groups torsion in APST has been calculated changing the HOSiC dihedral angle. The barrier for the internal rotation of 3065 cm-1 has been obtained. The optimized molecular structure of APST dimer calculated for trans conformer has a SiOSi angle of 143.2 degrees, and a SiOSi bond length of 0.164 nm. A complete vibrational assignment for both conformers as well as for trans-dimer is supported by the normal coordinate analysis, calculated IR intensities as well as Raman activities. On the basis of the results, the vibrational spectra of APST aqueous solution and APST polymer have been analyzed. The average error between the observed and calculated frequencies is 14 cm-1.     

Pressure-induced structural transition in n-pentane: a Raman study

Pressure-induced Raman spectroscopy studies on n-pentane have been carried out up to 17 GPa at ambient temperature. n-Pentane undergoes a liquid-solid transition around 3.0 GPa and a solid-solid transition around 12.3 GPa. The intensity ratio of the Raman modes related to all-trans conformation (1130 cm-1 and 2850 cm-1) to that of gauche conformation (1090 cm-1 and 2922 cm-1) suggests an increase in the gauche population conformers above 12.3 GPa. This is accompanied with broadening of Raman modes above 12.3 GPa. The high-pressure phase of n-pentane above 12.3 GPa is a disordered phase where the carbon chains are kinked. The pressure-induced order-disorder phase transition is different from the behavior of higher hydrocarbon like n-heptane.     

Conformer assignment of OH stretches in ethanol and isopropanol

Rigid harmonic asymetric top contour simulation is applied to assign the OH stretch transitions observed in the vapour phase infrared spectra of ethanol and isopropanol to gauche and trans conformers. For hetanol, the high frequency ν(OH) absorption must be assigned to the trans, for isopropanol the high frequency absorption is found to be due to the gauche conformer.     

Equilibrium and non-equilibrium kinetics of self-assembled surfactant monolayers: a vibrational sum-frequency study of dodecanoate at the fluorite-water interface

The adsorption, desorption, and equilibrium monomer exchange processes of sodium dodecanoate at the fluorite(CaF 2)-water interface have been studied. For the first time, we use in situ vibrational sum-frequency spectroscopy (VSFS) to gain insights into the mechanism and kinetics of monolayer self-assembly at the mineral-water interface. By exploiting the nonlinear optical response of the adsorbate, the temporal correlation of headgroup adsorption and alignment of the surfactant's alkyl chain was monitored. Because of the unique surface-specificity of VSFS, changes in the interfacial water structure were also tracked experimentally. The spectra clearly reveal that the structure of interfacial water molecules is severely disturbed at the start of the adsorption process. With the formation of a well-ordered adsorbate layer, it is partially reestablished; however, the molecular orientation and state of coordination is significantly altered. Even at very low surfactant concentrations, overcharging of the mineral surface (i.e., the adsorption of adsorbates past the point of electrostatic equilibrium) was observed. This points out the importance of effects other than electrostatic interactions and it is proposed that cooperative effects of both water structure and surfactant hemimicelle formation at the interface are key factors. The present study also investigates desorption kinetics of partially and fully established monolayers and a statistical model for data analysis is proposed. Additional experiments were performed in the presence of electrolytes and showed that uni- and divalent anions affect the nonequilibrium kinetics of self-assembled monolayers in strikingly different ways.     

Gauche- and trans-n-butane and their IR-induced interconversion in solid neon

Both conformers of n-butane are trapped from the gas phase in solid Ne at 4 K. Broad band infrared irradiation (mainly the CH-stretching region) induces interconversions, the quantum yield for the gauche — trans isomerization being approximately eight times higher than that for the reverse process. The energy difference between the conformers was found to be ?3.05 kJ mol^?1. The vibrational spectra of both conformers are given.     

Aziridinyl ketones and their heteroanalogs. 5. Synthesis and structures of 2-aryl-3-aroylaziridines

A number of trans- and cis-isomeric 1-R-2-aryl-3-aroylaziridines were synthesized, and their IR spectra were studied. Intramolecular hydrogen bonding is realized in the trans isomers when R = H, and they exist in the only possible conformation (intermediate between a gauche and a cisoid conformation). cis-Isomers II (R = alkyl) exist in solutions in the form of two conformers, viz., gauche and cisoid conformers, and the gauche conformer is thermodynamically preferable.See [1] for Communication A.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 11, pp. 1489–1494, November, 1980.     

On the mechanism of the hofmeister effect

Vibrational sum frequency spectroscopy was used to probe fatty amine monolayers spread on various electrolyte solutions. The spectra revealed ion specific changes in both monolayer ordering and water structure with the former following the Hofmeister series. Separate measurements of the surface potential as a function of ion tracked closely to changes in alkyl chain structure, but less closely to changes in water structure. The disruption of the monolayer ordering could be ascribed to the relative ability of the ions to penetrate past the hydrophilic surface of the monolayer's headgroups and into the more hydrophobic portion of the thin film. The corresponding trends observed in the surface water structure showed significant deviations from the Hofmeister series, leading to the conclusion that the changes in surface water structure, often credited with being the origin of Hofmeister effects, are probably not of primary importance. On the other hand, dispersion forces almost certainly play a large role in the order of the Hofmeister series.     

Characterization and control of lipid layer fluidity in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the distal lipid layer in hybrid bilayer membranes (HBMs) under water was investigated using vibrational sum frequency spectroscopy (VSFS). VSFS has unique sensitivity to order/disorder transitions in the lipid acyl chains and can determine T(m) for the lipid monolayers in HBMs. We recently reported the observation that T(m) is raised and the transition width is broadened for the overlying phospholipid monolayer in HBM systems formed on densely packed crystalline self-assembled monolayers (SAMs) as compared to that of vesicles in solution. In this report, we establish that T(m) for the lipid layer of HBMs can be controlled by proper choice of the SAM underlayer. The SAM underlayer of the HBM was systematically altered by using an alkane thiol, a saturated thiolipid, a mixed SAM of a saturated lipid-pyridine disulfide, and finally a mixed SAM of an unsaturated lipid-pyridine disulfide. T(m) was measured for two different chain length saturated phosphatidylcholine lipid overlayers on these SAMs. The values obtained show that Tm of the lipid layer of HBMs is sensitive to the composition and/or packing density of molecules in the underlying SAM.     

Probing molecular conformations with electron momentum spectroscopy: the case of n-butane.

High-resolution (e,2e) measurements of the valence electronic structure and momentum-space electron density distributions of n-butane have been exhaustively reanalyzed in order to cope with the presence of two stable structures in the gas phase, namely the all-staggered and gauche conformers. The measurements are compared to a series of Boltzmann-weighted simulations based on the momentum-space form of Kohn-Sham (B3LYP) orbital densities, and to ionization spectra obtained from high-level [ADC(3)] one-particle Green's Function calculations. Indubitable improvements in the quality of the simulated (e,2e) ionization spectra and electron momentum profiles are seen when the contributions of the gauche form of n-butane are included. Both the one-electron binding energies and momentum distributions consistently image the distortions and topological changes that molecular orbitals undergo due to torsion of the carbon backbone, and thereby exhibit variations which can be traced experimentally. With regard to the intimate relation of (e,2e) cross sections with orbital densities, electron momentum spectroscopy can therefore be viewed as a very powerful, but up to now largely unexploited, conformational probe. The study also emphasizes the influence of thermal agitation in photoionization experiments of all kind.     

Vibrational spectroscopy of mechanically compressed monolayers

Sum-frequency spectroscopy has been used to investigate the behavior of self-assembled monolayers in a solid-solid contact. Various alkanethiol layers on gold were observed before, during, and after compression to 660 MPa against a sapphire counterface. Well-ordered layers that differ only in the length of their alkane tails (C(8) versus C(18)) behave similarly. In contrast, defective and partly melted monolayers are more sensitive to stress than are their well-ordered analogues, and they are more prone to irreversible changes. In all cases, the intensity of methyl C-H stretching modes decreases with applied pressure, indicating a loss of net orientational order among the terminal methyl groups. The magnitude of this effect in well-ordered layers can be compared with the theoretical sensitivity of the resonant sum-frequency signal to molecular orientation. On these grounds, an increased population of terminal gauche conformers is identified as the disordering mechanism under pressure.     

Sum-frequency spectroscopic study of Langmuir monolayers of lipids having oppositely charged headgroups

Sum-frequency vibrational spectroscopy, with the help of surface pressure-area (π-A) isotherm, was used to study lipid Langmuir monolayers composed of molecules with positively and negatively charged headgroups as well as a 1:1 neutral mixture of the two. The spectral profiles of the CH(x) stretch vibrations are similar for all monolayers in the liquid-condensed (LC) phase. They suggest a monolayer structure of closely packed alkyl chains that are nearly all-trans and well oriented along the surface normal. In the liquid-expanded (LE) phase, the spectra of all monolayers appear characteristic of loosely packed chains with significant gauche defects. The OH stretch spectra of interfacial water for both positively and negatively charged monolayers are significantly enhanced in comparison with a neutral water interface, but the phase measurement of SFVS indicates that OH in the two cases points toward the bulk and the interface, respectively. The enhancement results mainly from surface-field-induced polar ordering of interfacial water molecules. For a charge-neutral monolayer composed of an equal number of positively and negatively charged lipid molecules, no such enhancement is observed. This mixed monolayer exhibits a wide range of LC/LE coexistence region extended to very low surface pressure and its CH(x) spectral profile in the coexistence region resembles that of the LC phase. This result suggests that in the LC/LE coexistence region, the mixed monolayer consists of coexisting LC and LE patches in which oppositely charged lipid molecules are homogeneously mixed and dispersed.     

Effect of ionization on infrared and electronic absorption spectra of methyl and ethyl formate in the gas phase and in astrophysical H2O ice: a computational study

This work reports infrared and electronic absorption spectra of trans and gauche conformers of neutral ethyl formate, trans and cis conformers of neutral methyl formate, their ions in the gas phase, and neutral ethyl and methyl formate in astrophysical H(2)O ice. The second-order M?ller-Plesset perturbation (MP2) method with TZVP basis set has been used to obtain ground-state geometries. An influence of ice on vibrational frequencies of neutral ethyl and methyl formate was obtained using integral equation formalism polarizable continnum model (IEFPCM). Significant shift in vibrational frequencies for neutral methyl and ethyl formate when studied in H(2)O ice and upon ionization is observed. Rotational and distortion constants for neutral ethyl and methyl formate from this work are in excellent agreement with the available experimental values. Electronic absorption spectra of conformers of ethyl and methyl formate and their ions are obtained using time-dependent density functional method (TDDFT). The nature of electronic transitions is also identified. We suggested lines especially good to detect these molecules in interstellar medium. Using these lines, we can identify the conformers of ethyl and methyl formate in gas phase and H(2)O ice in interstellar medium. This comparative study should provide useful guidelines to detect conformers of ethyl and methyl formate and their ions in gas phase and neutral molecules in H(2)O ice in different astronomical environment.     

Molecular dynamics models and thermodynamic characteristics of hydrogen bonds in 1,2-ethanediol

A correlation between the lifetimes of hydrogen bonds and the thermodynamic characteristics of their formation and breaking, and the experimental relaxation times of dielectric spectra and the energy characteristics of relaxation processes, is observed via molecular dynamics (MD) simulation of the rearranging of the network structure of 1,2-ethanediol. The MD torsional frequency of the transition of gauche conformer tGg′ at 224.1 cm_?1 and the experimental frequency of the band maximum of torsional vibrations at 230 cm_?1 in the infrared spectrum correlate with the oscillation frequency of molecules at 240 cm_?1 inside clusters in the Dissado–Hill (DH) model. The MD and DH models indicate a predominantly parallel alignment of the electric dipole moments of conformers tGg′ in the three-dimensional network of hydrogen bonds of the liquid 1,2-ethanediol phase.     

An ab initio study on the torsional surface of alkanes and its effect on molecular simulations of alkanes and a DPPC bilayer

Energies of 119 conformations of normal alkanes from butane to heptane were calculated at approximately the CCSD(T)/cc-pVQZ level. Energies of gauche (g) conformers relative to trans (t) decrease as chain length increases. In what is termed the "positive pentane effect", adjacent gauche conformers of the same sign are stabilized compared to nonadjacent conformers; e.g., for hexane the energies of tgt, tgg, and gtg are 0.600, 0.930, and 1.18 kcal/mol, respectively. Torsional terms in the CHARMM27 (C27) force field were fit to the calculated QM energies to yield a revised potential, C27r. Molecular dynamics simulations of normal alkanes (heptane, decane, tridecane, and pentadecane) with C27r yield higher populations of gauche states, increased transition rates, and improved agreement with experiment as compared to C27. In addition, C27r simulations of a hydrated DPPC lipid bilayer yield improved agreement with the experimental NMR deuterium order parameters for the aliphatic chain ends.