Advanced experimental methods toward understanding biophysicochemical interactions of interfacial biomolecules by using sum frequency generation vibrational spectroscopy

Sum frequency generation vibrational spectroscopy(SFG-VS)has been demonstrated to be a powerful technique to study the interfacial structures and interactions of biomolecules at the molecular level.Yet most previous studies mainly collected the SFG spectra in the frequency range of 1500–4000 cm-1,which is not always sufficient to describe the detailed interactions at surface and interface.Thorough knowledge of the complex biophysicochemical interactions between biomolecules and surface requires new ideas and advanced experimental methods for collecting SFG vibrational spectra.We introduced some advanced methods recently exploited by our group and others,including(1)detection of vibration modes in the fingerprint region;(2)combination of chiral and achiral polarization measurements;(3)SFG coupled with surface plasmon polaritons(SPPs);(4)imaging and microscopy approaches;and(5)ultrafast time-resolved SFG measurements.The technique that we integrated with these advanced methods may help to give a detailed and high-spatial-resolution 3D picture of interfacial biomolecules.     

Probing molecular orientation at bulk heterojunctions by polarization-selective transient absorption spectroscopy

A bulk heterojunction in organic solar cells is where charge separation and recombination occur. Molecular orientation at the interface is one of the key factors that dictate solar cell efficiency. Although X–ray scattering-based methods can determine donor/acceptor domain orientations between an anisotropic phase and an isotropic fullerene-based phase, the rise of nonfullerene solar cells presents a new challenge in delineating local molecular directions at the interface between two anisotropic donor/acceptor domains. Here, we determine interfacial molecular orientations of three high-efficiency small molecule solar cells(ZR1:Y6, B1:BO–4 Cl, and BTR:BO–4 Cl) using polarization-selective transient absorption spectroscopy. The polarization anisotropy of charge separation dynamics indicates an angle of ～90° between ZR1 and Y6 molecules at the interface, an angle close to 0° between B1 and BO–4 Cl, and random orientations between BTR and BO–4 Cl. These observations provide complementary information to X–ray scattering measurements and highlight polarization-selective transient absorption spectroscopy as a tool to probe interfacial structure and dynamics of key photophysical steps in energy conversion.     

Salt effect on molecular orientation at air/liquid methanol interface

The salt effects on molecular orientation at air/liquid methanol interface were investigated by the polarization-dependent sum frequency generation vibrational spectroscopy(SFG-VS). We clarified that the average tilting angle of the methyl group to be u = 308 58 at the air/pure methanol surface assuming a d-function orientational distribution. Upon the addition of 3 mol/L Na I, the methyl group tilts further away from the surface normal with a new u = 418 38. This orientational change does not explain the enhancement of the SFG-VS intensities when adding Na I, implying the number density of the methanol molecules with a net polar ordering in the surface region also changed with the Na I concentrations. These spectroscopic findings shed new light on the salt effects on the surfaces structures of the polar organic solutions. It was also shown that the accurate determination of the bulk refractive indices and Raman depolarization ratios for different salt concentrations is crucial to quantitatively interpret the SFG-VS data.     

Absolute Orientation of Molecules with Competing Hydrophilic Head Groups at the Air/Water Interface Probed with Sum Frequency Generation Vibrational Spectroscopy

The constructive or destructive spectral interference between the molecular groups oriented up and down at the interface in the sum-frequency generation （SFG） spectra provides a direct measurement of the absolute orientation of these molecular groups. This simple approach can be employed to interrogate absolute molecular orientations other than using the complex absolute phase measurement in the SFG studies. We used the -CN group in the p-cyanophenol （PCP） molecule as the internal phase standard, and we measured the phases of the SFG fields of the -CN groups in the 3,5-dimethyl-4-hydroxy-benzonitrile （35DMHBN） and 2,6-dimethyl-4-hydroxy-benzonitrile （26DMHBN） at the air/water interface by measuring the SFG spectra of the aqueous surfaces of the mixtures of the PCP, 35DMHBN, and 26DMHBN solutions. The results showed that the 35DMHBN had its -CN group pointing into the aqueous phase; while the 26DMHBN, similar to the PCP, had its -CN group pointing away from the aqueous phase. The tilt angles of the -CN group for both the 35DMHBN and 26DMHBN molecules at the air/water interface were around 25°-45° from the interface normal. These results provided insights on the understanding of the detailed balance of the competing factors, such as solvation of the polar head groups, hydrogen bonding and hydrophobic effects, etc., on influencing the absolute molecular orientation at the air/water interface.     

Microscopic Investigation of Ethylene Carbonate Interface: A Molecular Dynamics and Vibrational Spectroscopic Study

Ethylene carbonate(EC) liquid and its vapor-liquid interface were investigated using a combination of molecular dynamics(MD)simulation and vibrational IR, Raman and sum frequency generation(SFG)spectroscopies. The MD simulation was performed with a flexible and polarizable model of the EC molecule newly developed for the computation of vibrational spectra. The internal vibration of the model was described on the basis of the harmonic couplings of vibrational modes, including the anharmonicity and Fermi resonance coupling of C=O stretching. The polarizable model was represented by the charge response kernel(CRK),which is based on ab initio molecular orbital calculations and can be readily applied to other systems. The flexible and polarizable model can also accurately reproduce the structural and thermodynamic properties of EC liquid. Meanwhile, a comprehensive set of vibrational spectra of EC liquid, including the IR and Raman spectra of the bulk liquid as well as the SFG spectra of the liquid interface, were experimentally measured and reported. The set of experimental vibrational spectra provided valuable information for validating the model, and the MD simulation using the model comprehensively elucidates the observed vibrational IR, Raman, and SFG spectra of EC liquid. Further MD analysis of the interface region revealed that EC molecules tend to orientate themselves with the C=O bond parallel to the interface. The MD simulation explains the positive Im[χ~((2))](ssp) band of the C=O stretching region in the SFG spectrum in terms of the preferential orientation of EC molecules at the interface. This work also elucidates the distinct lineshapes of the C=O stretching band in the IR, Raman, and SFG spectra. The lineshapes of the C=O band are split by the Fermi resonance of the C=O fundamental and the overtone of skeletal stretching. The Fermi resonance of C=O stretching was fully analyzed using the empirical potential parameter shift analysis(EPSA) method. The apparently different lineshapes of the C=O stretching band in the IR, Raman, and SFG spectra were attributed to the frequency shift of the C=O fundamental in different solvation environments in the bulk liquid and at the interface. This work proposes a systematic procedure for investigating the interface structure and SFG spectra, including general modeling procedure based on ab initio calculations, validation of the model using available experimental data, and simultaneous analysis of molecular orientation and SFG spectra through MD trajectories. The proposed procedure provides microscopic information on the EC interface in this study, and can be further applied to investigate other interface systems, such as liquid-liquid and solid-liquid interfaces.     

Vibrational spectroscopic identification of isoprene,pinenes and their mixture

Here we show a study of vibrational spectroscopic identification of a few typical organic compounds which are known as the main sources of organic aerosols(OAs) particle matter in air pollution. Raman and IR spectra of isoprene, terpenoids, pinenes and their mixture are meticulously examined, showing distinguishable intrinsic vibrational spectroscopic fingerprints for these chemicals, respectively. As a reference, first-principles calculations of Raman and infrared activities are also conducted. It is interestingly found that, the experimental spectra are peak-to-peak consistent with the DFT(Density Functional Theory)-calculated vibrational activities. Also found is that, in a certain case such as for bpinene, a dimer model, rather than an isolated single molecular model, reproduces the experimental results, indicating unneglected intermolecular interactions. Starting with this study, we are endeavoring to advocate a database of Raman/IR fingerprint spectra for OA haze identification.     

Mixed Conduction in Tb2O3 Doped BaCeO3

BaxCe0.8Tb0.2O3-a （x=0.98-1.03） solid electrolytes were synthesized and characterized by using X-ray diffraction （XRD）. By using AC impedance spectroscopy and gas concentration cell electromotive force （EMF） measurements, the electrical conduction behavior of the specimens was investigated in different gases during 500-1000 ℃ The influence of nonstoichiometry in the specimens with x ≠ 1 on conduction properties was studied and compared with that in the specimen with x = 1. The results show that the specimens are all of perovskite-type orthorhombic structure. In 500-1000 ℃, electronic hole conduction is dominant in dry and wet oxygen, air or nitrogen. Protonic conduction is dominant in wet hydrogen and it is about two orders of magnitude higher than that in hydrogen-free atmospheres （oxygen, air and nitrogen）. The electrical conductivity of the same specimen in water vapor-saturated oxygen, air or nitrogen is slightly higher than that in corresponding gas without water vapor. The electrical conductivities of the nonstoichiometric specimens are higher than those of the stoichiometric one.     

MOLECULAR DYNAMICS SIMULATION ON VIERATIONAL SPECTROSCOPIC FEATURES OF HYDROGEN BONDS IN CRYSTALLINE POLYMERS

Introduction The molecular dynamics simulation technique has recently proved to be a suitable alternative approachfor simulation of vibrational spectroscopy. In this study, molecular dynamics was utilized to understandlow frequency vibrations in highly ordered poly(ρ-phenylene terephthalmide) (PPTA). A key structuralfeature of this polymer is the presence of hydrogen bonds. There is little question that this strong localized     

Substituent Effects on the Low-Frequency Vibrational Modes of Benzoic Acid and Related Compounds

Well-resolved absorption spectra of benzoic acid and its derivatives with one hydrogen atom replaced by a substituent group CH3, OH, NH2 or NO2 were reported in the frequency region between 6 and 67 cm^-1 at room temperature with terahertz time-domain spectroscopy （THz-TDS）. These substances can be distinguished easily based on the terahertz absorption spectra. The measurements suggested that even minor changes in the molecular configuration and chemical composition lead to distinct differences in THz spectrum. Density functional theory （DFT） method was used to assist the analysis and assignment of the individual THz absorption spectra of benzoic acid and its methyl derivatives. Observed THz responses of samples can be assigned to the collective vibrations associated with intermolecular hydrogen bonds.     

Surface of room temperature ionic liquid [bmim][PF6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy

Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids(RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy(SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the(N)–CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up(N)–CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.     

Temperature inversion enables superior stability for low-temperature Zn-ion batteries

It is challenging for aqueous Zn-ion batteries(ZIBs) to achieve comparable low-temperature(low-T) performance due to the easy-frozen electrolyte and severe Zn dendrites. Herein, an aqueous electrolyte with a low freezing point and high ionic conductivity is proposed. Combined with molecular dynamics simulation and multi-scale interface analysis(time of flight secondary ion mass spectrometry threedimensional mapping and in-situ electrochemical impedance spectroscopy method), the temperature indep...     

Microscopic heterogeneity in viscoelastic properties of molecular assembled systems

An important step in understanding molecular assembled systems is to examine the structure and physical properties at various length scales and clarify the correlation between them. However, while the structures of these systems have been extensively studied from nanoscopic to macroscopic scales, their viscoelastic properties have been often limited to bulk rheological measurements. By using optical tweezers and particle tracking, we here show the local viscoelastic properties and their spatial distributions for the following systems: worm-like micelle solution, supramolecular hydrogel and lyotropic liquid crystal, which are formed by self-assembly of amphiphilic molecules in water. We found that all systems studied possessed a spatial heterogeneity in their viscoelastic properties and this was originated from the heterogeneous structures. It is interesting to note that there is the heterogeneity with the characteristic length scale of sub-micrometer or micrometer scale, thereby structures, although the systems are formed by molecules with nanometer size. The findings of these studies should lead to a better understanding of the dynamics of such systems.     

SURFACE REARRANGEMENTS OF OXYGEN PLASMA TREATED POLYSTYRENE： SURFACE DYNAMICS AND HUMIDITY EFFECT

The time evolution of oxygen plasma treated polystyrene(PS)surfaces was investigated upon storing them in theair under controlled humidity conditions.The methods of water contact angle,X-ray photoelectron spectroscopy(XPS),sumfrequency generation(SFG)vibrational spectroscopy,and atomic force microscopy(AFM)were used to infer the surfaceproperties and structure.Chemical groups containing oxygen were formed on the PS surface with the plasma treatment,demonstrated by water contact angle and XPS.The surface polarity decayed markedly on time,as assessed by steady increasein the water contact angle as a function of storage time,from zero to around 60°.The observed decay is interpreted as arisingfrom surface rearrangement processes to burying polar groups away from the uppermost layer of the surfaces,which is incontact with air.On the other hand,XPS results show that the chemical composition in the first 3 nm surface layer isunaffected by the surface aging,and the depth profile of oxygen is essentially the same with time.A possible change of PSsurface roughness was examined by AFM,and it showed that the increase of water contact angle during surface aging couldnot be attributed to surface roughness.Thus,it is concluded that surface aging is attributable to surface reorganization andthe motion of oxygen containing groups is confined within the XPS probing depth.SFG spectroscopy,which is intrinsicallyinterface-specific,was used to detect the chemical structure of PS surface at the molecular level after various aging times.The results are interpreted as follows.During the aging of the plasma treated PS surfaces,the oxygen containing groupsundergo reorientation processes toward the polymer bulk and/or parallel to the surface,while the CH_2 moiety stands up onthe PS surface.Our results indicate that the surface configuration changes do not require large length scale segmentalmotions or migration of macromolecules.Motions that are responsible for surface configuration changes could be relativelysmall rotational motions.The aging behaviors under different relative humidity conditions were shown to be similar from18% to 91%,whereas the kinetics of surface polarity decays were faster in higher relative humidity.Here,the surfacerearrangement of polystyrene films that were previously treated by oxygen plasma and aged,and was investigated in terms ofcontact angle after the water immersion.The contact angles of the water-immersed samples were found to change andapproach the initial values before the immersion asymptotically.     

Microscopic study of binary mixtures between pyrrolidinium bis(triflorosulfonyl)imide and dimethyl sulfoxide/acetonitrile

Molecular interactions of a representative pyrrolidinium-based ionic liquid 1-butyl-1-methyl-pyrrolidinium bis(triflorosulfonyl)-imide([BMPyrr][TFSI]) with dimethyl sulfoxide(DMSO) and acetonitrile(AN) have been analyzed in this work. Attenuated total reflection Fourier transform infrared spectroscopy(ATR-FTIR) and density functional theory(DFT) calculations are used in the investigation, while excess infrared spectra and two-dimensional correlation spectroscopy are used to explore the data in detail. It has been found that the molecular solvents can interact with TFSI-(mainly with S=O and weakly with S–N–S group). AN interacts feebly with BMPyrr~+ as compared with the strong interaction of DMSO. The strength of the interactions depends on the electron donating ability of the solvent. Upon mixing, hydrogen bonds regarding C-Hs in cation and S–N–S in anion are weakened, while that regarding S=O in anion is strengthened. Among the C-Hs which are connected directly with the N of the cation, C1-H is the main interaction site for both DMSO and AN. This means that C1-H is the most acidic hydrogen in pyrrolidinium cation.     

Study on the drug resistance and the binding mode of HIV-1 integrase with LCA inhibitor

Human immunodeficiency virus type 1 (HIV-1) integrase (IN) is an essential enzyme in the lifecycle of this virus and also an important target for the study of anti-HIV drugs. The binding mode of the wild type IN core domain and its G140S mutant with L-Chicoric acid (LCA) inhibitor were investigated by using multiple conformation molecular docking and molecular dynamics (MD) simulation. Based on the binding modes, the drug resistance mechanism was explored for the G140S mutant of IN with LCA. The results indicate that the binding site of the G140S mutant of IN core domain with LCA is different from that of the core domain of the wild type IN, which leads to the partial loss of inhibition potency of LCA. The flexibility of the IN functional loop region and the interactions between Mg2 ion and the three key residues (i.e., D64, D116, E152) stimulate the biological operation of IN. The drug resistance also lies in several other important effects, such as the repulsion between LCA and E152 in the G140S mutant core domain, the weakening of K159 binding with LCA and Y143 pointing to the pocket of the G140S mutant. All of the above simulation results agree well with experimental data, which provide us with some helpful information for designing the drug of anti-HIV based on the structure of IN.     

Two-layer model description of polymer thin film dynamics

Experiments in the past two decades have shown that the glass transition temperature of polymer films can become noticeably different from that of the bulk when the film thickness is decreased below ca. 100 nm. It is broadly believed that these observations are caused by a nanometer interfacial layer with dynamics faster or slower than that of the bulk. In this paper, we examine how this idea may be realized by using a two-layer model assuming a hydrodynamic coupling between the interfacial layer and the remaining, bulk-like layer in the film. Illustrative examples will be given showing how the two-layer model is applied to the viscosity measurements of polystyrene and polymethylmethacrylate films supported by silicon oxide, where divergent thickness dependences are observed.     

Molecular Arrangement of a Bolaamphiphilic Anthrancene Derivative in Langmuir-Blodgett Film

Langmuir monolayer at the air/water interface is the basis to understand the molecular arrangement and to fabricate the organized molecular films1. Bolaamphiphiles describes the molecules in which two head functional groups are linked by one or two hydrophobic chains2,3. In comparison with the one-headed amphiphile, abundant configurations of Langmuir monolayer are expected in bolaamphiphiles. Generally, three kinds of configurations of the Langmuir monolayers of bolaamphiphile at the air/w…     

Orientation and Structure of Ionic Liquid Cation at Air/[bmim][BF4] Aqueous Solution Interface

The watermiscible room temperature ionic liquid 1butyl3methylimidazolium tetrafluorob orate （[bmim] [BF4]） is a model system for studying the interactions between ionic liquid and water molecules. In this work the orientational structure of the low concentrated aqueous solution of [bmim] [BF4] at the air/liquid interface was investigated by sum frequency gener ation vibrational spectroscopy. It has been found that at very low concentrations, the butyl chain exhibited a significant gauche defect, indicating a disordered conformation; and the cation ring oriented with a fairly small tilting angle at the surface. When the concentration increased, the cation ring tended to lie flat at the surface, and the gauche defects of the butyl chain decreased due to the intermolecular chainchain interactions and the consequent more ordered interfacial molecular arrangement. Additionally, the antisymmetric stretching mode in the PPP and SPS spectra exhibited a peak shift, showing that there exists more than one kind of orientation or chemical environment for the butyl CH3 group. These results may shed new light on understanding the surface behavior of watermiscible ionic liquids as well as the imidazolium based surfactants.     

下缘芳香取代结构对杯[4]芳烃Langmuir膜的影响

The Langmuir monolayer properties of lower rim aromatically substituted calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis（2-naphth-1＇-ylacetylaminoethoxy）-26,28-dihydroxylcalix[4]arene （BNAEC）, 5,11,17,23-tetra-tert- butyl-25,27-bis（2-benzoylamino ethoxy）-26,28-dihydroxylcalix[4]arene （BBAEC） and 5,11,17,23-tetra-tert-butyl- 25,27-bis（2-cinnamoylaminoethoxy）-26,28-dihydroxylcalix[4]arene （BCAEC）, have been studied. Film balance measurements and Brewster angle microscopy （BAM） observation demonstrate that all the compounds can form Langmuir monolayers with different molecular limiting areas. BNAEC or BBAEC monolayer is able to form condensed domains during compression, while BCAEC monolayer can never form condensed domain. BNAEC monolayer is more readily to form condensed domain than BBAEC monolayer. Moreover, BNAEC monolayer can form the total condensed phase during compression even when T=28℃, while BBAEC monolayer can not when T 〉 10 ℃. The results imply that different lower rim aromatic substitutions affect essentially the intermolecular interaction and molecular packing in the monolayer at air/water interface.