Femtosecond Sum Frequency Generation Research Articles

Dynamic protein hydration water mediates the aggregation kinetics of amyloid β peptides at interfaces

Protein hydration water is essential for protein misfolding and amyloid formation, but how it directs the course of amyloid formation has yet to be elucidated. Here, we experimentally demonstrated that femtosecond sum frequency generation vibrational spectroscopy (SFG–VS) and the femtosecond IR pump–SFG probe technique can serve as powerful tools for addressing this issue. Using amyloid β(1–42) peptide as a model, we determined the transient misfolding intermediates by probing the amide band spectral features and the local hydration water changes by measuring the ultrafast vibrational dynamics of the amide I band. For the first time, we established a correlation between the dynamic change in protein hydration water and aggregation propensity. The aggregation propensity depends on the dynamic change in the hydration water, rather than the static hydration water content of the initial protein state. Water expulsion enhances the aggregation propensity and promotes amyloid formation, while protein hydration attenuates the aggregation propensity and inhibits amyloid formation. The suppression of water expulsion and protein hydration can prevent protein aggregation and stabilize proteins. These findings contribute to a better understanding of the underlying effect of hydration water on amyloid formation and protein structural stability and provide a strategy for maintaining long–term stabilization of biomolecules.

Close Packing in Trans Conformers Promotes the Formation of Supramolecular Structures with C1 Symmetry.

Assemblies with C1 symmetry exhibit important applications in many fields such as enantioselective catalysis. However, their formation is challenging due to their large entropic disadvantage, and molecular information on their formation dynamics is limited because of the lack of effective characterization techniques. Here, using achiral amphiphilic molecules such as N-oleoyl ethanolamide (OEA) and its analogues as modeling assembly units, we demonstrated that the sss polarization signals, generated by femtosecond sum frequency generation vibrational spectroscopy (SFG-VS), provide a powerful tool to monitor the formation dynamics of the C1 symmetric supramolecular structures at the interfaces. The trans conformation of the assembly units can provide strong π-π interactions and thus produce enough enthalpy to drive the formation of C1 symmetric supramolecular structures. However, the cis conformation impedes the assembly of C1 symmetric structures and cannot generate sss and chiral polarization SFG signals. These findings may aid in rationally constructing ordered and functional superstructures and understanding the mechanism of chirality formation.

Local electric field in nanocavities dictates the vibrational relaxation dynamics of interfacial molecules.

Plasmonic nanocavities enable the generation of strong light-matter coupling and exhibit great potential in plasmon-mediated chemical reactions (PMCRs). Although an electric field generated by nanocavities (E n) has recently been reported, its effect on the vibrational energy relaxation (VER) of the molecules in the nanocavities has not been explored. In this study, we reveal the impact of an electric field sensed by molecules (para-substituted thiophenol derivatives) in a nanocavity (E f) on VER processes by employing advanced time-resolved femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) supplemented by electrochemical measurements. The magnitude of E n is almost identical (1.0 ± 0.2 V nm-1) beyond the experimental deviation while E f varies from 0.3 V nm-1 to 1.7 V nm-1 depending on the substituent. An exponential correlation between E f and the complete recovery time of the ground vibrational C[double bond, length as m-dash]C state (T 2) of the phenyl ring is observed. Substances with a smaller T 2 are strongly correlated with the reported macroscopic chemical reactivity. This finding may aid in enriching the current understanding of PMCRs and highlights the possibility of regulating vibrational energy flow into desired reaction coordinates by using a local electric field.

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites.

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

Visualizing Water Monomers and Chiral OH-(H2O) Complexes Infiltrated in a Macroscopic Hydrophobic Teflon Matrix.

Insights into the interaction of fluoroalkyl groups with water are crucial to understanding the polar hydrophobicity of fluorinated compounds, such as Teflon. While an ordered hydrophobic-like 2D water layer has been demonstrated to be present on the surface of macroscopically hydrophobic fluorinated polymers, little is known about how the water infiltrates into the Teflon and what is the molecular structure of the water infiltrated into the Teflon. Using highly sensitive femtosecond sum frequency generation vibrational spectroscopy (SFG-VS), we observe for the first time that monomeric H2O and chiral OH-(H2O) complexes are present in macroscopically hydrophobic Teflon. The species are inhomogeneously distributed inside the Teflon matrix and at the Teflon surface. No water clusters or single-file water "wires" are observed in the matrix. SFG free induction decay (SFG-FID) experiments demonstrate that the OH oscillators of physically absorbed molecular water at the surface dephase on the time scale of <230 fs, whereas the water monomers and hydrated hydroxide ions infiltrated in the Teflon matrix dephase much more slowly (680-830 fs), indicating that the embedded monomeric H2O and OH-(H2O) complexes are decoupled from the outer environment. Our findings can well interpret ultrafast water permeation through fluorous nanochannels and the charging mechanism of Teflon, which may tailor the desired applications of organofluorines.

Observing Nonpreferential Absorption of Linear and Cyclic Carbonate on the Silicon Electrode.

Silicon is reported to be a promising anode material due to its high storage capacity and excellent energy conversion rate. Molecular-level insight into the interaction between silicon electrodes and electrolyte solutions is essential for understanding the formation of a stable solid electrolyte interphase (SEI), but it is yet to be explored. In this study, we apply femtosecond sum frequency generation vibrational spectroscopy to investigate the initial adsorption of various pure and mixed electrolyte molecules on the silicon anode surface by monitoring the SFG signals from the carbonyl group of electrolyte molecules. When the silicon comes in contact with a pure carbonate solution, the linear carbonates of diethyl carbonate and ethyl methyl carbonate adopt two conformations with opposite C═O orientations on the silicon interface while the cyclic carbonates of ethylene carbonate and propylene carbonate almost adopt one conformation with C═O bonds pointing toward the silicon electrode. When the silicon comes in contact with the mixed linear and cyclic carbonate solutions, the total SFG intensity from the mixed solutions is approximately 2∼5 times weaker than those of pure cyclic carbonates. The C═O bonds of cyclic carbonates point toward the silicon electrode, while the C═O bonds of linear carbonates face toward the bulk solution at the silicon/mixed solution interface. No preferential absorption behaviors of the linear and cyclic carbonate electrolytes on the silicon electrode are observed. Such findings may help to understand the mechanism by which the SEI formed on the silicon anode is unstable.

Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra

Surface plasmon-enhanced vibrational spectroscopy has been demonstrated to be an important highly sensitive diagnostic technique, but its enhanced mechanism is yet to be explored. In this study, we couple femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with surface plasmon generated by the excitation of localized gold nanorods/nanoparticles and investigate the plasmonically enhanced factors (EFs) of SFG signals from poly(methyl methacrylate) films. Through monitoring the SFG intensity of carbonyl and ester methyl groups, we have established a correlation between EFs and the coupling of localized surface plasmon resonance with SFG and visible beams. It is found that the total enhanced factor is approximately proportional to the square of an enhanced factor of the SFG electromagnetic field and the fourth power of the enhanced factor of the visible electromagnetic field. The local field effect is roughly expressed to be the square of an enhanced factor of the visible electromagnetic field. This finding will help to guide the experimental design of plasmon-enhanced SFG to drastically improve the detection sensitivity and thus provide greater insight into the ultrafast dynamics near plasmonic surfaces.

Sum frequency generation vibrational spectra of perovskite nanocrystals at the single-nanocrystal and ensemble levels

Determination of molecular structures of organic-inorganic hybrid perovskite (OIHP) nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process, but its detection is still a bit challenging. In this study, we demonstrate that femtosecond sum frequency generation (SFG) vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter (∼10 nm) at the single-nanocrystal level. The SFG signals are monitored using the spectral features of the phenyl group in (R-MBA)PbBr3 and (R-MBA)2PbI4 nanocrystals (MBA: methyl-benzyl-ammonium). It is found that the SFG spectra exhibit a strong resonant peak at 3067±3 cm−1 (ν2 mode) and a weak shoulder peak at 3045±4 cm−1 (ν7a mode) at the ensemble level, whereas a peak of the ν2 mode and a peak at 3025±3 cm−1 (ν20b mode) at the single-nanocrystal level. The nanocrystals at the single-nanocrystal level tend to lie down on the surface, but stand up as the ensemble number and the averaged sizes increase. This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.

Molecular Coupling between Organic Molecules and Metal.

Molecular couplings at interfaces play important roles in determining the performance of nanophotonics and molecular electronics. In this Letter, using femtosecond sum frequency generation to trace free-induction decay of vibrationally excited aromatic thiol molecules immobilized on metal with and without the bridged methylene group(s), metal surface free electron-coupled and uncoupled phenyl C-H stretching vibrational modes were identified, with dephasing times of ∼0.28 and ∼0.60 ps, respectively. For thiols on Au with the bridged methylene group(s) (benzyl mercaptan and phenylethanethiol), both the coupled and uncoupled modes were observed; for thiol on Au without the bridged methylene group (thiophenol), only the coupled mode was observed. This indicates that the bridged methylene group(s) serving as a spacer can be used to adjust the molecular coupling between the phenyl vibration and surface free electrons. The experimental approach can be used to tune molecular couplings in advanced nanophotonics and molecular electronics.

Real-Time observation of protein transport across membranes by femtosecond sum frequency generation vibrational spectroscopy

Characterization of conformation kinetics of proteins at the interfaces is crucial for understanding the biomolecular functions and the mechanisms of interfacial biological action. But it requires to capture the dynamic structures of proteins at the interfaces with sufficient structural and temporal resolutions. Here, we demonstrate that a femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) system developed by our group provides a powerful tool for monitoring the real-time peptide transport across the membranes with time resolution of less than one second. By probing the real-time SFG signals in the amide I and amide A bands as WALP23 interacts with DMPG lipid bilayer, it is found that WALP23 is initially absorbed at the gel-phase DMPG bilayer with a random coil structure. The absorption of WALP23 on the surface leads to the surface charge reversal and thus changes the orientation of membrane-bound water. As the DMPG bilayer changes from gel phase into fluid phase, WALP23 inserts into the fluid-phase bilayer with its N-terminal end moving across the membrane, which causes the membrane dehydration and the transition of WALP23 conformation from random coil to mixed helix/loop structure and then to pure α-helical structure. The established system is ready to be employed in characterizing other interfacial fast processes, which will be certainly helpful for providing a clear physical picture of the interfacial phenomena.

Orientational Analysis of Interfacial Molecular Groups of a 2-Methoxyethyl Methacrylate Monomer Using Femtosecond Sum Frequency Generation Spectroscopy

This study employs femtosecond sum frequency generation (FS-SFG) spectroscopy to identify the functional groups at the air–liquid interface of the 2-methoxyethyl methacrylate (MEMA) monomer. Based on the collected spectra, the methoxy group (−OCH3), the methylene (−CH2) group from the ethyl side chain of MEMA, the α-methyl group (α-CH3), and the alkene-methylene (═CH2) groups are present at the interface. Tilt angle was determined by calculating the intensity ratio values from the fitting results, and then the ratios are compared to the simulated SFG curves. The results derived from the conventional polarization combination (SSP, PPP, and PSS) approach show average tilt angles of 60° for −OCH3 symmetric stretch vibration dipoles. Using the polarization mapping method, the tilt angle for the symmetric stretch of the −OCH3 group was 48° with an intensity ratio (SSP/PPP) of 17. The orientation distribution of the functional groups was also obtained from the amplitude ratios using the two methods. In conclusi...

Preferential Organization of Methacrylate Monomers and Polymer Thin Films at the Air Interface Using Femtosecond Sum Frequency Generation Spectroscopy

Monitoring and understanding monomer conformational changes as substituents are varied is an important step in investigating in situ polymerization and controlling the polymerization process as well as regulating the functionality of polymer surfaces to provide efficient polymer coatings. In this study, the preliminary stage involves characterization of the interfacial structures of methacrylate-based functional monomers and their polymer thin films at the air–liquid interface using femtosecond sum frequency generation spectroscopy (FSFGS). By varying the substituted ethyl group of the methacrylate monomers with hydroxy (−OH), chloro (−Cl), and phenoxy (−OPh), the alpha-methyl (α-CH3), alkene-methylene (alkene-CH2), and methylene (CH2) groups are observed to have preferential surface ordering toward the air interface. A peak positioned at ∼3000 cm–1 was observed in the spectrum of the 2-hydroxyethyl methacrylate (HEMA) monomer and assigned to alkene-CH2 group. This functional group signifies that the pure...

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.

Femtosecond sum frequency generation at the metal–liquid interface

We present a study of the C–H and C–N vibrational modes of acetonitrile molecules at the interface between the bulk liquid and a gold film using both time and frequency domain femtosecond sum frequency generation. Acetonitrile is adsorbed on the gold surface with the N atom towards the interface. Due to the surface roughness there are different adsorption sites, leading to an inhomogeneous distribution of vibrational frequencies of the C–N stretch vibration of different molecules. This is observed most clearly in the time domain SFG measurements, while the frequency domain measurements are much less sensitive to the inhomogeneity. In contrast to the C–N stretch mode, the C–H stretching modes are unaffected by the surface and display homogeneous line broadening.

Ultrafast Surface Dynamics Studied with Femtosecond Sum Frequency Generation

We present an investigation into femtosecond time-resolved vibrational sum frequency spectroscopy as a tool for studying ultrafast surface dynamics. It is demonstrated that the presence of experimentally observed new spectral features does not necessarily mean that a novel chemical species has appeared on the surface. In addition, we present a simple model for calculating the sum frequency intensity as a chemical reaction takes place on the surface via a transition state or reaction intermediate. These calculations show that a transition state can be observed and its lifetime can be determined.