Intermolecular Protein-Water Coupling Impedes the Coupling Between the Amide A and Amide I Mode in Interfacial Proteins.

The coupling between different vibrational modes in proteins is essential for chemical dynamics and biological functions and is linked to the propagation of conformational changes and pathways of allosteric communication. However, little is known about the influence of intermolecular protein-H2O coupling on the vibrational coupling between amide A (NH) and amide I (C═O) bands. Here, we investigate the NH/CO coupling strength in various peptides with different secondary structures at the lipid cell membrane/H2O interface using femtosecond time-resolved sum frequency generation vibrational spectroscopy (SFG-VS) in which a femtosecond infrared pump is used to excite the amide A band, and SFG-VS is used to probe transient spectral evolution in the amide A and amide I bands. Our results reveal that the NH/CO coupling strength strongly depends on the bandwidth of the amide I mode and the coupling of proteins with water molecules. A large extent of protein-water coupling significantly reduces the delocalization of the amide I mode along the peptide chain and impedes the NH/CO coupling strength. A large NH/CO coupling strength is found to show a strong correlation with the high energy transfer rate found in the light-harvesting proteins of green sulfur bacteria, which may understand the mechanism of energy transfer through a molecular system and assist in controlling vibrational energy transfer by engineering the molecular structures to achieve high energy transfer efficiency.

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.

Counterion-Mediated Hydrogen Bonding Making Poly(styrenesulfonate)-Based Strong Polyelectrolytes pH-Responsive.

Owing to the well-established fact that poly(styrenesulfonate) (PSS)-based strong polyelectrolytes are pH insensitive, their applications in smart materials have thus been severely limited. However, we demonstrate here that counterion-mediated hydrogen bonding (CMHB) makes the PSS brush pH-responsive. With decreasing pH, more hydrogen bonds are formed between the bound hydronium counterions and the sulfonate (-SO3-) groups in the PSS brush. At the microscale, the formation of more hydrogen bonds with decreasing pH leads to a more ordered structure and a larger tilt angle of the -SO3- groups in the PSS brush. On the other hand, a range of important physicochemical properties of the PSS brush, including hydration, stiffness, wettability, and adhesion, are responsive to pH, induced by the effect of CMHB on the PSS brush. Our work reveals a clear structure-property relationship for the pH-responsive PSS brush. This work not only provides a new understanding of the fundamental properties of the PSS brush but also greatly extends the applications of PSS-based strong polyelectrolytes.

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites.

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy.

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (diw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T1) of the OH stretching mode at 3200 cm-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine diw. By investigating the T1 values at the SiO2/NaCl solution interface, we established a time-space (T1-diw) relationship. We find that water has a T1 lifetime of ≥0.5 ps for diw ≤ 3 Å, while it displays bulk-like dynamics with T1 ≤ 0.2 ps for diw ≥ 9 Å. T1 decreases as diw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK15ß protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T1 values.

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.

Boosting Charge Transport in a 2D/3D Perovskite Heterostructure by Selecting an Ordered 2D Perovskite as the Passivator.

We demonstrate that an ordered 2D perovskite can significantly boost the photoelectric performance of 2D/3D perovskite heterostructures. Using selective fluorination of phenyl-ethyl ammonium (PEA) lead iodide to passivate 3D FA0.8 Cs0.2 PbI3 , we find that the 2D/3D perovskite heterostructures passivated by a higher ordered 2D perovskite have lower Urbach energy, yielding a remarkable increase in photoluminescence (PL) intensity, PL lifetime, charge-carrier mobilities (ϕµ), and carrier diffusion length (LD ) for a certain 2D perovskite content. High performance with an ultralong PL lifetime of ≈1.3 µs, high ϕµ of ≈18.56 cm2  V-1 s-1 , and long LD of ≈7.85 µm is achieved in the 2D/3D films when passivated by 16.67 % para-fluoro-PEA2 PbI4 . This carrier diffusion length is comparable to that of some perovskite single crystals (>5 µm). These findings provide key missing information on how the organic cations of 2D perovskites influence the performance of 2D/3D perovskite heterostructures.

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.

Protein-water coupling tunes the anharmonicity of amide I modes in the interfacial membrane-bound proteins.

The diagonal anharmonicity of an amide I mode of protein backbones plays a critical role in a protein's vibrational dynamics and energy transfer. However, this anharmonicity of long-chain peptides and proteins in H2O environment is still lacking. Here, we investigate the anharmonicity of the amide I band of proteins at the lipid membrane/H2O interface using a surface-sensitive pump-probe setup in which a femtosecond infrared pump is followed by a femtosecond broadband sum frequency generation vibrational spectroscopy probe. It is found that the anharmonicity of the amide I mode in ideal α-helical and ß-sheet structures at hydrophobic environments is 3-4 cm-1, indicating that the amide I mode in ideal α-helical and ß-sheet structures is delocalized over eight peptide bonds. The anharmonicity increases as the bandwidth of the amide I mode increases due to the exposure of peptide bonds to H2O. More H2O exposure amounts lead to a larger anharmonicity. The amide I mode of the peptides with large H2O exposure amounts is localized in one to two peptide bonds. Our finding reveals that the coupling between the amide I mode and the H2O bending mode does not facilitate the delocalization of the amide I mode along the peptide chain, highlighting the impact of H2O on energy transfer and structural dynamics of proteins.

Ordered Water Layer on the Macroscopically Hydrophobic Fluorinated Polymer Surface and Its Ultrafast Vibrational Dynamics.

Hydrophobic-like water monolayers have been predicted at the metal and some polar surfaces by theoretical simulations. However, direct experimental evidence for the presence of this water layer at surfaces, particularly at biomolecule and polymer surfaces, is yet to be validated at room temperature. Here we observe experimentally that an ordered molecular water layer is present at the hydrophobic fluorinated polymer such as polytetrafluoroethylene (PTFE) surface by using sum frequency generation vibrational spectroscopy. The macroscopic hydrophobicity of PTFE surface is actually hydrophilic at the molecular level. The macroscopically hydrophobic character of PTFE is indeed resulting from the hydrophobicity of the ordered two-dimension (2D) water layer, in which cyclic water tetramer structure is found. The water layer at humidity of ≤40% has a vibrational relaxation time of 550 ± 60 fs. The vibrational relaxation time in the frequency range of 3200-3400 cm-1 shows remarkable difference from the interfacial water at the air/H2O interface and the lipid/H2O interface. No discernible frequency dependence of the vibrational relaxation time is observed, indicating the homogeneous dynamics of OH groups in the water layer. These insights into the water layer at the macroscopically hydrophobic surface may contribute to a better understanding of the hydrophobic interaction and interfacial water dynamics.

Surface charge printing for programmed droplet transport.

The directed, long-range and self-propelled transport of droplets on solid surfaces is crucial for many applications from water harvesting to bio-analysis1-9. Typically, preferential transport is achieved by topographic or chemical modulation of surface wetting gradients that break the asymmetric contact line and overcome the resistance force to move droplets along a particular direction10-16. Nonetheless, despite extensive progress, directional droplet transport is limited to low transport velocity or short transport distance. Here we report the high-velocity and ultralong transport of droplets elicited by surface charge density gradients printed on diverse substrates. We leverage the facile water droplet printing on superamphiphobic surfaces to create rewritable surface charge density gradients that stimulate droplet propulsion under ambient conditions17 and without the need for additional energy input. Our strategy provides a platform for programming the transport of droplets on flat, flexible and vertical surfaces that may be valuable for applications requiring a controlled movement of droplets17-19.

Acidic Environment Significantly Alters Aggregation Pathway of Human Islet Amyloid Polypeptide at Negative Lipid Membrane.

The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) at cell membrane has a close relationship with the development of type 2 diabetes (T2DM). This aggregation process is susceptible to various physiologically related factors, and systematic studies on condition-mediated hIAPP aggregation are therefore essential for a thorough understanding of the pathology of T2DM. In this study, we combined surface-sensitive amide I and amide II spectral signals from the protein backbone, generated simultaneously in a highly sensitive femtosecond broad-band sum frequency generation vibrational spectroscopy system, to examine the effect of environmental pH on the dynamical structural changes of hIAPP at membrane surface in situ and in real time. Such a combination can directly discriminate the formation of ß-hairpin-like monomer and oligomer/fibril at the membrane surface. It is evident that, in an acidic milieu, hIAPP slows down its conformational evolution and alters its aggregation pathway, leading to the formation of off-pathway oligomers. When matured hIAPP aggregates are exposed to basic subphase, partial conversion from ß-sheet oligomers into ordered ß-sheet fibrillar structures is observed. When exposed to acidic environment, however, hIAPP fibrils partially converse into more loosely patterned ß-sheet oligomeric structures.

Misfolding of a Human Islet Amyloid Polypeptide at the Lipid Membrane Populates through ß-Sheet Conformers without Involving α-Helical Intermediates.

Amyloid formation has been implicated in many fatal diseases, but its mechanism remains to be clarified due to a lack of effective methods that can capture the transient intermediates. Here, we experimentally demonstrate that sum frequency generation vibrational spectroscopy can unambiguously discriminate the intermediates during amyloid formation at the lipid membrane in situ and in real time by combining the chiral amide I and achiral amide II and amide III spectral signals of the protein backbone. Such a combination can directly identify the formation of ß-hairpin-like monomers and ß-sheet oligomers and fibrils. A strong correlation between the amide II signals and the formation of ß-sheet oligomers and fibrils was found. With this approach, the structural evolution of human islet amyloid polypeptides (hIAPP) at negative lipid bilayers was elucidated. It was firmly confirmed that hIAPP populates through ß-sheet conformers without involving α-helical intermediates. The membrane-associated assembly of hIAPP proceeds by assembling with a ß-hairpin-like monomer at the lipid bilayer surface, rather than by inserting the preassembled ß-sheet oligomers in solution. This newly established protocol is ready to be utilized in revealing the mechanism of amyloid aggregation at the lipid membrane.

Specific Ion Effects on Protein Thermal Aggregation from Dilute Solutions to Crowded Environments.

We have investigated specific ion effects on protein thermal aggregation from dilute solutions to crowded environments. Ovalbumin and poly(ethylene glycol) have been employed as the model protein and crowding agent, respectively. Our studies demonstrate that the rate-limiting step of ovalbumin thermal aggregation is changed from the aggregation of unfolded protein molecules to the unfolding of the protein molecules, when the solution conditions are varied from a dilute solution to a crowded environment. The specific ion effects acting on the thermal aggregation of ovalbumin generated by kosmotropic and chaotropic ions are different. The thermal aggregation of ovalbumin molecules is promoted by kosmotropic anions in dilute solutions via an increase in protein hydrophobic interactions. In contrast, ovalbumin thermal aggregation is facilitated by chaotropic ions in crowded environments through accelerated unfolding of protein molecules. Therefore, there are distinct mechanisms causing the ion specificities of protein thermal aggregation between dilute solutions and crowded environments. The ion specificities are dominated by ion-specific hydrophobic interactions between protein molecules and ion-specific unfolding of protein molecules in dilute solutions and crowded environments, respectively.

Amide I SFG Spectral Line Width Probes the Lipid-Peptide and Peptide-Peptide Interactions at Cell Membrane In Situ and in Real Time.

The balance of lipid-peptide and peptide-peptide interactions at cell membrane is essential to a large variety of cellular processes. In this study, we have experimentally demonstrated for the first time that sum frequency generation vibrational spectroscopy can be used to probe the peptide-peptide and lipid-peptide interactions in cell membrane in situ and in real time by determination of the line width of amide I band of protein backbone. Using a "benchmark" model of α-helical WALP23, it is found that the dominated lipid-peptide interaction causes a narrow line width of the amide I band, whereas the peptide-peptide interaction can markedly broaden the line width. When WALP23 molecules insert into the lipid bilayer, a quite narrow line width of the amide I band is observed because of the lipid-peptide interaction. In contrast, when the peptide lies down on the bilayer surface, the line width of amide I band becomes very broad owing to the peptide-peptide interaction. In terms of the real-time change in the line width, the transition from peptide-peptide interaction to lipid-peptide interaction is monitored during the insertion of WALP23 into 1,2-dipalmitoyl- sn-glycero-3-phospho-(1'- rac-glycerol) (DPPG) lipid bilayer. The dephasing time of a pure α-helical WALP23 in 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho-(1'- rac-glycerol) and DPPG bilayer is determined to be 2.2 and 0.64 ps, respectively. The peptide-peptide interaction can largely accelerate the dephasing time.

In situ examination of a charged amino acid-induced structural change in lipid bilayers by sum frequency generation vibrational spectroscopy.

The interactions between amino acids (AAs) and membranes represent various short-range and long-range interactions for biological phenomena; however, they are still poorly understood. In this study, we used cationic lysine and arginine as AA models, and systematically investigated the interactions between charged AAs and lipid bilayers using sum frequency generation vibrational spectroscopy (SFG-VS) in situ and in real time. The AA-induced dynamic structural changes of the lipid bilayer were experimentally monitored using the spectral features of CD2, CD3, the lipid head phosphate, and carbonyl groups in real time. Time-dependent SFG changes in the structure of the lipid bilayer provide direct evidence for the different interactions of lysine and arginine with the membrane. It was found that the discrepancy between lysine and arginine in binding with the lipid bilayer is due to the nature of the terminal functional groups. Arginine exhibits a more drastic impact on the membrane than lysine. SFG responses of the acyl chains, phosphate groups, and carbonyl groups provide evidence that the interaction between AAs and the membrane most likely follows an electrostatics and hydrogen bond-induced defect model. This work presents an exemplary method for comprehensive investigations of interactions between membranes and other functionally significant substances.

Molecular interactions of organic molecules at the air/water interface investigated by sum frequency generation vibrational spectroscopy.

The molecular structure and dynamics of organic molecules at the aqueous interface have attracted a number of investigations owing to their importance and specific nature. However, there are relatively few studies on the direct characterization of the molecular interactions at the air/water interface because they are extremely difficult to measure in experiments. In this study, we use dibutyl ester molecules (R1CO2R2O2CR1) as a model of organic molecules, and investigate their molecular structure and interactions using sum frequency generation vibrational spectroscopy. We demonstrate that the molecular interactions can be estimated by measuring the intensity ratio of the symmetric stretching (ν1) and Fermi resonant bands (2ν2) of methyl groups. Here, dibutyl ester molecules are widely used as plasticizers in polymers to improve the properties of the plastics and polymers. It is found that the orientation angles of the tailed methyl groups at the air/water interface decrease from 34° to 19° when the chain length of R2 increases from 0 to 8. The total intermolecular interactions of the dibutyl ester molecules decrease as the chain length of R2 increases because the van der Waals interactions between the hydrocarbon chains increase, while the hydrogen bond interactions between the carbonyl group and water molecules decrease. Our study demonstrates the stability of ester-based plasticizers in polymers can be well predicted from the intensity ratio of the ν1 and 2ν2 bands of methyl group. Such an intensity ratio can be thus used as an effective vibrational optical ruler for characterizing molecular interactions between plasticizers and polymers.

Ultrafast Vibrational Dynamics of Membrane-Bound Peptides at the Lipid Bilayer/Water Interface.

Vibrational energy transfer (VET) of proteins at cell membrane plays critical roles in controlling the protein functionalities, but its detection is very challenging. By using a surface-sensitive femtosecond time-resolved sum-frequency generation vibrational spectroscopy with infrared pump, the detection of the ultrafast VET in proteins at cell membrane has finally become possible. The vibrational relaxation time of the N-H groups is determined to be 1.70(±0.05) ps for the α-helix located in the hydrophobic core of the lipid bilayer and 0.9(±0.05) ps for the membrane-bound ß-sheet structure. The N-H groups with strong hydrogen bonding gain faster relaxation time. By pumping the amide A band and probing amide I band, the vibrational relaxation from N-H mode to C=O mode through two pathways (direct coupling and through intermediate states) is revealed. The ratio of the pathways depends on the NH⋅⋅⋅O=C hydrogen-bonding strength. Strong hydrogen bonding favors the coupling through intermediate states.

Transport and Organization of Cholesterol in a Planar Solid-Supported Lipid Bilayer Depend on the Phospholipid Flip-Flop Rate.

Understanding the transport behavior of the cholesterol molecules within a cell membrane is a key challenge in cell biology at present. Here, we have applied sum frequency generation vibrational spectroscopy to characterize the transport and organization of cholesterol in different kinds of planar solid-supported lipid bilayers by combining achiral- and chiral-sensitive polarization measurements. This method allows us to distinguish the organization of cholesterol in tail-to-tail, head-to-tail, head-to-head, and side-by-side manners. It is found that the movement of cholesterol in the lipid bilayer largely depends on the flip-flop rate of the phospholipid. The flip-flop dynamics of the phospholipid and cholesterol are synchronous. In the solid-supported zwitterionic phosphocholine lipid bilayer, the cholesterol molecules flip quickly from the distal leaflet to the neutral proximal leaflet of the bilayer and form tail-to-tail organization on both leaflets. The phosphocholine lipid and cholesterol show the same flip-flop rate. However, when the proximal leaflet is prepared using negative glycerol phospholipids, cholesterol organizes itself by mainly forming an α-ß structure on the distal leaflet. Because of the strong interaction between the glycerol phospholipid and the substrate, no or only partial cholesterol molecules flip from the distal leaflet to the negatively charged proximal leaflet. However, the cholesterol molecules undergo flip-flop in the presence of salt solution because the ions weaken the interaction between the negative phospholipid and the substrate.

Accurate determination of interfacial protein secondary structure by combining interfacial-sensitive amide I and amide III spectral signals.

Accurate determination of protein structures at the interface is essential to understand the nature of interfacial protein interactions, but it can only be done with a few, very limited experimental methods. Here, we demonstrate for the first time that sum frequency generation vibrational spectroscopy can unambiguously differentiate the interfacial protein secondary structures by combining surface-sensitive amide I and amide III spectral signals. This combination offers a powerful tool to directly distinguish random-coil (disordered) and α-helical structures in proteins. From a systematic study on the interactions between several antimicrobial peptides (including LKα14, mastoparan X, cecropin P1, melittin, and pardaxin) and lipid bilayers, it is found that the spectral profiles of the random-coil and α-helical structures are well separated in the amide III spectra, appearing below and above 1260 cm(-1), respectively. For the peptides with a straight backbone chain, the strength ratio for the peaks of the random-coil and α-helical structures shows a distinct linear relationship with the fraction of the disordered structure deduced from independent NMR experiments reported in the literature. It is revealed that increasing the fraction of negatively charged lipids can induce a conformational change of pardaxin from random-coil to α-helical structures. This experimental protocol can be employed for determining the interfacial protein secondary structures and dynamics in situ and in real time without extraneous labels.