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Within cell plasma membranes, unsaturated lipids are asymmetrically distributed over the inner and outer leaflets, offering an attractive local structural feature. However, the mechanism to keep lipid transmembrane asymmetry and the closely related transmembrane movement (flip-flop) for unsaturated lipids remain poorly understood. Here, we applied sum frequency generation vibrational spectroscopy to investigate this lipid transmembrane asymmetry upon mimicking the cell membrane homeostatic processes. On the one hand, unsaturated lipids were found to hinder the flip-flop process and preserve lipid transmembrane asymmetry in model cell membranes, owing to the steric hindrance caused by their bent tails. On the other hand, local unsaturated lipids in the mixed unsaturated/saturated lipid bilayer were conducive to the formation of the local asymmetry. Therefore, lipid unsaturation can be recognized as an intrinsic key factor to form and maintain lipid transmembrane asymmetry in cell membranes. [ABSTRACT FROM AUTHOR]

Distinct annexin V binding behaviours in Ca2+-dependent and Ca2+-independent cases were comparatively investigated using sum frequency generation vibrational spectroscopy. It was discovered that binding affected the molecular arrangement of both membrane leaflets, and the initial Ca2+-independent binding went through a transition with annexin V reorientation to a more stable state upon adding Ca2+. [ABSTRACT FROM AUTHOR]

[Display omitted] • The specific surface-and phase-sensitive SFG spectroscopy revealed the absolute PS structure at the buried PS/silica interface. • The PS nanolayer near the substrate gradually formed during annealing. • The dewetting occurred at the non-adsorbed PS/the flattened PS nanolayer interface due to the autophobic effect. • As the solvent quality reduced, the dewetting rate of the PS film became slower. • This study provides a molecular-level understanding of the dewetting property of the polymer thin film. Dewetting is a common phenomenon observed from polymer thin films under stimuli, such as annealing, which directly affects the applications of polymer thin films. A key to understanding such dewetting behavior is to precisely probe the molecular structure evolution at the buried polymer/substrate interface during annealing. However, non-destructively probing the buried interface is very difficult at the molecular level and in situ. In this study, the specific surface- and phase-sensitive sum frequency generation (SFG) vibrational spectroscopy was used to investigate PS thin films on silica substrates during annealing. The results showed that the absolute orientation/conformation of PS at the film surface was significantly different from that of the flattened PS nanolayer formed near the substrate by dewetting. The dynamic molecular structure evolution of the PS nanolayer (the formation process of the flattened PS nanolayer) at the buried PS/silica interface was directly observed in situ. The dewetting occurred at the non-adsorbed (bulk, free) PS/the flattened PS nanolayer interface due to the autophobic effect. Furthermore, the effects of end groups and solvents on the PS dewetting rate were also elucidated. This study provides a molecular-level understanding of the PS structure at the buried PS/silica interface and its relationship to dewetting behavior. [ABSTRACT FROM AUTHOR]

In this research, sum frequency generation (SFG) vibrational spectroscopy, a surface and interface specific technique, was used to study chemical interactions occurring at the buried interface between two polymers in situ. More specifically, the chemical reaction at the buried poly (ethylene vinyl alcohol) (EVOH)/maleic anhydride (MAH) grafted polyethylene (MAHgEO) interface was monitored at the molecular level using SFG. This system was compared to a control EVOH/polyethylene (EO) (without MAH) system where no chemical reaction was observed at the interface. Results from the hydroxyl and carbonyl stretching regions contain contributions from the reaction products of EVOH with MAHgEO. SFG results suggest a chemical reaction occurs which increases interlayer adhesion, consistent with adhesion measurements. This work demonstrates that chemical reactions at buried interfaces between two polymers can be probed in situ nondestructively by SFG, and such interfacial reactions are important for the development of designed polymer/polymer interfaces with improved functionality and properties. Image 1 • Maleic anhydride grafted to polyethylene greatly enhanced adhesion at the polyethylene/poly (ethyl vinyl alcohol) interface. • Enhanced adhesion at the interface is due to the interfacial reaction between maleic anhydride and hydroxyl groups of EVOH. • Interfacial reaction was observed non-destructively at the buried polymer interface in situ. [ABSTRACT FROM AUTHOR]

• SFG revealed the interfacial structural evolution of PS under confinement. • The penetration of the PS chain at the d 8 -PS/PMMA interface occurred at ∼423 K. • The molecular ordering/orientation evolved during the penetration. • The tilt angle of the phenyl group increased while the twist angle decreased. The geometrical confinement directly affects many crucial properties of polymer thin films by interfacial interactions. A key to understanding the role of these interfacial interactions is to specifically probe the confined interface at the molecular level and in situ. However, the direct and nondestructive detection of interfacial polymer structures under confinement is very difficult. In this study, specific interface-sensitive sum frequency generation (SFG) spectroscopy was applied to study the deuterated PS (d 8 -PS) film confined between hydrogenated poly (methyl methacrylate) (PMMA) films at the molecular level and in situ. The results showed that the ordering/orientation of the PS chain (backbone and phenyl group) evolved at the buried d 8 -PS/PMMA interface during annealing. The tilt angle of the PS phenyl group increased while the twist angle decreased with elevated annealing temperature. Both overall SFG ssp and ppp intensities decreased after annealing at 433 K for 24 h, indicating that PS chains entangled with or penetrated the PMMA chains at the confined interface. This study qualitatively and quantitatively revealed the interfacial structure and structural evolution of the PS chain confined between PMMA films in situ during annealing, which is beneficial to the molecular-level understanding of the interfacial chain conformational relaxation of polymers under confinement. [Display omitted] [ABSTRACT FROM AUTHOR]

Sumfrequency generation (SFG) vibrational spectroscopy was appliedto study molecular interactions between amantadine and substrate supportedlipid bilayers serving as model cell membranes. Both isotopicallyasymmetric and symmetric lipid bilayers were used in the research.SFG results elucidated how the water-soluble drug, amantadine, influencedthe packing state of each leaflet of a lipid bilayer and how the drugsaffected the lipid flip-flop process. It is difficult to achieve suchdetailed molecular-level information using other analytical techniques.Especially, from the flip-flop rate change of isotopically asymmetriclipid bilayer induced by amantadine, important information on thedrug–membrane interaction mechanism can be derived. The resultsshow that amantadine can be associated with zwitterionic PC bilayersbut has a negligible influence on the flip-flop behavior of PC moleculesunless at high concentrations. Different effects of amantadine onthe lipid bilayer were observed for the negatively charged DPPG bilayer;low concentration amantadine (e.g., 0.20 mM) in the subphase couldimmediately disturb the outer lipid leaflet and then the lipid associatedamantadine molecules gradually reorganize to cause the outer leafletto return to the original orderly packed state. Higher concentrationamantadine (e.g., 5.0 mM) immediately disordered the packing stateof the outer lipid leaflet. For both the high and low concentrationcases, amantadine molecules only bind to the outer PG leaflet andcannot translocate to the inner layer. The presence of amantadinewithin the negatively charged lipid layers has certain implicationsfor using liposomes as drug delivery carriers for amantadine. Besides,by using PC or PG bilayers with both leaflets deuterated, we wereable to examine how amantadine is distributed and/or oriented withinthe lipid bilayer. The present work demonstrates that SFG resultscan provide an in-depth understanding of the molecular mechanismsof interactions between water-soluble drugs and model cell membranes. [ABSTRACT FROM AUTHOR]