Interfacial Hydrogen Bonds Research Articles

Fabrication of mesoporous SiO2@SLS composite to remove organic pollutants: hydrogen bond-induced intriguing changes of solubility

Because biomass lignin shows good adsorption activity, its derivatives such as alkali lignin and kraft lignin have been applied in water treatment; however, sodium lignosulfonate (SLS), mainly limited by its high water solubility, is not applicable in this respect. In this work, we fabricated SiO2@SLS mesoporous adsorbent using a facile solution coating method by one step. SLS coating SiO2 enables the former to be “separated” from water, accomplishing the soluble to insoluble conversion. This change makes the composite more suitable and efficient to remove organic pollutants from water than the SLS precursor. It is found that interfacial hydrogen bonds play a significant role in the conversion, which is further corroborated by density functional theory calculations. Various characterizations show that SiO2@SLS is composed of uniform particles around 80 nm in size. Abundant mesopores with diameters ranging from 2 to 3 nm are found on the surface of SiO2@SLS nanoparticles. The specific surface areas of SiO2@SLS were measured between 293.4 and 33.2 m2 g−1, depending on the SLS amount used in the preparing process. Originating from the bio-adsorbent lignin precursor, the SiO2@SLS composite shows excellent adsorption property. Representatively, SiO2@SLS-0.6 is able to efficiently remove methylene blue (MB), almost twice as much as SiO2-0 does; its adsorption capacity (Qm) reaches 206.2 mg g−1 according to the Langmuir model fitting; notably, SiO2@SLS-0.6 works very well even in a high-concentration MB solution (50 mg L−1), showing removal efficiencies at 90.0% (within 1 h) and 99.9% (720 h, i.e., 30 days). Plus having remarkable adsorption stability, our composite SiO2@SLS is anticipated to be promising in application of polluted water treatment.

Hydrogen-bonding power interfacial load transfer of carbon fabric/polypyrrole composite pseudosupercapacitor electrode with improved electrochemical stability

The design of a strong interface interaction is a critical issue to improve the electrochemical stability of polypyrrole (PPy) composite electrodes. In the natural world, hydrogen bond that powers the load transfer can be observed in spider silk. The challenges are to achieve interfacial hydrogen bond interaction to stiff the PPy to help the effective load transfer and alleviate the swelling/shrinkage of PPy chains. Here, a composite electrode was prepared by pyrrole (Py) vapor deposition on the air-plasma-activated carbon fabric (PCF) surface. The X-ray photoelectron spectroscopy and fourier transformed infrared spectroscopy results showed that the COOH groups appearing on the PCF and hydrogen bonds can be formed on the interface of PCF/PPy. Interfacial shear strength (IFSS) and scanning electronic microscope determined hydrogen bond on the interface of PCF/PPy enhance the adhesion and power the load transfer process from PPy to carbon fiber. These interfacial hydrogen bonds severed as adhesion points helped the load transfer effectively to the substrate and decreased the contact resistance, preventing the damage of PPy chains and improving electrochemical performances of PCF/PPy as the electrode for pseudosupercapacitor. The PCF/PPy supercapacitor showed excellent cycling stability (93% capacitance retention after 4000 cycles), while CF/PPy showed poor cycling stability (74% after 4000 cycles). The idea of hydrogen bond powered electrochemical stability can be of reference for the researchers to improve the cycling stability of other conductive polymers that are widely used in high-performance supercapacitor.

Nanoscale Morphology, Interfacial Hydrogen Bonding, Confined Crystallization and Greatly Improved Toughness of Polyamide 12/Polyketone Blends.

Nanostructured polyamide 12(PA12)/polyketone (PK) blends were fabricated by melt compounding. The nanoscale droplet and domain-in-domain morphologies depending on PK content were observed. When the content of PK was 10 vol %, the impact strength of the blend jumped from 6.8 to 111.9 kJ/m2 and further improved with an increasing content of PK. The toughening mechanism was found to be closely related with the morphology change from nanoscale droplet morphology to domain-in-domain morphology owing to the strong interfacial hydrogen bonding. The nanoscale morphology confinement and interfacial hydrogen bonding enhances the crystallization kinetics, while it lowers down the thermodynamic stability of the crystals. The toughening mechanisms were discussed based on these factors.

Ionically Cross-Linked Silk Microfibers/Alginate Tough Composite Hydrogels with Hierarchical Structures

Developing hydrogels with enhanced mechanical properties has attracted broad attention in recent years. In this work, we propose a facile procedure to prepare tough composite hydrogels by incorporating silk microfibers (mSF) into alginate ionically cross-linked networks. The mSF gives rise to ionic bonds with Ca2+ and interfacial hydrogen bonds because of the carboxyl groups on the surface of mSF, and the neighboring alginate chains are interlinked by mSF, which synergistically led to efficient energy dissipation and prevention of stress concentration. The attained composite hydrogels show superior elastic modulus (1.58 MPa), tensile strength (1.60 MPa), and unique adaptive interface response. Moreover, the mechanical properties of the composite hydrogels can be tailored by the concentration of mSF and the time immersing the composite hydrogels in CaCl2 solution post-cross-linking. Intriguingly, the mechanical properties can be further improved through a prestretching methodology to align the alginate chains along the stress direction, where the oriented hierarchical structures are formed and well-retained in the prestretched composite hydrogels. We envisage that this study provides a general strategy for designing composite hydrogels with both excellent and tunable mechanical properties, which enriches the use of alginate hydrogels for promising applications where high loading is needed.

Nitrile rubber/sliding graft copolymer damping material with significantly improved strength and damping performance

ABSTRACTNitrile rubber (NBR)/sliding graft copolymer (SGC) composites with significantly improved strength and damping property are successfully prepared. SGC is a novel kind of supramolecular material with sliding crosslink junctions. The micromorphology analyses of NBR/SGC composites indicate that the SGC phase with particle size less than 500 nm is fairly uniformly dispersed in the NBR matrix. As SGC content increases, the loss factors (tan δ) of NBR/SGC composites increase gradually. Specifically, the tan δ of NBR/SGC (100/40) is about 1.2 times higher than that of pristine NBR rubber. The tensile strength and elongation at break of NBR/SGC composites are unexpectedly improved after the addition of SGC. The significantly improved damping performance and tensile strength can be ascribed to the pulley effect of SGC and the strong interfacial hydrogen bonds between SGC and NBR. The high damping performance and good mechanical strength make the NBR/SGC composite a promising high‐performance damping material. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47188.

Unique Structures and Vibrational Spectra of Protic Ionic Liquids Confined in TiO2 Slits: The Role of Interfacial Hydrogen Bonds.

The ionic liquid (IL)/titanium dioxide (TiO2) interface exists in many application systems, such as nanomaterial synthesis, catalysis, and electrochemistry systems. The nanoscale interfacial properties in the above systems are a common issue. However, directly detecting the interfacial properties of nanoconfined ILs by experimental methods is still challenging. To help better learn about the interfacial issue, molecular dynamics simulations have been performed to explore the structures, vibration spectra, and hydrogen bond (HB) properties at the IL/TiO2 interface. Ethylammonium nitrate (EAN) ILs confined in TiO2 slit pores with different pore widths were studied. A unique vibrational spectrum appeared for EAN ILs confined in a 0.7 nm TiO2 slit, and this phenomenon is related to interfacial hydrogen bonds (HBs). An analysis of the HB types indicated that the interfacial NH3+ group of the cations was in an asymmetric HB environment in the 0.7 nm TiO2 slit, which led to the disappearance of the symmetric N-H stretching mode. In addition, the significant increase in the HB strength between NH3+ groups and the TiO2 surface slowed down the stretching vibration of the N-H bond, resulting in one peak in the vibrational spectra at a lower frequency. For the first time, our simulation work establishes a molecular-level relationship between the vibrational spectrum and the local HB environment of nanoconfined ILs at the IL/TiO2 interface, and this relationship is helpful for interface design in related systems.

Water Adsorption to Crystalline Cu2O Thin Films: Structural and Vibrational Properties

D2O adsorption to well-ordered Cu2O(111) thin films has been explored with scanning tunneling microscopy, thermal desorption and infrared absorption spectroscopy. The molecules bind associatively to the oxide surface, whereby the first D2O layer experiences slightly stronger adhesion due to interfacial hydrogen bonds. At higher coverage, amorphous solid water (ASW) films condense on the Cu2O(111) surface, as concluded from a dominant 155 K desorption peak and a broad vibrational band centered at 2540 cm–1. Development of ASW is also observed on bare Au(111), being used as substrate for Cu2O growth. In contrast to the oxide surface, no binding enhancement is detected for the D2O monolayer and desorption is accompanied by dewetting and transformation to crystalline ice. The different binding schemes of water reflect the hydrophilic versus hydrophobic character of Cu2O(111) and Au(111) supports, respectively.

Development of high damping acrylic rubber/sliding graft copolymer composites.

Aiming at fabricating high damping rubber composites, the acrylic rubber ACM was incorporated with sliding graft copolymer (SGC) materials. SGC is a novel supramolecular material with sliding crosslink junctions, and it acts as a high damping phase in ACM/SGC composites. Fourier transform infrared spectroscopy reveals the presence of two types of hydrogen bonds in ACM/SGC composites. Micro-structure analysis shows a clear sea-island phase structure. SGC particles disperse fairly uniformly in the ACM matrix. A wide interphase region exists between these two phases, indicating the good blend compatibility between ACM and SGC. The damping performance of ACM/SGC composites under dynamic shear strain and frequency condition significantly improved with the increase in SGC content. Specifically, the loss factor (tan δ) value of ACM/SGC (100/40) composite increased by 120% compared with that of neat ACM, according to the RPA results. The significantly improved damping property can be ascribed to the interfacial hydrogen bonds and the pulley effect of SGC molecules.

Interfacial Hydrogen Bonds and Their Influence Mechanism on Increasing the Thermal Stability of Nano-SiO₂-Modified Meta-Aramid Fibres.

For further analysis of the effect of nano-doping on the properties of high polymers and research into the mechanism behind modified interfacial hydrogen bonds, a study on the formation probability of nano-SiO2/meta-aramid fibre interfacial hydrogen bonds and the strengthening mechanism behind interfacial hydrogen bonds on the thermal stability of meta-aramid fibres using molecular dynamics is performed in this paper. First, the pure meta-aramid fibre and nano-SiO2/meta-aramid fibre mixed models with nanoparticle radiuses of 3, 5, 7 and 9 Å (1 Å = 10−1 nm) are built, and then the optimization process and dynamics simulation of the models are conducted. The dynamics simulation results indicate that the number of hydrogen bonds increase due to the doping by nano-SiO2 and that the number of interfacial hydrogen bonds increases with the nanoparticle radius. By analysing the hydrogen bond formation probability of all the atom pairs in the mixed model with pair correlation functions (PCFs), it can be observed that the hydrogen bond formation probability between the oxygen atom and hydrogen atom on the nanoparticle surface is the greatest. An effective way to increase the number of interfacial hydrogen bonds in nano-SiO2 and meta-aramid fibres is to increase the number of hydrogen atoms on the nano-silica surface and oxygen atoms in the meta-aramid fibre. By using the radial distribution function (RDF), the conclusion can be further drawn that the hydrogen bond formation probability is at a maximum when the atomic distance is 2.7–2.8 Å; therefore, increasing the number of atoms within this range can significantly increase the formation probability of hydrogen bonds. According to the results of chain movement, the existence of interfacial hydrogen bonds effectively limits the free movement of the molecular chains of meta-aramid fibres and enhances the thermal stability of meta-aramid fibres. The existence of interfacial hydrogen bonds is one of the important reasons for formation of the stable interface structure between nanoparticles and meta-aramid fibres. In addition, a nanoparticle with a small radius improves the interfacial hydrogen bond energy density and interfacial interaction energy density, enhancing the stability of the mixed model interface.

Membrane Interactions of hIAPP Monomer and Oligomer with Lipid Membranes by Molecular Dynamics Simulations.

Interaction of human islet amyloid polypeptide (hIAPP) peptides with cell membrane is crucial for the understanding of amyloid toxicity associated with Type II diabetes (T2D). While it is known that the hIAPP-membrane interactions are considered to promote hIAPP aggregation into fibrils and induce membrane disruption, the membrane-induced conformation, orientation, aggregation, and adsorption behaviors of hIAPP peptides have not been well understood at the atomic level. Herein, we perform all-atom explicit-water molecular dynamics (MD) simulations to study the adsorption, orientation, and surface interaction of hIAPP aggregates with different sizes (monomer to tetramer) and conformations (monomer with α-helix and tetramer with β-sheet-rich U-turn) upon adsorption on the lipid bilayers composed of both pure zwitterionic POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and mixed anionic POPC/POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine) (3:1) lipids. MD simulation results show that hIAPP monomer with α-helical conformation and hIAPP pentamer with β-sheet conformation can adsorb on both POPC and POPC/POPE bilayers via a preferential orientation of N-terminal residues facing toward the bilayer surface. The hIAPP aggregates show stronger interactions with mixed POPC/POPE lipids than pure POPC lipids, consistent with experimental observation that hIAPP adsorption and fibrililation are enhanced on mixed lipid bilayers. While electrostatic interactions are main attractive forces to drive the hIAPP aggregates to adsorb on both bilayers, the introduction of the more hydrophilic head groups of POPE lipids further promote the formation of the interfacial hydrogen bonds. Complement to our previous studies of hIAPP aggregates in bulk solution, this computational work increases our knowledge about the mechanism of amyloid peptide-membrane interactions that is central to the understanding the progression of all amyloid diseases.

Carbon Nanodots as High-Functionality Cross-Linkers for Bioinspired Engineering of Multiple Sacrificial Units toward Strong yet Tough Elastomers

It is still a huge challenge to implement multiple energy dissipation mechanisms into polymers toward strong yet tough elastomers. Here, we describe a biomimetic design for diene-rubber by incorporating carbon nanodots (CDs) into a chemically cross-linked network. The high-functionality CDs serve as both physical and chemical cross-linkers, which give rise to a covalent network that interlinks multiple chains with nonuniform lengths, and interfacial hydrogen bonds. Upon stretching, the hydrogen bonds preferentially detach, leading to the orientation of short covalent bridging, which contributes the forward onset of strain-induced crystallization. The subsequent rupture of short covalent bridging, together with the successive detachment of hydrogen bonds result in further orientation of hidden length, which enhances the crystallinity. Consequently, the samples exhibit an integrated improvement of strength and toughness, and intact stretchability. We envisage that this strategy may provide a new avenue to i...

Couplings Across the Vibrational Spectrum Caused by Strong Hydrogen Bonds: A Continuum 2D IR Study of the 7-Azaindole-Acetic Acid Heterodimer.

Strongly hydrogen-bonded motifs provide structural stability and can act as proton transfer relays to drive chemical processes in biological and chemical systems. However, structures with medium and strong hydrogen bonds are difficult to study due to their characteristically broad vibrational bands and large anharmonicity. This is further complicated by strong interactions between the high-frequency hydrogen-bonded vibrational modes, fingerprint modes, and low-frequency intradimer modes that modulate the hydrogen-bonding. Understanding these structures and their associated dynamics requires studying much of the vibrational spectrum. Here, mid-IR continuum spectroscopy of the cyclic 7-azaindole-acetic acid (7AI-AcOH) heterodimer reveals the vibrational relaxation dynamics and couplings of this complex hydrogen-bonded system. Within this dimer, the NH bond of 7AI exhibits a band at 3250 cm-1 caused by a medium strength hydrogen bond, while the strongly hydrogen-bonded OH modes of acetic acid exhibit a broad double-peaked vibrational feature spanning 1750 to 2750 cm-1. Transient IR and 2D IR experiments were performed using three excitation frequencies, centered on the high-frequency OH and NH modes, and probed with a mid-IR continuum to measure the spectral response from 1000 to 3500 cm-1. While the NH stretch is observed to relax in 300 fs, the strongly hydrogen-bonded OH modes relax within the time resolution of the experiment (sub-100 fs). The difference in the strength of the hydrogen bonds is also reflected in the coupling pattern in the fingerprint region observed with 2D IR spectroscopy. Here the NH is strongly coupled to fingerprint modes involving the 7AI monomer, while the OH vibrations are strongly coupled to vibrational modes across the entire dimer. Together, the results show strong coupling and rapid energy transfer across the hydrogen-bonded interface and through the structure of the 7-azaindole-acetic acid heterodimer, highlighting the need to study the full vibrational spectrum for strongly hydrogen-bonded systems.

Exploring the molecular basis of RNA recognition by the dimeric RNA-binding protein via molecular simulation methods

ABSTRACTRNA-binding protein with multiple splicing (RBPMS) is critical for axon guidance, smooth muscle plasticity, and regulation of cancer cell proliferation and migration. Recently, different states of the RNA-recognition motif (RRM) of RBPMS, one in its free form and another in complex with CAC-containing RNA, were determined by X-ray crystallography. In this article, the free RRM domain, its wild type complex and 2 mutant complex systems are studied by molecular dynamics (MD) simulations. Through comparison of free RRM domain and complex systems, it's found that the RNA binding facilitates stabilizing the RNA-binding interface of RRM domain, especially the C-terminal loop. Although both R38Q and T103A/K104A mutations reduce the binding affinity of RRM domain and RNA, the underlining mechanisms are different. Principal component analysis (PCA) and Molecular mechanics Poisson-Boltzmann surface area (MM/PBSA) methods were used to explore the dynamical and recognition mechanisms of RRM domain and RNA. R38Q mutation is positioned on the homodimerization interface and mainly induces the large fluctuations of RRM domains. This mutation does not directly act on the RNA-binding interface, but some interfacial hydrogen bonds are weakened. In contrast, T103A/K104A mutations are located on the RNA-binding interface of RRM domain. These mutations obviously break most of high occupancy hydrogen bonds in the RNA-binding interface. Meanwhile, the key interfacial residues lose their favorable energy contributions upon RNA binding. The ranking of calculated binding energies in 3 complex systems is well consistent with that of experimental binding affinities. These results will be helpful in understanding the RNA recognition mechanisms of RRM domain.

Role of Solvent Dynamics in Photoinduced Proton-Coupled Electron Transfer in a Phenol-Amine Complex in Solution.

Photoinduced proton-coupled electron transfer (PCET) plays an essential role in a wide range of energy conversion processes. Previous experiments on a phenol-amine complex in solution provided evidence of an electron-proton transfer (EPT) excited state characterized by both intramolecular charge transfer and proton transfer from the phenol to the amine. Herein we analyze hundreds of surface hopping trajectories to investigate the role of solvent dynamics following photoexcitation to the EPT state. This solvent dynamics leads to a significant decrease in the energy gap between the ground and EPT states, thereby facilitating decay to the ground state, and generates an electrostatic environment conducive to proton transfer on the EPT state. In addition to solvent reorganization, the geometrical properties at the hydrogen-bonding interface must be suitable to allow proton transfer. These mechanistic insights elucidate the underlying fundamental physical principles of photoinduced PCET processes.

Molecular-Level Understanding of Solvation Structures and Vibrational Spectra of an Ethylammonium Nitrate Ionic Liquid around Single-Walled Carbon Nanotubes

Molecular dynamics simulations have been performed to explore the solvation structures and vibrational spectra of an ethylammonium nitrate (EAN) ionic liquid (IL) around various single-walled carbon nanotubes (SWNTs). Our simulation results demonstrate that both cations and anions show a cylindrical double-shell solvation structure around the SWNTs regardless of the nanotube diameter. In the first solvation shell, the CH3 groups of cations are found to be closer to the SWNT surface than the NH3+ groups because of the solvophobic nature of the CH3 groups, while the NO3– anions tend to lean on the nanotube surface, with three O atoms facing the bulk EAN. On the other hand, the intensities of both C–H (the CH3 group of the cation) and N–O (anion) asymmetric stretching bands at the EAN/SWNT interface are found to be slightly higher than the corresponding bulk values owing to the accumulation and orientation of cations and anions in the first solvation shell. More interestingly, the N–O stretching band exhibits a red shift of around 10 cm–1 with respect to the bulk value, which is quite contrary to the blue shift of the O–H stretching band of water molecules at the hydrophobic interfaces. Such a red shift of the N–O stretching mode can be attributed to the enhanced hydrogen bonds (HBs) of the NO3– anions in the first solvation shell. Our simulation results provide a molecular-level understanding of the interfacial vibrational spectra of an EAN IL on the SWNT surface and their connection with the relevant solvation structures and interfacial HBs.

Using G0W0 Level Alignment to Identify Catechol’s Structure on TiO2(110)

We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol on rutile TiO2(110). Catechol is often used as an anchoring group for larger more complex organic and inorganic dyes on TiO2 and forms type II heterojunctions on TiO2(110). In particular, we compare quasiparticle (QP) G0W0 with hybrid exchange correlation functional (HSE) density functional theory (DFT) calculations for the catechol–rutile TiO2(110) interface. In so doing, we provide a theoretical interpretation of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) experiments for this prototypical system. Specifically, we demonstrate that the position, presence, and intensity of peaks associated with catechol’s HOMO, intermolecular OH–O bonds, and interfacial hydrogen bonds to the surface bridging O atoms (ObrH–C and ObrH–O) may be used to fingerprint deprotonation of catechol’s OH anchoring groups. Furthermore, our results suggest deprotonation of these groups, while being nearly isoenergetic at high coverages, may significantly increase the photovoltaic efficiency of catechol–TiO2(110) interfaces.

Simultaneous Reinforcement and Toughening of Carbon Nanotube/Cellulose Conductive Nanocomposite Films by Interfacial Hydrogen Bonding

Carbon nanotube (CNT)/cellulose nanocomposite films were prepared by a featured processing method, i.e., solution dispersion, slow gelation and hot-press drying, where an environmentally benign processing solvent (sodium hydroxide/urea aqueous solution) was used. The scanning electron microscopy and transmission electron microscopy demonstrated uniform CNT dispersion in the cellulose. The slow gelation and hot-press drying could effectively reduce the free volume and force the cellulose chains and CNTs to contact as close as possible, thus forming the strong interfacial hydrogen bonding between the residual oxygen-containing functional groups on the CNT surfaces and the hydroxyl groups in the cellulose chains, as confirmed by X-ray photoelectron spectroscopy and Fourier transformation infrared spectroscopy results. As a result, with a CNT loading of 5 wt %, the tensile strength and Young’s modulus of the cellulose nanocomposite films were increased by 55% and 21% relative to neat cellulose film. More inte...

Effect of interfacial hydrogen bonds on the structure and dynamics of confined water

Reactive molecular dynamics (MD) is used to simulate the equilibrium process of water confined between two fully hydroxylated α-quartz (001) surfaces with separation distances from 7 to 20 Å. Effect of different patterns of interfacial hydrogen bonds on the structure and dynamics of confined water is investigated. Density profiles, radial distribution functions, number of interfacial hydrogen bonds, and mean square displacements are calculated. The α-quartz (001) surface is cut from an α-quartz crystal at a certain depth to construct a surface with geminal silanols after being fully hydroxylated. The silanol groups on the surface are treated in two different ways in the MD simulations. One of the silanol groups are treated as to be fixed, and the other one is treated as no constraint for the movement of surface silanols. Our results show that different patterns of hydrogen bonds are formed at the interface between SiO2 surface and water. For the fixed silanol surface there is one type of strong hydrogen bonds interacting between the oxygen atoms of water and the hydrogen atoms of surface silanols, leading to the dipole moment of water molecules pointing out from the surface. For the movable silanol surface there are two types of strong hydrogen bonds formed at the interface. One is between the oxygen atoms of water and the hydrogen atoms of surface silanols, and the other is between the oxygen atoms of surface silanols and the hydrogen atoms of water. The number of hydrogen bonds of the first type is much less than those of the second type, leading to the dipole moment of water molecules pointing to the surface. Moreover, the total number of interfacial hydrogen bonds formed on the fixed silanol surfaces is larger than that on the movable silanol surfaces. The density profiles of the confined water indicate the formation of a strong layering of water in the vicinity of the fixed silanol surface, and the water layer is also more ordered with an ice-like structure, as compared with a dense water layer with a liquid-like structure in the case of movable silanol surfaces. Thus the mean square displacements of confined water show that, as compared with interfacial hydrogen bonds formed on the fixed silanol surfaces, the weaker and the lesser interfacial hydrogen bonds formed on the movable silanol surfaces may be responsible for more intense movement of confined water between the movable silanol surfaces. Our simulation suggests that the different pattern of interfacial hydrogen bonds could signifiantly affect the structure and dynamic behaviors of the confined water between two fully hydroxylated silica surfaces.

On the role of interfacial hydrogen bonds in "on-water" catalysis.

Numerous experiments have demonstrated that many classes of organic reactions exhibit increased reaction rates when performed in heterogeneous water emulsions. Despite enormous practical importance of the observed "on-water" catalytic effect and several mechanistic studies, its microscopic origins remains unclear. In this work, the second generation Car-Parrinello molecular dynamics method is extended to self-consistent charge density-functional based tight-binding in order to study "on-water" catalysis of the Diels-Alder reaction between dimethyl azodicarboxylate and quadricyclane. We find that the stabilization of the transition state by dangling hydrogen bonds exposed at the aqueous interfaces plays a significantly smaller role in "on-water" catalysis than has been suggested previously.

Analysis of the Strength of Interfacial Hydrogen Bonds between Tubulin Dimers Using Quantum Theory of Atoms in Molecules

Microtubules are key structural elements that, among numerous biological functions, maintain the cytoskeleton of the cell and have a major role in cell division, which makes them important cancer chemotherapy targets. Understanding the energy balance that brings tubulin dimers, the building blocks of microtubules, together to form a microtubule is especially important for revealing the mechanism of their dynamic instability. Several studies have been conducted to estimate various contributions to the free energy of microtubule formation. However, the hydrogen-bond contribution was not studied before as a separate component. In this work, we use concepts such as the quantum theory of atoms in molecules to estimate the per-residue strength of hydrogen bonds contributing to the overall stability that brings subunits together in pair of tubulin heterodimers, across both the longitudinal and lateral interfaces. Our study shows that hydrogen bonding plays a major role in the stability of tubulin systems. Several residues that are crucial to the binding of vinca alkaloids are shown to be strongly involved in longitudinal microtubule stabilization. This indicates a direct relation between the binding of these agents and the effect on the interfacial hydrogen-bonding network, and explains the mechanism of their action. Lateral contacts showed much higher stability than longitudinal ones (–462 ± 70 vs. –392 ± 59 kJ/mol), which suggests a dramatic lateral stabilization effect of the GTP cap in the β-subunit. The role of the M-loop in lateral stability in absence of taxol was shown to be minor. The B-lattice lateral hydrogen bonds are shown to be comparable in strength to the A-lattice ones (−462 ± 70 vs. –472 ± 46 kJ/mol). These findings establish the importance of hydrogen bonds to the stability of tubulin systems.