Dynamics of water of hydration near disaccharides strongly depends on solute topology: mapping density fluctuations, rotational anisotropy and h-bond exchange mechanism around disaccharides

comunicação em posterDisaccharides such as trehalose are abundant components of cells and may alter the phase behavior or dynamics of phospholipid bilayers: for example, trehalose is a cryoprotectant of lipid bilayers. The origin of this and other effects of disaccharides on membranes is still under debate. One possibility is that some disaccharides alter the dynamics of water of hydration relative to the bulk, and that interactions between the water of hydration of disaccharides and the phospholipids lead to changes in bilayer properties. We address this issue by investigating the dynamics of water near disaccharides kojibiose and trehalose using classical atomistic molecular dynamics simulations and transition state theory. Our results indicate that the cryoprotectant trehalose and the non-cryoprotectant kojibiose differ in the rotational dynamics of their water of hydration, with the subpopulation of water molecules nearest to the central linking oxygen being significantly slower for trehalose. Interestingly, this effect results from differences in both solute chemistry and topology: identical functional groups may interact differently with water depending on the orientation of neighboring groups, in agreement with existing reports on proteins. In contrast to observations on topologically simple solutes, our results indicate that topologically complex solutes such as disaccharides induce unexpected changes in the free energy landscape associated with rotation of water molecules. These results suggest that theoretical models to predict water dynamics near solutes, relevant for example to understand how water dynamics influences protein folding or diffusion through polysaccharide brushes, must account for both solute chemistry and topology

Detecting Weak Signals from Interfaces by High Accuracy Phase-Resolved SFG Spectroscopy

Much work over the last 25 years has demonstrated that the interface-specific, alloptical technique, vibrational sum frequency generation (v-SFG) spectroscopy, is often uniquely capable of characterizing the structure and dynamics of interfacial species. The desired information in such a measurement is the complex second order susceptibility which gives rise to the nonlinear response from interfacial molecules. The ability to detect molecular species yielding only small contributions to the susceptibility is meanwhile limited by the precision by which the spectral phase and amplitude can be determined. In this study we describe a new spectrometer design that offers unprecedented phase and amplitude accuracy while significantly improving the sensitivity of the technique. Combining a full collinear beam geometry with a technique enabling the simultaneous measurement of the complex sample and reference spectrum, uncertainties in the reference phase and amplitude are shown to be greatly reduced. Furthermore, we show that using balanced detection, the signal to noise ratio can be increased by one order of magnitude. The capabilities of the spectrometer are demonstrated by the isolation of a small isotropic surface signal from the bulk dominated nonlinear optical response of z-cut quartz. The achieved precision of our spectrometer enables measurements not currently feasible in v-SFG spectroscopy.Comment: 24 pages, 5 figure

Characterization of Water Dissociation on α\alpha-Al2_{2}O3_{3}(11ˉ02)(1\bar{1}02): Theory and Experiment

The interaction of water with $\alpha$-alumina (i.e. $\alpha$-Al$_{2}$O$_{3}$ surfaces is important in a variety of applications and a useful model for the interaction of water with environmentally abundant aluminosilicate phases. Despite its significance, studies of water interaction with $\alpha$-Al$_{2}$O$_{3}$ surfaces other than the $(0001)$ are extremely limited. Here we characterize the interaction of water (D$_{2}$O) with a well defined $\alpha$-Al$_{2}$O$_{3}$$(1\bar{1}02)$ surface in UHV both experimentally, using temperature programmed desorption and surface-specific vibrational spectroscopy, and theoretically, using periodic-slab density functional theory calculations. This combined approach makes it possible to demonstrate that water adsorption occurs only at a single well defined surface site (the so-called 1-4 configuration) and that at this site the barrier between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV. A subset of OD stretch vibrations are parallel to this dissociation coordinate, and thus would be expected to be shifted to low frequencies relative to an uncoupled harmonic oscillator. To quantify this effect we solve the vibrational Schr\"odinger equation along the dissociation coordinate and find fundamental frequencies red-shifted by more than 1,500 cm$^{\text{-1}}$. Within the context of this model, at moderate temperatures, we further find that some fraction of surface deuterons are likely delocalized: dissociatively and molecularly absorbed states are no longer distinguishable.Comment: Paper: 22 pages, 9 figures , ESI: 6 pages, 1 figur

The infuence of glycosidic linkage neighbors on disaccharide conformation in vacuum

Correct description of the free energy of conformation change of disaccharides is important in understanding a variety of biochemical processes and, ultimately, in the manufacture of better food and paper products. In this study, we determine the relative free energy of a series of 12 disaccharides in vacuum using replica exchange molecular dynamics (repMD) simulations. The chosen sugars and the novel application of this method allow the exploration of the role of glycosidic linkage neighbors in conformer stabilization. In line with expectations, we find that hydrogen bonding (and therefore energetically preferred conformations) are determined both by the nature of the glycosidic linkage (i.e., 1 f 2, 1 f 3, or 1 f 4), the C1 epimer of the of the nonreducing monosaccharide, and by the configuration of carbon atoms once removed from the glycosidic linkage. Contrary to suggestions by prior authors for repMD more generally, we also demonstrate that repMD provides enhanced sampling, relative to conventional MD simulations of equivalent length, for disaccharides in vacuum at 300 K.Fundação para a Ciência e a Tecnologia (FCT)SFRH/BPD/20555/2004/0GVLNational Science Foundation under Grant CHE-043132

Statics and dynamics of free and hydrogen-bonded OH groups at the air/water interface

We use classical atomistic molecular dynamics simulations of two water models (SPC/E and TIP4P/2005) to investigate the orientation and reorientation dynamics of two subpopulations of OH groups belonging to water molecules at the air/water interface at 300 K: those OH groups that donate a hydrogen bond (called “bonded”) and those that do not (called “free”). Free interfacial OH groups reorient in two distinct regimes: a fast regime from 0 to 1 ps and a slow regime thereafter. Qualitatively similar behavior was reported by others for free OH groups near extended hydrophobic surfaces. In contrast, the net reorientation of bonded OH groups occurs at a rate similar to that of bulk water. This similarity in reorientation rate results from compensation of two effects: decreasing frequency of hydrogen-bond breaking/formation (i.e., hydrogen-bond exchange) and faster rotation of intact hydrogen bonds. Both changes result from the decrease in density at the air/water interface relative to the bulk. Interestingly, because of the presence of capillary waves, the slowdown of hydrogen-bond exchange is signiffcantly smaller than that reported for water near extended hydrophobic surfaces, but it is almost identical to that reported for water near small hydrophobic solutes. In this sense water at the air/water interface has characteristics of water of hydration of both small and extended hydrophobic solutes.SARA Computing and Networking Services (www.sara.nl)Nederlandse Organisatie voor Wetenschappelijk Onderzoe

Ultrafast reorientation of dangling OH groups at the air-water interface using femtosecond vibrational spectroscopy

We report the real-time measurement of the ultrafast reorientational motion of water molecules at the water-air interface, using femtosecond time- and polarization-resolved vibrational sum-frequency spectroscopy. Vibrational excitation of dangling OH bonds along a specific polarization axis induces a transient anisotropy that decays due to the reorientation of vibrationally excited OH groups. The reorientation of interfacial water is shown to occur on subpicosecond time scales, several times faster than in the bulk, which can be attributed to the lower degree of hydrogen bond coordination at the interface. Molecular dynamics simulations of interfacial water dynamics are in quantitative agreement with experimental observations and show that, unlike in bulk, the interfacial reorientation occurs in a largely diffusive manner.This work is part of the research program of the Stichting Fundamenteel Onderzoek der Materie with financial support from the Nederlanse Organisatie voor Wetenschappelijk Onderzoek. We thank SARA Computing and Networking Services (www.sara.nl) for their support in using the Lisa Compute Cluster

Isolating the Nonlinear Optical Response of a MoS2_2 Monolayer under Extreme Screening of a Metal Substrate

Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices. Interaction with three-dimensional (3D) metals, like Au, profoundly affects their optical properties, posing challenges in characterizing the monolayer's optical responses within the semiconductor-metal junction. In this study, using precise polarization-controlled final-state sum frequency generation (FS-SFG), we successfully isolated the optical responses of a MoS$_2$ monolayer from a MoS$_2$/Au junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A or B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape illustrates the expected constant density of states (DOS) at the band edge of the 2D semiconductor, a feature often obscured by excitonic interactions in week-screening conditions such as in a free-standing monolayer. Extrapolation yields the onset of a direct quasiparticle bandgap of about $1.65\pm0.20$ eV, indicating a strong bandgap renormalization. This study not only enriches our understanding of the optical responses of a 2D semiconductor in extreme screening conditions but also provides a critical reference for advancing 2D semiconductor-based photocatalytic applications.Comment: 14 pages, 4 figures + supplemental materia

A femtosecond time resolved view of vibrationally assisted electron transfer across the metal/aqueous interface

Understanding heterogeneous charge transfer is crucial if we are to build the best electrolyzers, fuel cells and photoelectrochemical water splitting devices that chemistry allows. Because the elementary processes involved have timescales ranging from femto- to milliseconds, direct simulation is not generally possible. Model Hamiltonian approaches thus have a crucial role in gaining mechanistic insight. Current generations of such theories describe a reactant(s) or product(s) that interacts with electrolyte via a single effective interaction. Such approaches thus obscure the extent to which particular solvent fluctuations influence charge transfer. Here we demonstrate experimentally that for a prototypical system, a ferrocene terminated alkane thiol self-assembled monolayer (SAM) on gold in contact with aqueous electrolyte, charge transfer from the Au to the ferrocene can be induced by vibrational excitation of the ferrocene aromatic CH. Intriguingly the energy of the aromatic CH vibration, 0.38 eV, is a large fraction of the effective solvent interaction strength inferred for the ferrocene/ferrocenium system in prior electrochemical studies: 0.85 eV. Our results thus demonstrate the coupling of charge transfer to a specific solvent motion and more generally imply that solvent may affect reduction/oxidation rates in electrocatalysis by coupling to a few distinct solvent motions. Identifying these motions is crucial in rationalizing trends in reactivity with change in electrolyte and thus in pursuing electrolyte engineering from first principles.Comment: 18 pages, 6 figure