Chemical Equilibrium Probed by Two-Dimensional IR Spectroscopy: Hydrogen Bond Dynamics of Methyl Acetate in Water

Vibrational frequency fluctuations of poly(N,N-diethylacrylamide) in the vicinity of coil-to-globule transition studied by two-dimensional infrared spectroscopy and molecular dynamics simulations

Poly(N,N-diethylacrylamide) (PdEA), one of the thermoresponsive polymers, in aqueous solutions has attracted much attention because of its characteristic properties, such as coil-to-globule (CG) transition. We performed two-dimensional infrared spectroscopy and molecular dynamics (MD) simulations to understand the hydration dynamics in the vicinity of the CG transition at the molecular level via vibrational frequency fluctuations of the carbonyl stretching modes in the side chains of PdEA. Furthermore, N,N-diethylpropionamide, a repeating monomer unit of PdEA, is also investigated for comparison. From decays of the frequency-frequency time correlation functions (FFTCFs) of the carbonyl stretching modes, we consider that inhomogeneity of the hydration environments originates from various backbone configurations of PdEA. The degree of the inhomogeneity depends on temperature. Hydration water molecules near the carbonyl groups are influenced by the confinements of the polymers. The restricted reorientation of the embedded water, the local torsions of the backbone, and the rearrangement of the whole structure contribute to the slow spectral diffusion. By performing MD simulations, we calculated the FFTCFs and dynamical quantities, such as fluctuations of the dihedral angles of the backbone and the orientation of the hydration water molecules. The simulated FFTCFs match well with the experimental results, indicating that the retarded water reorientations via the excluded volume effect play an important role in the vibrational frequency fluctuations of the carbonyl stretching mode. It is also found the embedded water molecules are influenced by the local torsions of the backbone structure within the time scales of the spectral diffusion.

Perturbation of Fermi Resonance on Hydrogen-Bonded > C═O: 2D IR Studies of Small Ester Probes

We utilized linear and 2D infrared spectroscopy to analyze the carbonyl stretching modes of small esters in different solvents. Particularly noteworthy were the distinct carbonyl spectral line shapes in aqueous solutions, prompting our investigation of the underlying factors responsible for these differences. Through our experimental and theoretical calculations, we identified the presence of the hydrogen-bond-induced Fermi resonance as the primary contributor to the varied line shapes of small esters in aqueous solutions. Furthermore, our findings revealed that the skeletal deformation mode plays a crucial role in the Fermi resonance for all small esters. Specifically, the first overtone band of the skeletal deformation mode intensifies when hydrogen bonds form with the carbonyl group of esters, whereas such coupling is rare in aprotic organic solvents. These spectral insights carry significant implications for the utilization of esters as infrared probes in both biological and chemical systems.

Forced Interactions: Ionic Polymers at Charged Surfactant Interfaces

Characterizing electrostatic interactions at heterogeneous interfaces is critical for developing a fundamental description of the dynamic processes at charged interfaces. Water-in-oil reverse micelles (RMs) offer a high degree of tunability across composition, polarity, and temperature, making them ideal systems for studying interactions at heterogeneous liquid-liquid interfaces. In the present study, we use a combination of ultrafast two-dimensional infrared spectroscopy and molecular dynamics (MD) simulations to determine the picosecond interfacial dynamics in RMs containing binary compositions of sorbitan monostearate and anionic or cationic cosurfactants, which are used to tune the ratio of charged to nonionic surfactants at the interface. The positively charged polyethylenimine (PEI) polymer is encapsulated within the RMs, and the carbonyl stretching mode of sorbitan monostearate reports on the interfacial hydrogen-bond populations and dynamics. The results show that hydrogen-bond populations are altered through the inclusion of both negatively and positively charged cosurfactants. Charged surfactants increase interfacial water penetration into the surfactant layer, and the surface localization of polymers decreases water penetration. Local hydrogen-bond dynamics undergo a slowdown with the inclusion of charged surfactants, and the encapsulation of polymers results in similar effects, irrespective of the charge.

Resolving the Impact of Hydrogen Bonding on the Phylloquinone Cofactor through Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared spectroscopy (2DIR) was applied to phylloquinone (PhQ), an important biological cofactor, to elucidate the impact of hydrogen bonding on the ultrafast dynamics and energetics of the carbonyl stretching modes. 2DIR measurements were performed on PhQ dissolved in hexanol, which served as the hydrogen bonding solvent, and hexane, which served as a non-hydrogen bonding control. Molecular dynamics simulations and quantum chemical calculations were performed to aid in spectral assignment and interpretation. From the position of the peaks in the 2DIR spectra, we extracted the transition frequencies for the fundamental, overtone, and combination bands of hydrogen bonded and non-hydrogen bonded carbonyl groups of PhQ in the 1635-1680 cm^-1 region. We find that hydrogen bonding to a single carbonyl group acts to decouple the two carbonyl units of PhQ. Through analysis of the time-resolved 2DIR data, we find that hydrogen bonding leads to faster vibrational relaxation as well as an increase in the inhomogeneous broadening of the carbonyl groups. Overall, this work demonstrates how hydrogen bonding to the carbonyl groups of PhQ presents in the 2DIR spectra, laying the groundwork to use PhQ as a 2DIR probe to characterize the ultrafast fluctuations in the local environment of natural photosynthetic complexes.

Phosphine oxides as NMR and IR spectroscopic probes for geometry and energy of PO···H–A hydrogen bonds

In this work we evaluate the possibility to use the NMR and IR spectral properties of P=O group to estimate the geometry and strength of hydrogen bonds which it forms...

Carbonyl stretch of CH⋯O hydrogen-bonded methyl acetate in supercritical trifluoromethane

Infrared spectroscopy in the gas phase was used to study the formation reaction of the CH⋯O hydrogen bonding complex involving the CH group of trifluoromethane, as a hydrogen donor, and the carbonyl group of methyl acetate, as a hydrogen acceptor, under different (T, p) conditions. The hydrogen-bonded carbonyl stretch of the molecular pair was monitored in dilute mixtures of methyl acetate in trifluoromethane at near-critical temperatures, from gas- to liquid-like densities. In the gas region, it was possible to discriminate the carbonyl signal of the hydrogen-bonded complex from that of the free ester and have access to their relative concentration. The equilibrium constant of the hydrogen bonding reaction and the standard enthalpy and entropy changes in the process were determined using the spectroscopic data. CH⋯O bonding was favored by lowering temperature or pressurizing F₃CH in the mixture, remaining essentially no free carbonyl groups about the critical density. The carbonyl band of the hydrogen-bonded pair appeared as a single symmetric peak up to liquid-like densities, suggesting that the 1:1 methyl acetate-trifluoromethane complex has the most abundant stoichiometry. Spectral features as frequency shift and bandwidth of the hydrogen-bonded carbonyl were studied as a function of temperature and solvent-density. A bathochromic (red) vibrational shift was registered for the bound carbonyl band against density, with a sudden change in behavior in the near-critical region, while the width of this band remains mostly unresponsive.

Interfacial H-Bond Dynamics in Reverse Micelles: The Role of Surfactant Heterogeneity

Characterizing the hydrogen bond structure and dynamics at surfactant interfaces is essential for understanding how microscopic interactions translate to bulk microemulsion properties. Heterogeneous blends containing tens or hundreds of surfactants are common in the industry, but the most fundamental studies have been carried out on micelles composed of a single surfactant species. Therefore, the effect of surfactant heterogeneity on the interfacial structure and dynamics remains poorly understood. Here, we use ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations to characterize sub-picosecond solvation dynamics as a function of the surfactant composition in ∼120 nm water-in-oil reverse micelles. We probe the ester carbonyl vibrations of nonionic sorbitan surfactants, which are located precisely at the interface between the polar and nonpolar regions, and as such, report on the interfacial water dynamics. We show a 7% increase in hydrogen bond populations together with a 37% slowdown of interfacial hydrogen bond dynamics in heterogeneous mixtures containing hundreds of species, compared to more uniform compositions. Simulations, which are in semiquantitative agreement with experiments, indicate that structural diversity leads to decreased packing efficiency, which in turn drives water further into the otherwise hydrophobic region. Interestingly, this increase in hydration is accompanied by a slowdown of dynamics, indicating that water molecules solvating surfactants are conformationally constrained. These studies demonstrate that the composition and heterogeneity are key factors in determining interfacial properties.

Quantifying conformations of ester vibrational probes with hydrogen-bond-induced Fermi resonances

Solvatochromic shifts of local vibrational probes report on the strength of the surrounding electric fields and the probe's hydrogen bonding status. Stretching vibrational mode of the ester carbonyl group is a popular solvatochromic reporter used in the studies of peptides and proteins. Small molecules, used to calibrate the response of the vibrational probes, sometimes involve Fermi resonances (FRs) induced by inter-molecular interactions. In the present work, we focus on the scenario where FR does not appear in the infrared spectrum of the ester carbonyl stretching mode in aprotic solvents; however, it is intensified when a hydrogen bond with the reporter is established. When two molecules form hydrogen bonds to the same carbonyl oxygen atom, FR leads to strong hybridization of the involved modes and splitting of the absorption peak. Spectral overlap between the Fermi doublets associated with singly and doubly hydrogen-bonded carbonyl groups significantly complicates quantifying different hydrogen-bonded conformations. We employed a combination of linear and third-order (2DIR) infrared spectroscopy with chemometrics analysis to reveal the individual line shapes and to estimate the occupations of the hydrogen-bonded conformations in methyl acetate, a model small molecule. We identified a hydrogen-bond-induced FR in complexes of methyl acetate with alcohols and water and found that FR is lifted in larger molecules used for control experiments-cholesteryl stearate and methyl cyanoacetate. Applying this methodology to analyze acetonitrile-water solutions revealed that when dissolved in neat water, methyl acetate occupies a single hydrogen-bonding conformation, which is in contrast to the conclusions of previous studies. Our approach can be generally used when FRs prevent direct quantification of the hydrogen bonding status of the vibrational probe.

Accurate prediction of bulk properties in hydrogen bonded liquids: amides as case studies

In this contribution we show that it is possible to build accurate force fields for small organic molecules allowing the reliable reproduction of a large panel of bulk properties, which are seldom addressed in the same context. Starting from the results obtained in recent studies, we developed a protocol for charge estimation and virtual site generation for the amide class of molecules. The parametrization of electrostatic properties is based on population analysis and orbital localization of quantum mechanical computations rooted in density functional theory and the polarizable continuum model, without any additional external information. The new protocol, coupled to other recent studies in our group targeted at an accurate fitting of internal degrees of freedom, makes available a method for building force fields from scratch (excluding for the moment intermolecular van der Waals interactions) with focus on reproducing the structure and dynamics of hydrogen bonded liquids, yielding results that are in line or better than those delivered by current general force fields. The approach is tested on the demanding series formed by formamide and its two N-methyl derivatives, N-methylformamide and N,N-dimethylformamide. We show that the atomistic structure of the liquids arising from classical molecular dynamics (MD) simulations employing the new force field is in full agreement with X-ray and neutron diffraction experiments and the corresponding spatial distribution functions are in remarkable agreement with the results of ab initio MD simulations. It is noteworthy that the latter result has never been obtained before without using ad hoc (and system dependent) scale factors and that, in addition, our parameter-free procedure is able to reproduce static dielectric constants over a wide range of values without sacrificing the force field accuracy with respect to other observables. Finally, we are able to explain the trend of static dielectric constants followed by the three amides in terms of properties obtained from the simulations, namely hydrogen bond patterns and reorientational lifetimes.

Synthesis-Free Phase-Selective Gelator for Oil-Spill Remediation

A new deep eutectic solvent (DES) was developed as a phase-selective gelator for oil-spill remediation. The newly designed nonionic DES is based on a combination of an amide (N-methylacetamide) and a long chain carboxylic acid (lauric acid) and does not require any synthetic procedure besides mixing. Our studies show that the DES works as gelator by forming a gel between lauric acid and the hydrocarbon, whereas the amide serves to form the DES and dissolves in water during the gelation process. In addition, the DES material has gelation properties comparable to those considered as state-of-the-art. Overall, the newly developed material shows a promising future in oil recovery methodologies.

Two-Dimensional Infrared Spectroscopy Reveals Cosolvent-Composition-Dependent Crossover in Intermolecular Hydrogen-Bond Dynamics

Cosolvents have versatile composition-dependent applications in chemistry and biology. The simultaneous presence of hydrophobic and hydrophilic groups in dimethyl sulfoxide (DMSO), an industrially important amphiphilic cosolvent, when combined with the unique properties of water, plays key roles in the diverse fields of pharmacology, cryoprotection, and cell biology. Moreover, molecules dissolved in aqueous DMSO exhibit an anomalous concentration-dependent nonmonotonic behavior in stability and activity near a critical DMSO mole fraction of 0.15. An experimental identification of the origin of this anomaly can lead to newer chemical and biological applications. We report a direct spectroscopic observation of the anomalous behavior using ultrafast two-dimensional infrared spectroscopy experiments. Our results demonstrate the cosolvent-concentration-dependent nonmonotonicity arises from nonidentical mechanisms in ultrafast hydrogen-bond-exchange dynamics of water above and below the critical cosolvent concentration. Comparison of experimental and theoretical results provides a molecular-level mechanistic understanding: a distinct difference in the stabilization of the solute through dynamic solute-solvent interactions is the key to the anomalous behavior.

Vibrational Stark Effects of Carbonyl Probes Applied to Reinterpret IR and Raman Data for Enzyme Inhibitors in Terms of Electric Fields at the Active Site

IR and Raman frequency shifts have been reported for numerous probes of enzyme transition states, leading to diverse interpretations. In the case of the model enzyme ketosteroid isomerase (KSI), we have argued that IR spectral shifts for a carbonyl probe at the active site can provide a connection between the active site electric field and the activation free energy (Fried et al. Science 2014, 346, 1510-1514). Here we generalize this approach to a much broader set of carbonyl probes (e.g., oxoesters, thioesters, and amides), first establishing the sensitivity of each probe to an electric field using vibrational Stark spectroscopy, vibrational solvatochromism, and MD simulations, and then applying these results to reinterpret data already in the literature for enzymes such as 4-chlorobenzoyl-CoA dehalogenase and serine proteases. These results demonstrate that the vibrational Stark effect provides a general framework for estimating the electrostatic contribution to the catalytic rate and may provide a metric for the design or modification of enzymes. Opportunities and limitations of the approach are also described.

2D-IR spectroscopy of hydrogen-bond-mediated vibrational excitation transfer

Inter-molecular vibrational energy transfer in the hydrogen-bonded complexes of methyl acetate and 4-cyanophenol is studied by dual-frequency 2D-IR spectroscopy.

Modeling Vibrational Spectra of Ester Carbonyl Stretch in Water and DMSO Based on Molecular Dynamics Simulation

On the basis of molecular dynamics simulation, we model the ester carbonyl stretch FTIR signals of methyl acetate in D2O and DMSO. An ab initio map is constructed at the B3LYP/6-311++G** level to relate the carbonyl stretch frequency to the external electric field. Using this map, fluctuating Hamiltonian of the carbonyl stretch is constructed from the MD simulation trajectory. The IR spectra calculated based on this Hamiltonian are found to be in good agreement with the experiment. For methyl acetate in D2O, hydrogen bonding on alkoxy oxygen causes a blue shift of frequency, while that on carbonyl oxygen causes a red shift. Two peaks observed in FTIR signals originate from the balance of these two effects. Furthermore, in both D2O and DMSO solutions, correlations are found between the instantaneous electric field on C═O and the frequencies. Broader line width of the signal in D2O suggests a more inhomogeneous electric field distribution due to the complicated hydrogen-bonding environment.

Kinetics of exchange between zero-, one-, and two-hydrogen-bonded states of methyl and ethyl acetate in methanol

It has recently been shown that the ester carbonyl stretching vibration can be used as a sensitive probe of local electrostatic field in molecular systems. To further characterize this vibrational probe and extend its potential applications, we studied the kinetics of chemical exchange between differently hydrogen-bonded (H-bonded) ester carbonyl groups of methyl acetate (MA) and ethyl acetate (EA) in methanol. We found that, while both MA and EA can form zero, one, or two H-bonds with the solvent, the population of the 2hb state in MA is significantly smaller than that in EA. Using a combination of linear and nonlinear infrared measurements and numerical simulations, we further determined the rate constants for the exchange between these differently H-bonded states. We found that for MA the chemical exchange reaction between the two dominant states (i.e., 0hb and 1hb states) has a relaxation rate constant of 0.14 ps(-1), whereas for EA the three-state chemical exchange reaction occurs in a predominantly sequential manner with the following relaxation rate constants: 0.11 ps(-1) for exchange between 0hb and 1hb states and 0.12 ps(-1) for exchange between 1hb and 2hb states.

Ester carbonyl vibration as a sensitive probe of protein local electric field

The ability to quantify the local electrostatic environment of proteins and protein/peptide assemblies is key to gaining a microscopic understanding of many biological interactions and processes. Herein, we show that the ester carbonyl stretching vibration of two non-natural amino acids, L-aspartic acid 4-methyl ester and L-glutamic acid 5-methyl ester, is a convenient and sensitive probe in this regard, since its frequency correlates linearly with the local electrostatic field for both hydrogen-bonding and non-hydrogen-bonding environments. We expect that the resultant frequency-electric-field map will find use in various applications. Furthermore, we show that, when situated in a non-hydrogen-bonding environment, this probe can also be used to measure the local dielectric constant (ε). For example, its application to amyloid fibrils formed by Aβ(16-22) revealed that the interior of such β-sheet assemblies has an ε value of approximately 5.6.

Infrared spectroscopic study on hydration and chiral interaction of temperature-responsive polymer with L-proline moieties

We studied the hydration of a temperature-responsive polymer containing L-proline moieties (poly(acryloyl-L-proline methyl ester), PAProM) by using infrared spectroscopy. Red shifts of ν(C-H) bands and blue shifts of amide and ester carbonyl bands of PAProM during temperature-induced phase separation indicate that the alkyl, amide, and ester groups are partially dehydrated. The population of the amide carbonyls forming hydrogen bonds (H-bonds) with two water molecules decreased from 63 to 33%, while that of the ester carbonyls forming one H bonding decreased from 100 to 84%. We labeled the methyl groups of PAProM by introducing deuterium (poly(acryloyl-L-proline methyl-d3 ester, PAProMd3) to clarify hydration change of the labeled groups. Red shifts of three ν(C-D) bands appearing at 2000-2200 cm(-1) clearly showed that the methyl groups at the end of side chains also dehydrated as well as the alkyl groups on the main chain. As for the effects of additives, methanol raised the phase separation temperature (Tp) of PAProM. The IR spectra show that the average number of H bonds to the amide and ester carbonyls decreases with increasing methanol concentration and that the water molecules surrounding the alkyl groups of PAProM are replaced by methanol molecules. The increase in Tp suggests that the favorable effect of the latter is superior to the unfavorable effect of the former. On the other hand, malic acid (MA) reduced Tp of PAProM. Moreover, a chiral interaction occurs; that is, Tp was lower in the presence of d-isomer than L-isomer. The analysis of the amide band revealed that the d-isomer associates more effectively with the amide carbonyls of PAProM than the L-isomer does.

Refined OPLS all-atom force field parameters for n-pentadecane, methyl acetate, and dimethyl phosphate

OPLS All-Atom (OPLS/AA) is a generic all-atom force field which was fine-tuned to accurately reproduce condensed phase properties of organic liquids. Its application in modeling of lipid membranes is, however, limited mainly due to the inability to correctly describe phase behavior and organization of the hydrophobic core of the model lipid bilayers. Here we report new OPLS/AA parameters for n-pentadecane, methyl acetate, and dimethyl phosphate anion. For the new force field parameters, we show very good agreement between calculated and numerous reference data, including liquid density, enthalpy of vaporization, free energy of hydration, and selected transport properties. The new OPLS/AA parameters have been used in successful submicrosecond MD simulations of bilayers made of bacterial glycolipids whose results will be published elsewhere shortly.