Aqueous Trimethylamine N-oxide Solutions Research Articles

Counteracting Effects of Trimethylamine N-Oxide against Urea in Aqueous Solutions: Insights from Theoretical Two-Dimensional Infrared Spectroscopy.

The study of small osmolytes in their aqueous solutions has gained significant attention because of their relevance to structure and thermodynamics of proteins in aqueous media. Special attention has been given to the binary and ternary aqueous solutions of urea and trimethylamine N-oxide (TMAO). Urea is a well-known protein denaturant, while TMAO protects proteins in their native states. Interestingly, TMAO counteracts urea's ability to denature proteins when present in solutions with approximately half of the concentration of urea. Vibrational spectroscopy can improve our understanding of the molecular origin of this counteracting effect because of its sensitivity to local structure and dynamics. We present results of theoretical linear vibrational and two-dimensional infrared (2DIR) spectroscopy of water in the binary and ternary aqueous solutions of TMAO and urea. The 2DIR spectra are calculated using the electronic structure/molecular dynamics approach. The non-Condon effects in spectral transitions are incorporated in the theoretical calculations of 2DIR spectra. It is found that TMAO disrupts the local structure of water, while urea leaves it essentially unaffected. The 2DIR results show that both TMAO and urea slow down the dynamics of spectral diffusion of water. The extent of slowing down is found to be particularly significant for both hydration and bulk water in the presence of TMAO which can be attributed to strong hydrogen bonds between the water and TMAO molecules. The water molecules present in the hydration layer of the solutes in the ternary solutions are found to relax at even slower rates compared to that in their binary solutions in water. The hydrogen bonds between TMAO and urea are found to be not stable. Thus, the counteracting effect of TMAO against urea is seen to take place mainly through water-mediated interactions. Such TMAO-induced effects giving rise to more structured and slower hydrogen-bonded network are successfully captured through 2DIR spectroscopic calculations.

Interactions of glycine and its oligomers with the trimethylamine N-oxide and urea in aqueous solution at T = 298.15 K: Enthalpic measurement and computer simulation

Enthalpies of solution of glycine and its oligomers in aqueous solutions of trimethylamine N-oxide (TMAO) or urea have been measured by calorimetry at 298.15 K. The results obtained were used to calculate the heterogeneous enthalpic interaction coefficients between glycine and its oligomers and the molecule of TMAO or urea in water and the transfer enthalpies (ΔtrHm) of the glycine and its oligomers from water to aqueous solutions of TMAO or urea. The results are discussed in terms of solute-solute and solute-solvent interactions. The influence of trimethylamine N-oxide (TMAO) and urea on the structure of water has been investigated by molecular dynamics simulation. The results show that water affected by TMAO forms stronger H-bonds and is more ordered than pure water. The results obtained by molecular dynamics simulation support transfer enthalpic changing trend of glycine and its oligomers in aqueous solutions of trimethylamine N-oxide (TMAO) or urea.

Dynamical Effects of Trimethylamine N-Oxide on Aqueous Solutions of Urea.

Trimethylamine N-oxide (TMAO) stabilizes protein structures, whereas urea destabilizes proteins, and their opposing effects can be counteracted at a 1:2 ratio of TMAO to urea. To investigate how they affect solution dynamics, molecular dynamics simulations have been carried out for aqueous solutions of TMAO and urea at different concentrations. In the binary solutions, urea mainly slows the diffusion of waters that are hydrogen bonded to it (i.e., hydration water), whereas TMAO dramatically slows the diffusion of both hydration water and bulk water because of long-lived TMAO-water hydrogen bonds. In the ternary solutions, because TMAO decreases the diffusion rate of bulk water, the lifetimes of not only water-water but also urea-water hydrogen bonds increase. In addition, the constant forming and breaking of short lifetime hydrogen bonds between urea and water appears to impart energy into the bulk, whereas the long lifetime hydrogen bonds between TMAO and water slows down the bulk, resulting in the compensating effects on bulk water in the ternary solution. This suggests that the counteracting effects of TMAO on urea denaturation may be both to make longer lived hydrogen bonds to water and to counter the energizing effects of urea on bulk water.

Physicochemical properties of L-carnitine in aqueous solution and its interaction with trimethylamine N-oxide, sodium chloride and dextrose: Volumetric and calorimetric insights

Abstract L -carnitine plays physiologically important roles specifically to decrease triglycerides, very low density lipoprotein (VLDL) and cholesterol. Considering these facts, it is believed to be helpful in the reduction of cardiovascular diseases and diabetes. Physicochemical properties of L -carnitine in aqueous or mixed aqueous solutions are scarcely available in literature. Such properties will be useful in understanding the interactions of L -carnitine with other cellular components like trimethylamine-N-oxide (TMAO), dextrose ( D -glucose) and NaCl. The values of densities (ρ) and speeds of sound (u) of L -carnitine were measured over the molality range of m = 0.05 mol·kg−1 to m = 0.3 mol·kg−1 at temperature T = 293.15 K to T = 313.15 K in water and in aqueous solutions of TMAO, dextrose, and NaCl. Using these data, values of apparent molar volume (V2,m,φ) and apparent molar adiabatic compressibility (Ks,2,m,φ) have been determined. The results obtained from these measurements have been used to derive the standard partial molar volume ( V 2 , m o ), standard partial molar adiabatic compressibility ( K s , 2 , m o ), standard partial molar volume of transfer ( Δ tr V 2 , m o ) and standard partial molar isentropic compressibilities of transfer ( Δ tr K S , 2 , m o ) from water to aqueous solutions of TMAO, dextrose, and NaCl. Through Isothermal Titration Calorimetry (ITC), the enthalpies of dilution ( Δ dil H m ) of L -carnitine in water (over a temperature range of T = 293.15 K to T = 313.15 K) and in aqueous solutions of TMAO, dextrose, and NaCl have been determined. The values of standard molar enthalpy of dilution ( Δ dil H m o ) and their transfer values ( Δ tr Δ dil H m o ) have been derived from these measurements. The results have been discussed in terms of possible intermolecular interactions. The data suggest that there is overall balancing of hydrophobic and hydrophilic interactions in system containing L -carnitine and TMAO in aqueous solution, but hydrophobic effect dominates when the co-solute is NaCl or dextrose.

Ab Initio Simulations of Water Dynamics in Aqueous TMAO Solutions: Temperature and Concentration Effects.

We use ab initio molecular dynamics simulation to study the effect of hydrophobic groups on the dynamics of water molecules in aqueous solutions of trimethylamine N-oxide (TMAO). We show that hydrophobic groups induce a moderate (<2-fold) slowdown of water reorientation and hydrogen-bond dynamics in dilute solutions, but that this slowdown rapidly increases with solute concentration. In addition, the slowdown factor is found to vary very little with temperature, thus suggesting an entropic origin. All of these results are in quantitative agreement with prior classical molecular dynamics simulations and with the previously suggested excluded-volume model. The hydrophilic TMAO headgroup is found to affect water dynamics more strongly than the hydrophobic moiety, and the magnitude of this slowdown is very sensitive to the strength of the water-solute hydrogen-bond.

A new force field including charge directionality for TMAO in aqueous solution

We propose a new force field for trimethylamine N-oxide (TMAO), which is designed to reproduce the long-lived and highly directional hydrogen bond between the TMAO oxygen (OTMAO) atom and surrounding water molecules. Based on the data obtained by ab initio molecular dynamics simulations, we introduce three dummy sites around OTMAO to mimic the OTMAO lone pairs and we migrate the negative charge on the OTMAO to the dummy sites. The force field model developed here improves both structural and dynamical properties of aqueous TMAO solutions. Moreover, it reproduces the experimentally observed dependence of viscosity upon increasing TMAO concentration quantitatively. The simple procedure of the force field construction makes it easy to implement in molecular dynamics simulation packages and makes it compatible with the existing biomolecular force fields. This paves the path for further investigation of protein-TMAO interaction in aqueous solutions.

Density and Volumetric Properties of Aqueous Solutions of Trimethylamine N-Oxide in the Temperature Range from (278.15 to 323.15) K and at Pressures up to 100 MPa

Densities of aqueous solutions of trimethylamine N-oxide (TMAO) were measured over the concentration range (0.0875 to 4.3251) mol·kg–1 at temperatures (278.15, 288.15, 298.15, 308.15, and 323.15) K and pressures (0.101, 10, 25, 50, 75, and 100) MPa. Volumetric properties such as apparent molar volume of TMAO, Vϕ,2, molar isothermal compression, KT,m, molar isobaric expansion, EP,m, and internal pressure, Pint, of its aqueous solutions were calculated depending on concentration, temperature, and pressure. The volumetric partial properties of TMAO at infinite dilution in water (V2∞, KT,2∞, and EP,2∞) were also determined. The results were discussed from the standpoint of solute–solute and solute–solvent interactions.

Transfer enthalpies of amino acids and glycine peptides from water to aqueous solutions of trimethylamine N-oxide at 298.15 K

• Enthalpies of solution have been measured by calorimetry at 298.15 K. • The heterogeneous enthalpic interaction coefficients were calculated. • The results are discussed in terms of solute–solute and solute–solvent interactions. • The structure of amino acid has a more important effect on the interaction. Enthalpies of solution of glycine, l -alanine, l -serine, l -threonine, l -valine, diglycine and triglycine in aqueous solutions of trimethylamine N -oxide (TMAO) have been measured by calorimetry at 298.15 K. The results obtained were used to calculate the heterogeneous enthalpic interaction coefficients ( h AT ) between the amino acids or glycine peptides and the molecule of TMAO in water and the transfer enthalpies Δ tr H m of the amino acids or glycine peptides from water to aqueous solutions of TMAO. It has been observed that the transfer enthalpies of the amino acids and glycine peptides from water to aqueous solutions of TMAO are monotonically positive over the investigated concentration ranges. The relative order of h AT and Δ tr H m for amino acids are all l -serine > l -threonine > glycine > l -alanine > l -valine, whereas the sequence of glycine peptides is triglycine > diglycine > glycine. The results are discussed in terms of solute–solute and solute–solvent interactions.

The effect of aqueous solutions of trimethylamine-N-oxide on pressure induced modifications of hydrophobic interactions

To understand the mechanism of protein protection by the osmolyte trimethylamine-N-oxide (TMAO) at high pressure, using molecular dynamics (MD) simulations, solvation of hydrophobic group is probed in aqueous solutions of TMAO over a wide range of pressures relevant to protein denaturation. The hydrophobic solute considered in this study is neopentane which is a considerably large molecule. The concentrations of TMAO range from 0 to 4 M and for each TMAO concentration, simulations are performed at five different pressures ranging from 1 atm to 8000 atm. Potentials of mean force are calculated and the relative stability of solvent-separated state over the associated state of hydrophobic solute are estimated. Results suggest that high pressure reduces association of hydrophobic solutes. From computations of site-site radial distribution function followed by analysis of coordination number, it is found that water molecules are tightly packed around the nonpolar particle at high pressure and the hydration number increases with increasing pressure. On the other hand, neopentane interacts preferentially with TMAO over water and although hydration of neopentane reduces in presence of this osmolyte, TMAO does not show any tendency to prevent the pressure-induced dispersion of neopentane moieties. It is also observed that TMAO molecules prefer a side-on orientation near the neopentane surface, allowing its oxygen atom to form favorable hydrogen bonds with water while maintaining some hydrophobic contacts with neopentane. Analysis of hydrogen-bond properties and solvation characteristics of TMAO reveals that TMAO can form hydrogen bonds with water and it reduces the identical nearest neighbor water molecules caused by high hydrostatic pressures. Moreover, TMAO enhances life-time of water-water hydrogen bonds and makes these hydrogen bonds more attractive. Implication of these results for counteracting effect of TMAO against protein denaturation at high pressures are discussed.

Interactions of some short peptides with the osmolyte trimethylamine N-oxide in aqueous solution: Physico-chemical insights

The apparent molar volumes, V2,ϕ, apparent molar adiabatic compressibilities, KS,2,ϕ, and heats of dilution q of glycyl-glycine (gly-gly), glycyl-glycyl-glycine (gly-gly-gly), glycyl leucine (gly-leu), and glycyl-glycyl-leucine (gly-gly-leu) have been determined in aqueous trimethylamine N-oxide (TMAO) at T=298.15K. These data have been used to calculate the values of infinite dilution standard partial molar volume V2,m∘, standard partial molar isentropic compressibility KS,2,m∘, standard molar enthalpy of dilution ΔdilH°, and the corresponding transfer thermodynamic properties from water to aqueous TMAO solutions. The results on transfer thermodynamic properties of peptides from water to aqueous TMAO solutions have been interpreted in terms of ion–ion, ion–polar, hydrophilic–hydrophilic, hydrophilic–hydrophobic, and hydrophobic–hydrophobic group interactions. The results obtained in this work suggest strengthening of polar solute–solvent interactions in case of peptides compared to those in case of amino acids and support the experimental observations on the effect of TMAO on the thermodynamic and conformational stability of proteins.

Entropic Stabilization of Proteins by TMAO

The osmolyte trimethylamine N-oxide (TMAO) accumulates in the cell in response to osmotic stress and increases the thermodynamic stability of folded proteins. To understand the mechanism of TMAO induced stabilization of folded protein states, we systematically investigated the action of TMAO on several model dipeptides (leucine, L(2), serine, S(2), glutamine, Q(2), lysine, K(2), and glycine, G(2)) in order to elucidate the effect of residue-specific TMAO interactions on small fragments of solvent-exposed conformations of the denatured states of proteins. We find that TMAO preferentially hydrogen bonds with the exposed dipeptide backbone but generally not with nonpolar or polar side chains. However, interactions with the positively charged Lys are substantially greater than with the backbone. The dipeptide G(2) is a useful model of the pure amide backbone; interacts with TMAO by forming a hydrogen bond between the amide nitrogen and the oxygen in TMAO. In contrast, TMAO is depleted from the protein backbone in the hexapeptide G(6), which shows that the length of the polypeptide chain is relevant in aqueous TMAO solutions. These simulations lead to the hypothesis that TMAO-induced stabilization of proteins and peptides is a consequence of depletion of the solute from the protein surface provided intramolecular interactions are more favorable than those between TMAO and the backbone. To test our hypothesis, we performed additional simulations of the action of TMAO on an intrinsically disordered Aβ(16-22) (KLVFFAE) monomer. In the absence of TMAO, Aβ(16-22) is a disordered random coil. However, in aqueous TMAO solution, Aβ(16-22) monomer samples compact conformations. A transition from random coil to α-helical secondary structure is observed at high TMAO concentrations. The coil to α-helix transition is highly cooperative especially considering the small number of residues in Aβ(16-22). Our work highlights the potential similarities between the action of TMAO on long polypeptide chains and entropic stabilization of proteins in a crowded environment due to excluded volume interactions. In this sense, the chemical chaperone TMAO is a nanocrowding particle.

Thermodynamics of the interactions of a homologous series of some amino acids with trimethylamine N-oxide: Volumetric, compressibility, and calorimetric studies

The values of apparent molar volume V 2 ,ϕ and apparent molar compressibility K S, 2 ,ϕ of glycine, l-alanine, dl-α-amino-n-butyric acid, l-valine, and l-leucine have been determined in the aqueous solution of 1 mol · kg −1 and 2 mol · kg −1 trimethylamine N-oxide (TMAO) solutions by density and sound velocity measurements. Isothermal titration calorimetry has been employed to determine the values of heats of dilution q of the aqueous solutions of these amino acids in TMAO at temperatures from T = 288.15 K to T = 308.15 K. These data have been used to calculate values of the infinite dilution standard partial molar volume ( V 2 , m ∘ ), standard partial molar isentropic compressibility ( K S , 2 , m ∘ ) and limiting enthalpy of dilution (Δ dilH °) of the amino acids in aqueous TMAO solutions. The standard partial molar volumes of transfer ( Δ tr V 2 , m ∘ ), isentropic compressibility of transfer ( Δ tr K S , 2 , m ∘ ), and enthalpy of dilution of transfer (Δ tr Δ dilH °) of amino acids from water to aqueous TMAO solutions have been calculated from the measured quantities for these thermodynamic quantities. The linear correlation of V 2 , m ∘ for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups ( NH 3 + , COO −), CH 2 groups, and the other alkyl chains of the amino acids to V 2 , m ∘ . The results for the partial molar properties of transfer from water to aqueous TMAO solutions have been interpreted in terms of ion–ion, ion–polar, hydrophilic–hydrophobic, and hydrophobic–hydrophobic group interactions. The volume, compressibility, and enthalpy of dilution of transfer data suggest that hydrophobic interactions predominant in this system. It is inferred that the effect of TMAO on protein stability need not be necessarily due to preferential hydration, it can also interact with the hydrophobic groups of the constituents of protein affecting its stability.

Counteraction of Urea by Trimethylamine N-Oxide Is Due to Direct Interaction

Trimethylamine N-oxide (TMAO) is a naturally occurring osmolyte that stabilizes proteins, induces folding, and counteracts the denaturing effects of urea, pressure, and ice. To establish the mechanism behind these effects, isotopic substitution neutron-scattering measurements were performed on aqueous solutions of TMAO and 1:1 TMAO-urea at a solute mole fraction of 0.05. The partial pair distribution functions were extracted using the empirical potential structure refinement method. The results were compared with previous results obtained with isosteric tert-butanol, as well as the available data from spectroscopy and molecular-dynamics simulations. In solution, the oxygen atom of TMAO is strongly hydrogen-bonded to, on average, between two and three water molecules, and the hydrogen-bond network is tighter in water than in pure water. In TMAO-urea solutions, the oxygen atom in TMAO preferentially forms hydrogen bonds with urea. This explains why the counteraction is completed at a 2:1 urea/TMAO concentration ratio, independently of urea concentration. These results strongly support models for the effect of TMAO on the stability of proteins based on a modification of the simultaneous equilibria that control hydrogen bonding between the peptide backbone and water or intramolecular sites, without any need for direct interaction between TMAO and the protein.