Hydrogen-bond dynamics of water in presence of an amphiphile, tetramethylurea: signature of confinement-induced effects

Abstract

Various experimental and simulation studies have suggested that the presence of amphiphilic molecules in aqueous solutions substantially perturbs the tetrahedral hydrogen-bond (H-bond) network of neat liquid water. Such structural perturbation is expected to impact H-bond lifetime of liquid water. Tetramethylurea (TMU) is an example of an amphiphile because it possesses both hydrophobic and hydrophilic moieties. Molecular dynamics simulations of (water þ TMU) binary mixtures at various compositions have been performed in order to investigate the microscopic mechanism through which the amphiphiles influence the H-bond dynamics of liquid water at room temperature. Present simulations indicate lengthening of both water -water H-bond lifetime and H-bond structural relaxation time upon addition of TMU in aqueous solution. At the highest TMU mole fraction studied, H-bond lifetime and structural relaxation time are, respectively, , 4 and , 8 times longer than those in neat water. This is comparable with the slowing down of H-bond dynamics for water molecules confined in cyclodextrin cavities. Simulated relaxation profiles are multi-exponential in character at all mixture compositions, and simulated radial distribution functions suggest enhanced water -water and water -TMU interactions upon addition of TMU. No evidence for complete encapsulation of TMU by water H-bond network has been found.

Are N-methyl groups of Tetramethylurea (TMU) Hydrophobic? A composition and temperature-dependent fluorescence spectroscopic investigation of TMU/water binary mixtures

“…This enhanced H-bonding leads to more compact or stiffened solution structure 17,18 leading to the lowering of absorption frequency at low TMU concentrations. However, simulation study 6 shows that the average H-bond persistence time between two H-bonding specie in this system is in the order of sub-hundred picoseconds. Consequently, the excited state of C153 which has a lifetime ∼2-4 ns in these solutions experiences a large number of environmental fluctuations, forcing the emission energy to sense only the average solution polarity.…”

“…We mentioned the absorption spectral red-shift as anomalous because addition of TMU (static dielectric constant, ε 0 ≈ 23) 43 into water (ε 0 ≈ 80) reduces the average polarity of the medium and thus the extra stabilization of the fluorophore C153 at low X T MU (reflected by the red-shift) cannot arise from the increased solution polarity. Note here that computer simulation study of TMU/water binary mixtures 6 suggests enhancement of solution structure. The H-bonding interaction of water-water and water−TMU has been found to be increased upon addition of TMU in water.…”

“…Support of such a view can be accessed from simulation studies which suggest that aggregation of TMU molecules in aqueous mixtures takes place at low TMU concentrations. [4][5][6] It also suggests that at X T MU = 0.005 the aggregated domains are sufficiently longer-lived to be detected via the excitation wavelength dependent study.…”

“…Another important characteristic of TMU is the strong protein denaturing ability. 2,3 Interestingly, computer simulation studies [4][5][6] have shown TMU aggregation via hydrophobic interactions among the methyl (−CH 3 ) groups in aqueous solutions at very low concentrations. Disagreement among different studies stimulates further research in this area.…”

“…We have also observed the slowing down of water-water H-bond persistence time in presence of TMU in water. 6 In concentrated TMU solutions, the lengthening of H-bond dynamical timescales due to solute-induced confinement of water is reminiscent of water in explicit confined environment. 35,36 Here, we explore whether the hydrophobic part containing four N-methyl groups of TMU acts in the similar manner in aqueous solutions as we had observed before in aqueous alcoholic solutions.…”