Hydration and self-aggregation of a neutral cosolute from dielectric relaxation spectroscopy and MD simulations: the case of 1,3-dimethylurea

Abstract

The influence of the amphiphile 1,3-dimethylurea (1,3-DMU) on the dynamic properties of water was studied using dielectric relaxation spectroscopy. The experiment provided evidence for substantial retardation of water reorientation in the hydration shell of 1,3-DMU, leading to a separate slow-water relaxation in addition to contributions from bulk-like and fast water as well as from the solute. From the amplitudes of the resolved water modes effective hydration numbers were calculated, showing that each 1,3-DMU molecule effectively freezes the reorientation of 1-2 water molecules. Additionally, a significant amount of solvent molecules, decreasing from ∼39 at infinite dilution to ∼3 close to the solubility limit, is retarded by a factor of ∼1.4 to 2.3, depending on concentration. The marked increase of the solute amplitude indicates pronounced parallel dipole alignment between 1,3-DMU and its strongly bound HO molecules. Molecular dynamics (MD) simulations of selected solutions revealed a notable slowdown of water rotation for those solvent molecules surrounding the methyl groups of 1,3-DMU and strong binding of ∼2HO by the hydrophilic carbonyl group, corroborating thus the experimental results. Additionally, the simulations revealed 1,3-DMU self-aggregates of substantial lifetime.

Microglobule formation and a microscopic order parameter monitoring the phase transition of aqueous poly( N -isopropylacrylamide) solution

“…The practically constant value of Z ib ≈ 1.3 probably means that these irrotationally bound water dipoles are not released from the pNIPAm chains even above T S . A similar number of strongly bound water molecules was also found for 1,3dimethylurea in a combined DRS and molecular dynamics study [47] and assigned to H 2 O dipoles strongly interacting with the carbonyl oxygen of the solute. This may also be the case for pNIPAm with its amide group in the side chains.…”

Evidence for cooperative Na+ and Cl− binding by strongly hydrated l-proline

“…The difference Z ib = Z t À Z s indicates the corresponding number of irrotationally bound (ib) solvent molecules (more exactly, a polarization equivalent to Z ib solvent dipole moments, m w eff 57 ) per mole solute which apparently disappeared from the spectrum. Based on previous results for osmolytes and related compounds 28,[31][32][33]56 we argue that these strongly bound H 2 O molecules contribute to the solute relaxation. Almost certainly, they do not form stiff, long-lived complexes with the solute but adopt the rotational dynamics of Pro which is dominated by the lifetime of the solute-solvent hydrogen bonds.…”

“…4 shows the effective dipole moment, m eff (Pro), of Pro obtained from S 1 with eqn (2), which decreases linearly from 19.3 D at c(Pro) = 0 to 17.2 D at 5.6 M. These data are comparable to the results of Rodrguez-Arteche et al 21 but considerably larger than the value of m eff (Pro) = 12.0 D predicted by DFT calculations (Gaussian at the B3LYP/cc-pVDZ level with the C-PCM solvation model) 53,54 for a L-proline molecule embedded in water. 55 In analogy to previous osmolyte studies, 28,[31][32][33]56 this indicates strong hydration of Pro with parallel alignment of solute and solvent dipoles. Support for this view comes from DFT calculations of ProÁnH 2 O clusters ( Fig.…”

“…21 is buried under the broad wings of the two CC modes, can be attributed to retarded (slow) water molecules hydrating Pro for reasons explained below and detailed for similar systems elsewhere. 28,[31][32][33] No fast water mode at B500 GHz 31,52 could be resolved for Pro(aq) [and {Pro + NaCl}(aq)] but its presence is obvious from the large values of e N (Tables 1 and 2) and accordingly was taken into account in the calculation of the total amplitude of bulk-like water, S b = S 3 + e N (c(Pro)) À e N (0), where e N (0) = 3.52, (Fig. S3, ESI †).…”

“…28,30 This technique, which probes the response to a harmonic electric field of frequency, n, in terms of the complex permittivity,ê(n) = e 0 (n) À ie 00 (n) (e 0 (n) is the relative permittivity; e 00 (n) is the dielectric loss), 27 has been successfully used to study the hydration of biomolecules. [31][32][33] On the other hand, modern RISM theory accounts for the geometry and partial-charge distribution of solute and solvent molecules and thus provides detailed information on solutesolvent interactions at the molecular level. [34][35][36] RISM calculations cannot provide information on dynamics but are much less computationally demanding than MD simulations at comparable accuracy for structural and thermodynamic data, enabling exploration of the entire phase space covered by the experiments.…”