Structure and dynamics of urea/water mixtures investigated by vibrational spectroscopy and molecular dynamics simulation

Abstract

Urea/water is an archetypical "biological" mixture and is especially well-known for its relevance to protein thermodynamics as urea acts as a protein denaturant at high concentration. This behavior has given rise to an extended debate concerning urea's influence on water structure. On the basis of a variety of methods and of definitions of the water structure, urea has been variously described as a structure-breaker, a structure-maker, or as remarkably neutral toward water. Because of its sensitivity to microscopic structure and dynamics, vibrational spectroscopy can help resolve these debates. We report experimental and theoretical spectroscopic results for the OD stretch of HOD/H2O/urea mixtures (linear IR, 2DIR, and pump-probe anisotropy decay) and for the CO stretch of urea-D4/D2O mixtures (linear IR only). Theoretical results are obtained using existing approaches for water and a modification of a frequency map developed for acetamide. All absorption spectra are remarkably insensitive to urea concentration, consistent with the idea that urea only very weakly perturbs the water structure. Both this work and experiments by Rezus and Bakker, however, show that water's rotational dynamics are slowed down by urea. Analysis of the simulations casts doubt on the suggestion that urea immobilizes particular doubly hydrogen bonded water molecules.

FIG. 1

Experimental and theoretical spectra for OD stretch of dilute HOD in H₂O, as a function of urea concentration. Spectra are peak normalized. For clarity, spectra at 4 M urea are offset by 0.05, and spectra at 8 M urea are offset by 0.12.

FIG. 2

Experimental and theoretical spectra for CO stretch mode of urea-D₄ in D₂O. Spectra are peak normalized. For clarity, spectra at 4 M urea are offset by 0.05, and spectra at 8 M urea are offset by 0.1.

FIG. 3

Anisotropy decay of water OD for 8% HOD in H₂O for 3.4 M and 7.8 M urea solutions. Experimental values (symbols) were taken from the study by Rezus and Bakker. Theoretical results are shown for isolated HOD in bulk water (dashed line) and for 8% HOD in bulk water and in 4 M and 8 M urea (solid lines).

FIG. 4

P2 rotational OD-bond time-correlation functions, averaged either over all OD’s in the simulation (solid lines) or over only OD’s initially doubly hydrogen bonded to urea.

FIG. 5

P2 rotational OD-bond time-correlation functions at 4 M urea as a function of the number of urea molecules solvated by a given water at t = 0. Urea solvation is defined using a urea carbon–water oxygen cutoff of 5.5 Å.

FIG. 6

Theoretical OD-stretch 2DIR spectra for HOD/H₂O as a function of urea concentration. Spectra are normalized to the 1-0 peak intensity. Red regions are positive; blue regions are negative.

FIG. 7

Nodal slopes from OD-stretch 2DIR spectra for HOD/H₂O as a function of delay time for various urea concentrations. Nodal slope is determined over a range of 60 cm⁻¹, centered on the 1-0 peak.

FIG. 8

Water OD-stretch frequency-frequency time-correlation functions at 4 M urea as a function of the number of urea molecules solvated by a given water at t = 0. Urea solvation is defined using a urea carbon–water oxygen cutoff of 5.5 Å.