Depolarized light scattering and dielectric response of a peptide dissolved in water

Abstract

The density and orientational relaxation of bulk water can be separately studied by depolarized light scattering (DLS) and dielectric spectroscopy (DS), respectively. Here we ask the question of what are the leading collective modes responsible for polarization anisotropy relaxation (DLS) and dipole moment relaxation (DS) of solutions involving mostly hydrophobic solute-water interfaces. We study, by atomistic molecular dynamics simulations, the dynamics and structure of hydration water interfacing N-Acetyl-leucine-methylamide (NALMA) dipeptide. The DLS response of the solution is consistent with three relaxation processes: bulk water, rotations of single solutes, and collective dipole-induced-dipole polarizability of the solutes, with the time-scale of 130-200 ps. No separate DLS response of the hydration shell has been identified by our simulations. Density fluctuations of the hydration layer, which largely contribute to the response, do not produce a dynamical process distinct from bulk water. We find that the structural perturbation of the orientational distribution of hydration waters by the dipeptide solute is quite significant and propagates ∼ 3 − 5 hydration layers into the bulk. This perturbation is still below that produced by hydrated globular proteins. Despite this structural perturbation, there is little change in the orientational dynamics of the hydration layers, compared to the bulk, as probed by both single-particle orientational dynamics and collective dynamics of the dipole moment of the shells. There is a clear distinction between the perturbation of the interfacial structure by the solute-solvent interaction potential and the perturbation of the interfacial dynamics by the corresponding forces.

Hydration shells of proteins probed by depolarized light scattering and dielectric spectroscopy: Orientational structure is significant, positional structure is not

“…2 the DLS loss spectrum of SPC/E water reported previously by us. 29 The TIP3P force field overestimates the intensity of the lower-frequency DLS peak compared to the SPC/E model, but gives a fair account of the experimental low-frequency tail of the loss spectrum. The oscillatory feature in the time correlation function φ (t), well resolved for SPC/E water, is nearly absent for TIP3P water (Fig.…”

“…8 are significantly different from a similar analysis done for the solution of a N-Acetyl-leucine-methylamide dipeptide (NALMA) in SPC/E water. 29 Just a small increase, ∼10%, of the dipolar relaxation time in the first solvation shell compared to the bulk was reported for the longest exponential decay. Further, τ (a) was found to slightly decrease compared to the bulk due to a compensation between slowing down of the first layer by a slight speed-up of the second layer.…”

“…33 In dielectric experiment, the latter condition requires measurements at frequencies exceeding the frequency of the solute rotational diffusion (τ M 0 ) −1 to dynamically freeze the solute response. 29 The correlation function we calculate here becomes…”

“…A relatively weak decay of the average relaxation time with increasing the shell size is a signature of a deep propagation of the orientational structural perturbation produced in the water layers by the protein. 15,29 The structural effect of the protein on water is reflected by static dipolar correlations. To illustrate the spatial range of these correlations, Fig.…”

Fluctuation Theory of Ionic Solvation Potentials

“…[13] These boundary layer effects motivate treatment of interfacial water as a chemically distinct species. [17][18][19] Describing the electric field experienced by molecular solutes can be done within the Maxwell theory only by altering to the usual macroscopic boundary conditions to account for these effects. [20] This paper explores a theory of solvent density response that is distinct from the Maxwell dielectric picture.…”