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. 2005 Jul 7;109(26):12966-75.
doi: 10.1021/jp051137k.

Molecular view of water dynamics near model peptides

Daniela Russo  1 , Rajesh K Murarka, John R D Copley, Teresa Head-Gordon
Affiliations

Molecular view of water dynamics near model peptides

Daniela Russo et al. J Phys Chem B. .

Abstract

Incoherent quasi-elastic neutron scattering (QENS) has been used to measure the dynamics of water molecules in solutions of a model protein backbone, N-acetyl-glycine-methylamide (NAGMA), as a function of concentration, for comparison with results for water dynamics in aqueous solutions of the N-acetyl-leucine-methylamide (NALMA) hydrophobic peptide at comparable concentrations. From the analysis of the elastic incoherent structure factor, we find significant fractions of elastic intensity at high and low concentrations for both solutes, which corresponds to a greater population of protons with rotational time scales outside the experimental resolution (>13 ps). The higher-concentration solutions show a component of the elastic fraction that we propose is due to water motions that are strongly coupled to the solute motions, while for low-concentration solutions an additional component is activated due to dynamic coupling between inner and outer hydration layers. An important difference between the solute types at the highest concentration studied is found from stretched exponential fits to their experimental intermediate scattering functions, showing more pronounced anomalous diffusion signatures for NALMA, including a smaller stretched exponent beta and a longer structural relaxation time tau than those found for NAGMA. The more normal water diffusion exhibited near the hydrophilic NAGMA provides experimental support for an explanation of the origin of the anomalous diffusion behavior of NALMA as arising from frustrated interactions between water molecules when a chemical interface is formed upon addition of a hydrophobic side chain, inducing spatial heterogeneity in the hydration dynamics in the two types of regions of the NALMA peptide. We place our QENS measurements on model biological solutes in the context of other spectroscopic techniques and provide both confirming as well as complementary dynamic information that attempts to give a unifying molecular view of hydration dynamics signatures near peptides and proteins.

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Figures

Figure 1
Figure 1
Incoherent structure factor spectrum for 3.0 M deuterated NAGMA in H2O at 25 °C, measured at 35 μeV (open symbols) for (a) Q ≈ 1.25 Å-1 and (b) Q ≈ 0.84 Å-1. The solid line is the total fit component resulting from the convolution of the two Lorentzian functions and the flat background. The Lorentzian fits to the spectra (dashed lines) show good separation of widths and intensities and are typical of the qualities of the fits for all spectra measured in this study. The residuals show that the quality of fit is good in the energy range of the experiment.
Figure 2
Figure 2
Half-width at half-maximum of the Lorentzian function, Γtrans(Q), plotted vs Q2 corresponding to the translational motions of protons for NAGMA and NALMA at different solute concentrations: (a) 3.0 M deuterated NAGMA and 1.0 and 2.0 M deuterated NALMA in H2O and (b) 1.0 M deuterated NAGMA and 0.5 M deuterated NALMA in H2O.
Figure 3
Figure 3
Half-width at half-maximum of the Lorentzian function, Γrot, plotted vs Q2 corresponding to the rotational motions of protons for NAGMA and NALMA at different solute concentrations: (a) 3.0 M deuterated NAGMA and 1.0 and 2.0 M deuterated NALMA in H2O and (b) 1.0 M deuterated NAGMA and 0.5 M deuterated NALMA in H2O.
Figure 4
Figure 4
(a) EISF of hydration water plotted vs Q for 1.0 and 3.0 M NAGMA and 0.5, 1.0, and 2.0 M NALMA concentrations. (b) EISF of hydration water for 2.0 M NALMA and 1.0 M NAGMA concentrations with the fit to eq 7 of the text. The solid lines are the fits, with the fraction of immobile protons p and the radius a given in Table 1.
Figure 5
Figure 5
Self-intermediate scattering function from experiment at Q ≈ 0.44 Å-1 and Q ≈ 1.54 Å-1 (symbols) and the stretched exponential fit (line): (a) 3.0 M NAGMA and (b) 1.0 M NAGMA.
Figure 6
Figure 6
Stretched exponential parameter β, from the fit to experiment at different Q values for NAGMA and NALMA as a function of concentration. We show the error bar in regard to the fit for the highest concentration used for NAGMA and NALMA and similar error bars were found for the other concentrations as well.
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