Global and internal macromolecular motions in E. coli. The data were collected using three neutron spectrometers at the ILL and ISIS neutron sources. (a) Global motions. HWHM of the Lorentzian associated with the apparent self-diffusion of macromolecules, Γ_sd, as a function of Q² at 284 (squares) and 303 K (triangles). The Γ_sd profile revealed long-range translational motions, as indicated by the increase in the linewidth with Q and the extrapolation to 0 for Q → 0. We found a translational diffusion coefficient, D = (1.06 ± 0.11) × 10⁻⁷ cm² s⁻¹ at 303 K, which was similar to the value of 1.2 × 10⁻⁷ cm² s⁻¹ found for haemoglobin in red blood cells at 310 K (Doster & Longeville 2007). (b) Localized diffusive motions. HWHM of the Lorentzian associated to ‘fast’, a few picoseconds, motions, Γ_f, as a function of Q², at 280 (squares) and 300 K (triangles). The Γ_f profile revealed restricted jump-diffusion motions, occurring in approximately 4 ps as given by the value of the plateau at high Q. (c,d) Localized reorientational motions. (c) HWHM of the Lorentzian associated to ‘slow’, a few tens of picoseconds, motions, Γ_s, as a function of Q², at 280 (squares) and 300 K (triangles). (d) HWHM of the Lorentzian associated to ‘very slow’, a few hundred picoseconds, motions, Γ_vs, as a function of Q², at 284 (squares) and 303 K (triangles).

Translational and rotational diffusive motions of E. coli water. (a) HWHM of the translational Lorentzian, Γ_T, as a function of the scattering vector squared, Q², at 281 (squares) and 301 K (triangles). Γ_T was best fitted (solid lines) using a jump-diffusion model, which describes diffusion between sites for the water protons, with a mean residence time at each site (Bée 1988). The translational diffusion coefficients were similar to those of pure water at corresponding temperatures, with residence times about twice longer. The higher residence times may reflect the longer times spent by the protons in the first hydration shell. (b) HWHM of the broad Lorentzian, Γ_R + Γ_T, as a function of Q², at 281 (squares) and 301 K (triangles). The rotational correlation times, τ_cor,R = 1/Γ_R, were close to the values extracted for the buffer under the same conditions and of the same order as the values measured for pure water. We concluded, therefore, that proton exchange between water layers takes place with the diffusion rates of pure-like water.

Solvent isotope effect on macromolecular dynamics in E. coli. MSD, 〈u²〉, as a function of temperature, T, from E. coli containing H₂O (triangles) and E. coli containing D₂O (squares). Macromolecular flexibility, expressed as 〈u²〉 values, is smaller in D₂O. Structural resilience (Zaccai 2000), corresponding to a mean effective force constant, 〈k′〉, was extracted from the inverse of the slope of 〈u²〉 as a function of T in the two solvent conditions. 〈k′〉 was found to be smaller in D₂O than in H₂O.