A QUASI-ELASTIC NEUTRON SCATTERING STUDY OF HYDROGEN DYNAMICS IN AQUEOUS POLYMER SOLUTIONS

[摘要] We have used quasi-elastic neutron scattering techniques to study the dynamics of solvent water protons in H$sb 2$O solutions of deuterated poly(ethylene oxide) (Polyox) at room temperature. Using a slightly modified technique, we also have obtained values for the mean-square vibrational amplitude of polymer protons in powder and D$sb 2$O solutions of Polyox and trypsin at temperatures between 300K and 75K. To study the dynamics of solvent water in deuterated Polyox (dPolyox) solutions, we used a high energy-resolution spectrometer ($Gammasb{m res}approx$ 100$mu$eV) so that we could measure the width of the quasi-elastic line for various values of the momentum transfer, Q. The Q-dependence of the quasi-elastic linewidth shows that the diffusive properties of water in these Polyox/water solutions is slightly affected at low (5 polymer wt%) and intermediate (10-20 wt%) polymer concentrations. At high polymer concentration (38 wt%), the water clearly has a reduced diffusion coefficient and an increased residence time compared to bulk water and this may be attributed to the nature of the interaction of the water molecules with the polymer. We were motivated to study the mean-square vibrational amplitude of protons in powder and D$sb 2$O solutions of Polyox and trypsin because of recent computer simulations on biological macromolecules. We measured the vibrational mean-square amplitude by using a low energy-resolution spectrometer ($Gammasb{m res}approx$ 700$mu$eV) to measure the intensity of the quasi-elastic peak in a relatively short time compared to standard high energy-resolution techniques. Our results on these 20 wt% solutions at 300K are consistent with conventional quasi-elastic neutron scattering results on the Polyox and with computer simulations of trypsin in solution. The vibrational amplitude was proportional to temperature throughout the temperature range studied, and the proportionality constant is interpreted in terms of an effective spring constant for the motion of the hydrogens. These spring constants are smaller than those for some common chemical bonds indicating that there are significant contributions to the motion of the hydrogens besides vibrational motion.

[发布机构] Rice University