Hydration water dynamics of a completely hydrophobic oligopeptide

The dynamics of hydration water of a completely deuterated penta-alanine peptide has been studied by incoherent quasi-elastic neutron scattering. Measurements have been made at different hydration levels (7%, 30%, 50%, 90%), and on the dried powder (0%) which contains one structural water molecule. The dynamical contribution of this first hydration molecule of water is characteristic of a slow rotational motion with a relaxation time, tau(1), of 2.2 ps, similar to what is found in supercooled water dynamics. Adding two more hydration water molecules (7%) the rotational motion of the first water is coupled with the new diffusive motion and the dynamics profile can be, in first approximation, described through a rotational jump model. The results suggest a behavior similar to that of bulk water at 2 degreesC. At higher levels of hydration, the mobility of new molecules of water approaches that of bulk water, with a rotation relaxation time of 1 ps and a confined diffusing motion. However the residence time value, tau(0), is of the same order of magnitude as supercooled water at T = -10 degreesC. (C) 2003 Elsevier Science B.V. All rights reserved.