ROTATIONAL AND TRANSLATIONAL WATER DIFFUSION IN THE HEMOGLOBIN HYDRATION SHELL - DIELECTRIC AND PROTON NUCLEAR-RELAXATION MEASUREMENTS

The dynamic properties of water in the hydration shell of hemoglobin h ave been studied by means of dielectric permittivity measurements and nuclear magnetic resonance spectroscopy. The temperature behavior of t he complex permittivity of hemoglobin solutions has been measured at 3 .02, 3.98, 8.59, and 10.80 GHz. At a temperature of 298 K the average rotational correlation time tau of water within a hydration shell of 0 .5-nm thickness is determined from the activation parameters to be 68 +/- 10 ps, which is 8-fold the corresponding value of bulk water. Solv ent proton magnetic relaxation induced by electron-nuclear dipole inte raction between hemoglobin bound nitroxide spin labels and water proto ns is used to determine the translational diffusion coefficient D(T) o f the hydration water. The temperature dependent relaxation behavior f or Lamor frequencies between 3 and 90 MHz yields an average value D(29 8K) = (5 +/- 2) X 10(-10)m2 s-1, which is about one-fifth of the corre sponding value of bulk water. The decrease of the water mobility in th e hydration shell compared to the bulk is mainly due to an enhanced ac tivation enthalpy.