Time-domain reflectrometry studies of water binding and structural flexibility in chymotrypsin

Time-domain dielectric spectroscopy has been employed to probe the hydration properties and structural flexibility of chymotrypsin (EC 3.4.21.1). The dielectric properties of the hydrated protein above 100 MHz have been used to identify two categories of protein-bound water, the first being irrotationally bound to the protein with a second, relatively weakly bound, having a rotational freedom comparable with that of normal bulk water. A dielectric dispersion observed, centred at 12 MHz, has been attributed to the relaxation of the polar components of the protein structure. This dielectric loss became increasingly significant above a transition in the hydration dependence, where water is relatively weakly bound to the chymotrypsin. This is discussed in terms of the formation of water clusters on the protein surface which screen electrostatic interactions between protein-charged groups.