Effects of Internal Motion at the Macromolecular Binding Site

Effects of Internal Motion at the Macromolecular Binding Site 

In certain biological applications of the halide probe technique, for example when a halide ion may be liganded to a Hg-label attached to a macromolecule, the halide ion at the binding site may experience a certain amount of internal freedom of motion with respect to the overall motion of the macromolecule. A schematic illustration of this situation is given in Fig. 8.1. 

Studies of the effect of internal rotation on molecular correlation functions have been made by Woessner [411], Zeidler [62], Levine, Partington and Roberts [412] and Wallach [63]. The latter work considers internal rotation around several rotational axes in a mercury halide label anchored to a macromolecule. The resulting effects are that when the rate of internal rotation (1/t ) is much faster than the over-all motion of the macromolecule (t f the effective quadrupole interaction in the Hg-X bond is reduced by a factor of 

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