Electrooptical analysis of alpha-chymotrypsin at physiological salt concentration

The electric dichroism of alpha-chymotrypsin has been measured in a buffer containing 0.1 M Na+, 10 mM Mg2+ and 25 mM Tris-cacodylate pH 7.2. The reduced dichroism as a function of the electric field strength can be represented by the orientation function for permanent dipoles and is not consistent with the orientation function for induced dipoles. After correction for the internal directing field, the dipole moment is 1.1 x 10(-27) Cm (+/-10%), corresponding to 340 D, at 20 degrees C. The assignment of the permanent dipole moment is confirmed by the shape of the dichroism rise curves, which require two exponentials with amplitudes of opposite sign for fitting. The dichroism decay time constants measured in the range of temperatures between 2 and 30 degrees C indicate a temperature induced change of the structure, which is equivalent to an increase of the hydrodynamic radius from r = 26.6 Angstrom at 2 degrees C to 28.5 Angstrom at 30 degrees C. Our results demonstrate that electrooptical investigations of proteins with a high time resolution can be extended to physiological salt concentrations without serious problems by use of appropriate instruments.