MEASUREMENT AND COMPUTATION OF THE DIPOLE-MOMENT OF GLOBULAR-PROTEINS.4. ALPHA-CHYMOTRYPSIN AND GAMMA-CHYMOTRYPSIN

The dipole moments of alpha- and gamma-chymotrypsins are determined us ing the dielectric constant measurement. The results are roughly compa rable to those of electric dichroism measurements despite the principl e and methodology of these two techniques are entirely different. Neve rtheless, the differences which exist between them seem to be beyond t he experimental error. The cause of disagreement appears to be, at lea st partially, due to the difficulty of finding the correct internal fi eld. A new theory which is based on an ellipsoidal particle surrounded by a hydration shell is discussed. The model was found to improve the agreement markedly. Additionally, in order to corroborate the observe d dipole moments with numerical computations, the dipole moments of al pha- and gamma-chymotrypsins were calculated using protein data bases. The dipole moment of small proteins consists of two major components, the moment due to fixed surface charges and the core moment due to po lar chemical bonds. The calculation of the surface charge dipole momen t consists of two parts: (1) computation of the pK shifts of polar gro ups in proteins and (2) computation of the dipole moments using correc ted pK's. The core moment was calculated as the vectorial summation of polar group moments in the backbone and side chains. The agreement be tween measured and calculated dipole moments is excellent.