The structure and dipole moment of globular proteins in solution and crystalline states: Use of NMR and x-ray databases for the numerical calculationof dipole moment

The large dipole moment of globular proteins has been well known because of the detailed studies using dielectric relaxation and electro-optical metho ds. The search for the origin of these dipolemoments, however, must be base d on the detailed knowledge on protein structure with atomic resolutions. A t present, we have two sources of information on the structure of protein m olecules: (1) x-ray databases obtained in crystalline state: (2) NMR databa ses obtained in solution state. While x-ray databases consist of only one m odel, NMR databases, because of the fluctuation of the protein folding in s olution, consist of a number of models, thus enabling the computation of di pole moment repeated for all these models. The aim of this work, using thes e databases, is the detailed investigation on the interdependence between t he structure and dipole moment of protein molecules. The dipole moment of p rotein molecules has roughly two components: one dipole moment is due to su rface charges and the other, core dipole moment, is due to polar groups suc h as N-H and C double bondO bonds. The computation of surface charge dipole moment consists of two steps: (A) calculation of the pK shifts of charged groups for electrostatic interactions and (B) calculation of the dipole mom ent using the pK corrected for electrostatic shifts. The dipole moments of several proteins were computed using both NMR and x-ray databases. The dipo le moments of these two sets of calculations are, with a few exceptions, in good agreement with one another and also with measured dipole moments. (C) 2001 John Wiley & Sons. Inc. Biopolymers 58: 398-409, 2001.