Electric dipole moments of globular proteins: measurement and calculation with NMR and X-ray databases

The electric dipole moments of small proteins are computed using NMR as well as X-ray databases and compared with experimental results. The dipole moment of protein molecules consists of a surface charge dipole moment and the core dipole moment due to polar side chains. The computation of surface charge dipole moment consists of two steps: (A) calculation of the pK shifts of polar groups and (B) calculation of the dipole moment using pKs compensated for electrostatic shifts. Unlike X-ray databases, NMR databases consist of a number of models due to the variability of the conformation of protein molecules in solution. The structure of proteins is presented graphically by two- or three-dimensional diagrams. Of particular interest is the graphic presentation using a superposition diagram. The dipole moments calculated by NMR and X-ray databases are in good agreement with measured dipole moments.