A quantum-mechanical model for the determination of internal electric fields at protein active sites from the Stark effect on persistent spectral holes

The Stark effect of spectral holes burnt in protoporphyrin IX (H2-PPIX) substituted myoglobin is employed to determine quantitatively the internal electric fields generated by protein charge distribution at the H2-PPIX site. The procedure for extracting the internal field in molecular resolution from the measured hole profiles involves the numerical diagonalization of the H2-PPIX Hamiltonian. The resulting values for the internal fields depend on the number of electronic states used in the diagonalization. In this paper, we extend earlier work by carrying out the diagonalization by successively adding more electronic states based on their energy in order to determine the convergence behavior of the internal-field magnitudes. We show that a different sequence of adding states improves the convergence behavior of the internal fields.