Oxygen binding to iron-porphyrin: A density functional study using both LSD and LSD + GC schemes

Abstract

Equilibrium structures of iron-porphyrin (FeP) and its O2 complex, computed with Car-Parrinello molecular dynamics, are in excellent agreement with experimental data on synthetic heme models. Geometry optimization can affect significantly the relative energies among spin states of these systems. The Fe - O bond, best described as FeIII-O2-, increases its strength from 9 to 15 kcal/mol upon binding an imidazole axial ligand (Im). Our results are consistent with an open-shell singlet as the ground state of FeP(Im)(O2), in competition with a low-lying closed-shell singlet state and, as the FeOO angle increases, with a triplet state. The conformation in which the imidazole plane and the O - O axis projection lie in the same N - Fe - N porphyrin quadrant is found to be stable, although easy rotation of the O2 molecule around the Fe - O axis at room temperature is predicted. Comparison of the performance between the local spin density approximation (LSD) and the gradient-corrected scheme (LSD + GC) is provided. © 1998 John Wiley & Sons, Inc.