A force field for molecular mechanics studies of iron porphyrins

Abstract

The development of parameters for the MM2(87) force field to model the iron porphyrins has been described. The mean values of the bond lengths and bond angles of iron porphyrins in the Cambridge Structural Database have been used as a starting point in developing the force field, which was then refined using two representative structures, each of penta- and hexa-coordinated high-and low-spin Fe^II and Fe^III porphyrins, intermediate-spin penta- and hexa-coordinate Fe^III, and intermediate-spin tetra-coordinate Fe^II porphyrins. The bond lengths were usually reproduced to within 0.015 Å of the observed values, bond angles to within 1.5° and torsional angles to within 4°. The use of the force field has been exemplified in an investigation of the conformation of intermediate spin 5,10,15,20-tetraphenylporphyriniron(II)[Fe(TPP)] using quenched dynamics techniques and the generic structure algorithm of Gerber, Gubernator and Müller (Helv. Chim. Acta, 1988, 71, 1429) for ring systems. Both approaches have shown that the two lowest-energy conformations available to the molecule have, respectively, a planar and an S₄-ruffled porphyrin core.