A FORCE-FIELD FOR MOLECULAR MECHANICS STUDIES OF IRON PORPHYRINS

The development of parameters for the MM2(87) force field to model the iron porphyrins has been described. The mean values of the bond lengt hs and bond angles of iron porphyrins in the Cambridge Structural Data base have been used as a starting point in developing the force field, which was then refined using two representative structures, each of p enta- and hexa-coordinated high- and low-spin Fe-II and Fe-III porphyr ins, intermediate-spin penta- and hexa-coordinate Fe-III, and intermed iate-spin tetra-coordinate Fe-II porphyrins. The bond lengths were usu ally reproduced to within 0.015 Angstrom of the observed values, bond angles to within 1.5 degrees and torsional angles to within 4 degrees. The use of the force field has been exemplified in an investigation o f the conformation of intermediate-spin 5,10,15,20-tetraphenylporphyri niron(II) [Fe(TPP)] using quenched dynamics techniqes and the generic Structure algorithm of Gerber, Gubernator and Muller (Helv. Chim. Acta , 1988, 71, 1429) for ring systems. Both approaches have shown that th e two lowest-energy conformations available to the molecule have, resp ectively, a planar and an S-4-ruffled porphyrin core.