Iron normal mode dynamics in a porphyrin-imidazole model for deoxyheme proteins.

Iron vibrational modes of a deoxyheme protein model (2-methylimidazole)(tetraphenylporphinato)iron(II), [Fe(TPP)(2-MeImH)], have been studied by refining normal mode calculations to nuclear resonance vibrational spectroscopy (NRVS) data. The NRVS measurements give quantitative frequencies and iron amplitudes of all modes with significant Fe vibrational motion. Modes with in-plane displacement of iron are distinguished from those involving out-of-plane motion by measurements on oriented single-crystal samples. Normal modes having large overlaps with in-plane nu(42), nu(50), and nu(53) modes of the porphyrin core are identified, as well as several modes with large iron-imidazole stretch components. An out-of-plane mode at 78 cm(-1) shows significant doming of the porphyrin core, but the largest Fe doming motion arises from the coupling of phenyls and imidazole at 25 cm(-1).