Quantifying Vibrational Participation in Biological Reactions

Nuclear resonance vibrational spectroscopy (NRVS) extracts the complete vibrational spectrum of a Mössbauer probe nucleus from high-resolution X-ray absorption measurements near the nuclear resonance. We determine the frequency, amplitude, and direction of ^57Fe vibrational modes in a series of iron porphyrins, which model the active sites of heme proteins. These measurements allow us to characterize low frequency vibrational participants in biological reactions with diatomic ligands. Normal mode calculations can indicate the extent to which these vibrations resemble "doming" of the heme. However, the experimental data provide a direct estimate of the force constant for Fe displacement normal to the plane of the porphyrin, which does not rely on a specific normal mode interpretation. The result suggests that Fe displacement is an important element in protein control of ligand binding reactions.