Reactive Vibrational Dynamics of Iron in Heme

Abstract

Nuclear resonance vibrational spectroscopy (NRVS) measurements supported by DFT calculations identify vibrational modes of the iron atom in halide derivatives of iron porphyrins. These compounds capture many essential aspects of heme geometry and vibrations. The smaller (porphine) models simplify the vibrational spectrum and enable accurate analysis using DFT methods. NRVS identifies both doming and Fe-halide stretching components of the reaction coordinate with confidence. Correlation analysis between 4-coordinate and 5-coordinate compounds suggests significant mixing between Fe-ligand and heme modes. Measurements and calculations on larger porphyrins reveal the effect of peripheral groups. Measurements on oriented porphine halide crystals definitively identify the contribution of in plane and out of plane Fe motion. The frequency of heme doming vibrations varies in a systematic manner with peripheral substitution and halide mass, which will allow us to evaluate their contributions to vibrational signals that follow femtosecond laser excitation. Such measurements will ultimately enable quantitative estimates of the energetics of molecular distortions that modulate reaction rates in heme proteins.