X-ray absorption spectroscopy of myoglobin and iron prophyrin derivatives

Abstract

Diverse interpretations of the structure of carboxymyglobin and its photodissociation products have led us to a reexamination of problems of data collection and analysis of iron porphyrins and heme proteins by x-ray absorption spectroscopy. We have been especially concerned with the assignment of statistical weight to individual wave functions that contribute to the EXAFS spectral region. In the case of iron porphyrin derivatives, a single axial first shell atom contributes only a small fraction of the total wave form, and its definition may be fraught with uncertainties. We have therefore attempted to calculate least squares best fits of the sum of all significant wave form contributions to the total EXAFS spectrum of a series of iron porphyrin model compounds and myoglobin derivatives. This has required inclusion of wave forms for all porphyrin ring atoms in addition to substituent groups directly bonded to the porphyrin ring. Axial ligands have been more difficult to establish, except in the case of tightly bonded atoms, and those whose waveforms are enhanced by multiple scattering. Radial distances estimated for pyrrole nitrogens, alphacarbons, and meso-carbons, are all well described by single scattering theory, and correspond closely to crystallographically estimated distances. Apparent radial distance for beta-carbons, and gamma-carbons, are shorter than crystallographic distances by about 0.2 and 0.6 A, respectively. This is expected from multiple scattering phase shifts. The radial distances estimated for pyrrole nitrogens are sensitive to spin state of the iron, as predicted from crystallographic studies. This is clearly observed both with model compounds (derivatives of tetraphenyl porphyrin and octaethylporphyrin) as well as with myoglobin derivatives.