Structural dynamics of human deoxyhemoglobin and hemochrome investigated by nuclear gamma resonance absorption (Mössbauer) spectroscopy.

Mössbauer spectra of human deoxyhemoglobin and hemochrome frozen solutions, selectively enriched with 57Fe in either the alpha chains or the beta chains, were measured from 4.2 to 250 K. The Lamb-Mössbauer factor, ln f = 4II2 [x2]gamma/lambda 2, was calculated from these spectra and gives the structural dynamics of the iron atom (heme). Large differences in the mean-squared displacement, [x2]gamma, of the heme between hemochrome and deoxyhemoglobin at temperatures above 210 K were observed, demonstrating that when iron is bound to a rigid part of the protein (i.e., distal histidine in hemochrome), motions of the heme are suppressed (i.e., the dynamics are decreased). Comparison of the motions of these two hemoglobins proves that molecular diffusion can be neglected in an analysis of the dynamics below approximately 250 K.