Fe-heme conformations in ferric myoglobin

X-ray absorption near-edge structure (XANES) spectra of ferric myoglobin fr om horse heart have been acquired as a function of pH (between 5.3 and 11.3 ). At pH = 11.3 temperature-dependent spectra (between 20 and 293 K) have b een collected as well. Experimental data solve three main conformations of the Fe-heme: the first, at low pH, is related to high-spin aquomet-myoglobi n (Mb(+)OH(2)). The other two, at pH 11.3, are related to hydroxymet-myoglo bin (Mb(+)OH(-)), and are in thermal equilibrium, corresponding to high- an d low-spin Mb(+)OH(-). The structure of the three Fe-heme conformations has been assigned according to spin-resolved multiple scattering simulations a nd fitting of the XANES data. The chemical transition between Mb(+)OH(2) an d high-spin Mb(+)OH(-), and the spin transition of Mb(+)OH(-), are accompan ied by changes of the Fe coordination sphere due to its movement toward the heme plane, coupled to an increase of the axial asymmetry.