Effect of the vinyl-globin interactions on the temperature-dependent broadening of the Soret spectra: a study with horse myoglobin and Scapharca dimeric hemoglobin reconstituted with unnatural 2,4-heme derivatives.

The temperature dependence of the Soret absorption spectra has been measured over the range 80 to 300 K on deoxygenated and carbonmonoxy horse heart myoglobin and Scapharca inaequivalvis dimeric hemoglobin reconstituted with proto- or with meso- and deutero-heme, in which the vinyl groups have been replaced with ethyl groups or hydrogen atoms, respectively. In the meso- and deutero-derivatives of both proteins the linewidth of the absorption spectra is narrower and less sensitive to thermal broadening effects than in the proto-derivatives. Moreover, the broadening effects are larger in the deoxygenated proteins with respect to the liganded adducts. The quantitative analysis of these effects shows that the change in linewidth is due to a marked decrease in the extent of coupling between the heme vibronic transitions and the protein low-frequency motions. The relevance of the vinyl groups in the dynamics of the heme-globin interaction is highlighted by this experimental approach which shows that the protein is capable of transmitting structural information to the heme by coupling the ensemble of the low-frequency modes to the stereochemistry of the vinyl itself. This mechanism, which entails adjustment of the equilibrium between vinyl torsional conformers, represents an additional pathway for the control of the heme reactivity in addition to the iron-histidine link.