Dynamic properties of some beta-chain mutant hemoglobins

The thermal behavior of the Soret band relative to the carbonmonoxy derivatives of some beta-chain mutant hemoglobins is studied in the temperature range 300-10 K and compared to that of wild-type carbonmonoxy hemoglobin. The band profile at various temperatures is modeled as a Voigt function that accounts for homogeneous broadening and for the coupling with high- and low-frequency vibrational modes, while inhomogeneous broadening is taken into account with a gaussian distribution of purely electronic transition frequencies. The various contributions to the over-all bandwidth are singled out with this analysis and their temperature dependence, in turn, gives information on structural and dynamic properties of the system studied. In the wild-type and mutant hemoglobins, the values of homogeneous bandwidth and of the coupling constants to high-frequency vibrational modes are not modified with respect to natural human hemoglobin, thus indicating that the local electronic and vibrational properties of the heme-CO complex are not altered by the recombinant procedures. On the contrary, differences in the protein dynamic behavior are observed. The most relevant are those relative to the "polar isosteric" beta Val-67(E11)-->Thr substitution, localized in the heme pocket, which results in decreased coupling with low-frequency modes and increased anharmonic motions. Mutations involving residue beta Lys-144(Hc1) at the C-terminal and residue beta Cys-112(G14) at the alpha 1 beta 1 interface have a smaller effect consisting in an increased coupling with low-frequency modes. Mutations at the beta-N-terminal and at the alpha 1 beta 2 interface have no effect on the dynamic properties of the same heme pocket.