DYNAMIC PROPERTIES OF SOME BETA-CHAIN MUTANT HEMOGLOBINS

The thermal behavior of the Soret band relative to the carbonmonoxy de rivatives of some beta-chain mutant hemoglobins is studied in the temp erature range 300-10 K and compared to that of wild-type carbonmonoxy hemoglobin. The band profile at various temperatures is modeled as a V oigt function that accounts for homogeneous broadening and for the cou pling with high- and low-frequency vibrational modes, while inhomogene ous broadening is taken into account with a gaussian distribution of p urely electronic transition frequencies. The various contributions to the overall bandwidth are singled out with this analysis and their tem perature dependence, in turn, gives information on structural and dyna mic properties of the system studied. In the wildtype and mutant hemog lobins, the values of homogeneous bandwidth and of the coupling consta nts 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 protei n dynamic behavior are observed. The most relevant are those relative to the ''polar isosteric'' beta Val-67(E11) --> Thr substitution, loca lized 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 consistin g 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 o n the dynamic properties of the heme pocket. (C) 1995 Wiley-Liss, Inc.