Thermal and solvent perturbation as probes of conformational heterogeneity of haptoglobin 1-1.

The shifts which occur in the ultraviolet spectra of haptoglobin 1-1 (Hp) and a conformational isomer with lower affinity for hemoglobin (Hp), in response to temperature and solvent changes, have been measured by difference spectra. The thermal difference spectra provide evidence for a different participation of tryptophyl residues in the conformational stability of Hp and Hp. Solvent perturbations have allowed an estimation to be made of the number of surface chromophores in the native Hp and Hp. These results are compatible with the existence of two Hp conformational isomers with different free energy, separated by a high-potential barrier.