The extent to which ligation and quaternary structure modify the heme-heme pocket configuration is determined by generating and analyzing transient resonance Raman spectra from various photolyzed and partially photolyzed hemoglobins (Hb). From small frequency shifts in Raman band I (−1355 cm−1) it is determined that ligation induces a configurational change about the heme. The extent to which ligation modifies the heme pocket is influenced by the quaternary structure. With respect to the structural parameter responsible for variations in the π* orbital electron density of the porphyrin, the degree of alteration of the heme pocket configuration relative to deoxy-Hb(T) follows the sequence: liganded Hb(R) > liganded Hb(R) + IHP > liganded Hb(T) [α chain > β chain] > deoxy-Hb(R). This progression of configurations also forms a sequence with respect to the “retentiveness” of the heme pocket as reflected in the ligand dynamics associated with geminate recombination. The results indicate that the heme-heme pocket of the R-state Hb's, relative to those of the T-state species, favors ligand retention in a dynamic, as well as thermodynamic, sense. The analysis of these and other related data implicates a ligation and quaternary structure modulated electronic and/or electrostatic interaction between the π system of the porphyrin and the surrounding heme pocket as the basis for this variation in ligand dynamics as well as for the energetics of cooperativity.
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