**4. Conclusion**

The photoacoustic data for the ligand photo-dissociation from Mb shows that the structural volume changes associated with the O2 diffusion from the Mb active site are similar to those determined previously for CO in agreement with the crystallographic data. On the other hand, the time constant for O2 escape from the distal pocket to the surrounding solvent is two to three time faster than that for CO suggesting a distinct migration pathway for diatomic ligands in Mb. Our PAC study also indicates that IHP binding to Hb-CO complex alters the volume and enthalpy changes associated with the CO photo-dissociation from the heme iron indicating that the transition between the fully ligated (CO)4Hb and partially ligated (CO)3Hb complex is associated with the reorientation of IHP molecule within the central cavity and/ or charged amino acid residues interacting with IHP.
