migration in Cygb, Ngb and GbX, we replaced the non-fluorescent Fe protoporphyrin IX (FePPIX) with the fluorescent analogue Zn protoporphyrin IX (ZnPPIX) and characterized the ligand migration by monitoring the quenching of the ZnPPIX fluorescent emission by methyl viologen and the quenching of the ZnPPIX triplet state by oxygen. We observed an increase in the Ksv value for methyl viologen quenching in protein variants that are missing the internal disulfide bond, suggesting that the presence of the intraprotein disulfide bond regulates the accessibility of the heme binding pocket. Interestingly the lifetime of the triplet state for ZnPPIX incorporated in hexacoordinate vertebrate globins is not sensitive to changes in the heme environment as the observed lifetime is comparable to that determined previously for myoglobin.