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Reactions of excited triplet states of metal-substituted myoglobins

The small-molecule (i.e. oxygen) penetrability into proteins carries important implications for both the structural character and dynamic properties of proteins. We investigated aspects of protein dynamics by studying triplet-state quenching of Zn and Pd derivatives of myoglobin by oxygen. These two derivatives make an interesting comparison because the Zn derivative of myoglobin is five coordinated and is out-of-plane (as is deoxy myoglobin), whereas Pd porphyrin is planar (as is oxy myoglobin). Therefore relaxation of the polypeptide chain in the excited state analogues of the two functional conformations of myoglobin can be studied. In case of Znmyoglobin, the spectra did not exhibit an isosbestic point and the decay kinetics to the ground state dependtd upon the wavelength of measurement. The decay profiles could be fit by a double exponential function with a good correlation. In the case of Pd-myoglobin an isosbestic point was observed in the emission and the decay profile could be fit with a single exponential function. The comparison of the quenching constants for the two metal substituted derivatives suggests that there is no significant difference between the oxygen penetrability of the Zn and the Pd substituted myoglobins.