Kinetics of carboxymyoglobin and oxymyoglobin studied by picosecond spectroscopy

Abstract

Picosecond studies of carboxymyoglobin (MbCO) and oxymyoglobin (MbO2) reveal that excitation at 530 nm induces photodissociation at less than 8 ps. The kinetic and structural changes were monitored by following absorbance changes at selected wave-lengths in the Soret (B) band and in the Q band. Within the 10 ps-0.45 ns period of time over which our experiments were conducted, the absorbance changes in the Soret and Q bands for MbCO and MbO2 correspond to the conventional long-term, steady-state deoxymyoglobin difference spectra (Mb-MbCO and Mb-MbO2), as determined by comparison of isosbestic, maximum, and minimum points. In addition, MbCO exhibits a decay to a steady state in the Soret band (monitored at 440 nm). The onset of the decay immediately follows photodissociation and has a rate of (8 +/- 3) X 10(9) s-1 (tau = 125 +/- 50 ps). During the 10 ps-0.45 ns observation window, relaxation is not seen for MbO2 in the Soret band, nor is relaxation observed in the Q band for either MbCO or MbO2. We conclude from these results that the steady state that we observed for MbCO and MbO2 is most likely the stable form of deoxymyoglobin, and the relaxational differences between MbCO and MbO2 observed in the Soret band indicate that the electronic destabilization after ligand detachment is very different for these molecules. We believe that these relaxational differences may be related to differences in tertiary structural changes, or due to the fact that the MbCO (S = 0) molecule passes through an intermediate spin Mb (S = 1) state before relaxing the the Mb (S = 2) state.