PROTEIN CONFORMATIONAL RELAXATION FOLLOWING PHOTODISSOCIATION OF CO FROM CARBONMONOXYMYOGLOBIN - PICOSECOND CIRCULAR-DICHROISM AND ABSORPTION STUDIES

Picosecond time-resolved polarization spectroscopy is used to study relaxation dynamics in myoglobin following photoelimination of CO from carbonmonoxymyoglobin. Evolution of the transient circular dichroism signal of the N band of myoglobin (probed at 355 nm) to that characteristic of equilibrium myoglobin requires almost-equal-to 300 ps. This time scale is significantly longer than that corresponding to the photoinitiated bond cleavage. Transient linear dichroism of the Soret band and picosecond time-resolved magnetic circular dichroism measurements of the Q band demonstrate that the circular dichroism kinetics do not result from either time-dependent changes in the orientation of the transition moments of the heme ring or the doming of the heme that accompanies the out-of-plane motion of the iron. Finally, transient absorption data of the near-IR optical transition of photogenerated myoglobin suggest that the circular dichroism data are not a measure of the tilting of the proximal histidine. The circular dichroism data are discussed in terms of a relaxation in the tertiary structure of the protein following dissociation.