Complex nonexponential relaxation in myoglobin after photodissociation of MbCO: measurement and analysis from 2 ps to 56 υs

Photodissociation of carbon monoxide myoglobin (MbCO) creates Mb in nonequilibrium conformation and triggers a global conformational charge. The driving force for the change is localized initially in the iron-porhyrin coordinate. The dynamics of the iron-prophyrin displacement at physiologically relevant temperatures have been followed by probing the evolution of band III, a weak iron-prophyrin charge-transfer transition near 13110 cm^t-1 (763 nm) that is sensitive to the out-of-plane position of the iron. This functionally important motion is highly nonexponential, stretching over 5 decades in time in 70:30 (w/w) glycerol/water mixtures at 301 ±1 K. The relaxation is well described by a stretched exponential function that has been modified to account for the limiting rate associated with the primary relaxation process.