Direct observation of two-channel photodissociation of carbon monoxide from the hemoglobin subunits

Abstract Determining dynamics of bond breaking between carbon monoxide (CO) and heme proteins is essential to understand the interplay between protein function and dynamics, which is one of the fundamental challenges of physical biology. There is an ongoing debate about the mechanism of CO photodissociation from the heme iron. Here we use picosecond to millisecond transient mid-infrared spectroscopy to determine the dynamics of CO photodissociation from the isolated human hemoglobin chains. We find that the breaking of the Fe–CO bond is not a single-step process as is commonly accepted, but rather at least a two-step process, which includes both the known prompt sub-50-fs CO dissociation event and the additional slower, ~15 ps CO dissociation process discovered in this study. These results offer the direct experimental proof of the CO photodissociation mechanism containing several dissociative states.