Protein response to photodissociation of CO from carbonmonoxymyoglobin probed by time-resolved infrared spectroscopy of the amide I band.

Protein conformational changes coupled to the ligation reactions of carbon monoxide in myoglobin (Mb) are detected by time-resolved infrared spectroscopy. An apparatus based on a tunable diode laser operating in the region of 1650 cm-1 is used to probe changes in the amide I absorption band of the protein in response to photodissociation and subsequent rebinding of CO. The time course of changes in the amide I band is shown to follow the recombination of photolyzed CO with Mb. A time-resolved difference spectrum in the amide I region is generated by tuning the diode laser probe source. The features in the IR difference spectrum are assigned to the motions of the polypeptide backbone associated with the global relaxation of the protein from the ligated to the deoxy conformation. A static difference spectrum generated by subtracting FTIR spectra of carbonmonoxy-Mb and deoxy-Mb is essentially identical to the transient spectrum, indicating that the protein relaxation is complete with the 100-ns time resolution of the experiment.