Dynamics of nitric oxide rebinding to ferric myoglobin in D2O probed by femtosecond mid-IR spectroscopy

The dynamics of diatomic ligands such as O₂, CO, and NO binding to ferrous (Fe²⁺) heme iron of myoglobin have been studied widely due to the physiological importance of reversible binding. However, the dynamics of NO with ferric (Fe³⁺) heme iron of myoglobin is not much studied due to the high reactivity of the heme iron toward NO. The appearance of the multiple bands at 1914, 1927, and 1942 cm^-1 in the FT-IR spectrum of wild-type ferric myoglobin (Mb^IIINO) reveals the conformational heterogeneity, most probably arising from different electrostatic interactions between bound NO and His-64. Since IR spectrum is very sensitive to the protein structure and its environment, IR spectroscopy of the ligand bound myoglobin is an excellent method for probing the ligand binding kinetics of heme proteins, in particular, conformer-dependent rebinding rates. The coupling of protein motion to the rebinding process in Mb^IIINO is probed by the geminate recombination (GR), and the nature of the underlying protein dynamics that control ligand binding is discussed.