Vibrational relaxation of NO stretching modes in ferrous NO and ferric NO in model heme

Abstract

Femtosecond IR-pump–IR-probe spectroscopy was used to measure the vibrational lifetimes (T1) of NO stretching modes of ferrous NO near 1600cm−1 and ferric NO near 1900cm−1 at room temperature. The T1 of NO bound to the heme, ranging from 3.5 to 34ps, is much shorter in ferrous NO. The vibrational relaxation (VR) of NO was independent of solvent used and excess imidazole concentration, suggesting that intramolecular VR into the internal vibrational modes of the probed molecule may be the dominant pathway for VR of the bound NO. With estimated T1 of the bound NO, we simulated transient spectra of NO bound to ferrous hemoglobin (HbII) after photodeligation of HbIINO and discussed the influence of the hot band on the determination of the dynamics of geminate rebinding of NO to HbII using the change in the magnitude of the fundamental band.