Vibrational Relaxation of Cyanate or Thiocyanate Bound to Ferric Heme Proteins Studied by Femtosecond Infrared Spectroscopy

Femtosecond vibrational spectroscopy was used to measure the vibrational population relaxation time (T1) of different anions bound to ferric myoglobin (Mb) and hemoglobin (Hb) in D2O at 293 K. The T1 values of the anti-symmetric stretching (ν1) mode of NCS in the NCS − bound to Mb (MbNCS) and Hb (HbNCS) in D2O are 7.2 ± 0.2 and 6.6 ± 0.2 ps, respectively, which are smaller than that of free NCS − in D2O (18.3 ps). The T1 values of the ν1 mode of NCO in the NCO − bound to Mb (MbNCO) and Hb (HbNCO) in D2O are 2.4 ± 0.2 and 2.6 ± 0.2 ps, respectively, which are larger than that of free NCO− in D2O (1.9 ± 0.2 ps). The smaller T1 values of the ν1 mode of the heme-bound NCS suggest that intramolecular vibrational relaxation (VR) is the dominant relaxation pathway for the excess vibrational energy. On the other hand, the longer T1 values of the ν1 mode of the heme-bound NCO suggest that intermolecular VR is the dominant relaxation pathway for the excess vibrational energy in the ν1 mode of NCO − in D2O, and that intramolecular VR becomes more important in the vibrational energy dissipation of the ν1 mode of NCO in Mb NCO and HbNCO.