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

Abstract

Femtosecond vibrational spectroscopy was used to measure the vibrational population relaxation time (<TEX>$T_1$</TEX>) of different anions bound to ferric myoglobin (<TEX>$Mb^{III}$</TEX>) and hemoglobin (<TEX>$Hb_{III}$</TEX>) in <TEX>$D_2O$</TEX> at 293 K. The <TEX>$T_1$</TEX> values of the anti-symmetric stretching (<TEX>${\nu}_1$</TEX>) mode of NCS in the <TEX>$NCS^-$</TEX> bound to <TEX>$Mb^{III}$</TEX> (<TEX>$Mb^{III}$</TEX>NCS) and <TEX>$Hb_{III}$</TEX> (<TEX>$Hb_{III}$</TEX>NCS) in <TEX>$D_2O$</TEX> are <TEX>$7.2{\pm}0.2$</TEX> and <TEX>$6.6{\pm}0.2$</TEX> ps, respectively, which are smaller than that of free NCS. in <TEX>$D_2O$</TEX> (18.3 ps). The <TEX>$T_1$</TEX> values of the <TEX>${\nu}_1$</TEX> mode of NCO in the <TEX>$NCO^-$</TEX> bound to <TEX>$Mb^{III}$</TEX> (<TEX>$Mb^{III}$</TEX>NCO) and <TEX>$Hb_{III}$</TEX> (<TEX>$Hb_{III}$</TEX>NCO) in <TEX>$D_2O$</TEX> are <TEX>$2.4{\pm}0.2$</TEX> and <TEX>$2.6{\pm}0.2$</TEX> ps, respectively, which are larger than that of free <TEX>$NCO^-$</TEX> in <TEX>$D_2O$</TEX> (<TEX>$1.9{\pm}0.2$</TEX> ps). The smaller <TEX>$T_1$</TEX> values of the <TEX>${\nu}_1$</TEX> 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 <TEX>$T_1$</TEX> values of the <TEX>${\nu}_1$</TEX> mode of the heme-bound NCO suggest that intermolecular VR is the dominant relaxation pathway for the excess vibrational energy in the <TEX>${\nu}_1$</TEX> mode of <TEX>$NCO^-$</TEX> in <TEX>$D_2O$</TEX>, and that intramolecular VR becomes more important in the vibrational energy dissipation of the <TEX>${\nu}_1$</TEX> mode of NCO in <TEX>$Mb^{III}$</TEX>NCO and <TEX>$Hb_{III}$</TEX>NCO.