Vibrational Dynamics of Carbon Monoxide at the Active Sites of Mutant Heme Proteins

Abstract

Picosecond mid-IR pump-probe measurements of vibrational relaxation (VR) of CO bound to the active sites of wild-type and mutant myoglobins (Mb) reveal that an approximately linear relationship exists between the protein matrix-induced CO frequency shift and the VR rate. This relation parallels a similar linear relationship seen in a series of heme model compounds where Fe was replaced by Ru and Os. The VR rate of CO in the Mb is sensitive only to the magnitude of protein-induced carbonyl frequency shifts and apparently is not sensitive to the specific details of how the shift is induced, e.g., hydrogen bonding to CO or electrostatic interactions in the heme pocket. CO VR is insensitive to substantial changes in protein structure that do not affect the CO vibrational frequency. These observations suggest that the mechanism of carbon monoxide VR in heme proteins such as Mb occurs by through-π-bond anharmonic coupling, which, as shown in prior work, is also the dominant coupling in the model compounds. The experiments indicate that differing protein structures influence VR of CO bound at the active site not by opening and closing channels for vibrational energy flow from CO to the protein but by affecting the rate of energy flow from CO to heme. The rates are determined by the extent of back-bonding, which determines the magnitude of through-π-bond anharmonic coupling between CO and heme. The back-bonding and, therefore, the extent of anharmonic coupling are influenced by the electric fields in the heme pocket, which likely differ in the proteins studied here.

Neuroglobin dynamics observed with ultrafast 2D-IR vibrational echo spectroscopy

“…The dynamics of MbCO and MbCO mutants have been measured by using vibrational echo spectroscopy (see refs. 16,[18][19][20][21][32][33][34]. The backgroundsubtracted normalized FTIR spectra of CO bound to the mutants are shown in Fig.…”

Vibrational population relaxation of carbon monoxide in the heme pocket of photolyzed carbonmonoxy myoglobin: Comparison of time-resolved mid-IR absorbance experiments and molecular dynamics simulations

“…** The surviving features correspond to 13 CO in its ground vibrational state ( ϭ 0) and trapped within a protein docking site (20 shift between the ground (1 4 0) and hot band (2 4 1) vibrational transitions of 13 CO in the gas phase (57). Consequently, B* 1 and B* 2 arise from 13 CO generated in its first excited vibrational state ( ϭ 1). The spectral evolution of 13 CO is shown more clearly in Fig.…”

“…Consequently, B* 1 and B* 2 arise from 13 CO generated in its first excited vibrational state ( ϭ 1). The spectral evolution of 13 CO is shown more clearly in Fig. 3, where the raw data are omitted and the best-fit spectra † † are overlaid rather than offset.…”

“…The absence of temperature-dependent relaxation up to 300 K suggests that these ''bath'' modes have frequencies greater than Ϸ400 cm Ϫ1 (13). This argument excludes the protein's lowest frequency collective modes and indicates that the bath modes responsible for vibrational relaxation correspond to localized vibrations.…”

“…In addition, Fayer and coworkers found that (i) the T 1 population relaxation time for the carbonyl ligand stretch is largely independent of temperature from 20 to 300 K (12); (ii) the vibrational lifetimes vary among different vibrational substates of the same protein (13); (iii) changes in the vibrational lifetime of bound CO are highly correlated with a change in the vibrational frequency of CO (13); and (iv) intermolecular transfer of vibrational energy is relatively unimportant in the relaxation process of bound CO (13).…”