Vibrational dynamics of carbon monoxide at the active site of myoglobin: Picosecond infrared free-electron laser pump-probe experiments

The vibrational lifetimes of CO stretching modes of CO bound to different conformational substates of myoglobin, and CO bound to a water-soluble bare Fe:porphyrin, Fe tetraphenylporphyrin sulfate, were measured by picosecond infrared (IR) pump-probe experiments using the Stanford Free Electron Laser. At room temperature, two substates of carboxymyoglobin (Mb-CO), denoted A₀ and A₁, yielded lifetimes of 26.6 ± 1 and 18.2 ± 1 ps in a poly(vinyl alcohol) matrix. In glycerol:water solution, the A₁-state lifetime of Mb-CO was 17.4 ± 1 ps. These lifetimes do not depend much on temperature in the 20-300 K range. The lifetime of the bare Fe:porphyrin was 17 ± 3 ps. Results obtained on these and other heme-CO systems are used to show that vibrational relaxation is slower with CO whose frequency is close to the ∼1970 cm^-1 value characteristic of proteins and model compounds with CO nearly perpendicular to the heme plane, and faster with CO with lower frequencies characteristic of hindered CO. It is also shown that different conformational substates of the same protein can have different vibrational relaxation rates at the active site and that different substituents on the perimeter of the porphyrin may significantly affect the vibrational relaxation.