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 differ ent conformational substates of myoglobin, and CO bound to a water-sol uble 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 carboxymyo gIobin (Mb-CO), denoted A(0) and A(1), yielded lifetimes of 26.6 +/- 1 and 18.2 +/- 1 ps in a poly(vinyl alcohol) matrix. In glycerol:water solution, the A(1)-state lifetime of Mb-CO was 17.4 +/- 1 ps. These li fetimes do not depend much on temperature in the 20-300 K range. The L ifetime of the bare Fe:porphyrin was 17 +/- 3 ps. Results obtained on these andother heme-CO systems are used to show that vibrational relax ation is slower with CO whose frequency is close to the similar to 197 0 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 diffe rent conformational substates of the same protein can have different v ibrational relaxation rates at the active site and that different subs tituents on the perimeter of the porphyrin may significantly affect th e vibrational relaxation.