The escape process of carbon monoxide from myoglobin to solution at physiological temperature

The carbon monoxide (CO) docking sites involved in the ligand escape process from the iron atom in hem of myoglobin (Mb) to solution at physiological temperature were studied on the basis of the effect of xenon (Xe) on the ligand escape rate by the transient grating (TG) technique. The TG method provides a direct measurement of the changes in molecular volume. The apparent CO escaping rate and the volume contraction increase with increasing Xe pressure. The pressure dependence of the rate is consistent with that of the Xe population at the Xe(1) site. This result clearly shows that CO is trapped at the Xe(1) site before escaping to solvent in a Xe-free solution at room temperature. It is shown that only CO but not the trapped Xe is released by the photoexcitation of the Xe-trapped MbCO. A dissociation scheme is proposed to explain the enhancement of the escaping rate by the presence of Xe(1). There are two branches for the CO escaping pathway. The dominant part of the dissociated CO escapes to the solvent through the Xe(1) trapping site under the Xe-free condition, and there are at least three intermediate states along this pathway. When a Xe atom blocks the Xe(1) site, the CO escapes through another route.