Simulation of carboxymyoglobin photodissociation

Computer-generated models of the structures of deoxymyoglobin and CO-bound myoglobin show that the CO molecule is not packed so tightly by the amino acid residues of the distal pocket to prevent its motion away from the iron atom upon photodissociation. A simulation of low temperature photodissociation by energy minimization techniques shows that the CO moves to a position approximately 4 A away from the iron atom due to the van der Waals forces. The final position of the CO molecule requires far larger motion of the carbon atom than the oxygen atom and thereby suggests that the isotope dependence of the molecular tunneling is a consequence of the orientation of the photodissociated CO.