Laser Photolysis Investigations of Ligand Binding With Models of the Active Site of Respiratory Hemoproteins: Kinetic and Thermodynamic Aspects

During the past 15 years, laser photolysis has been the method of choice for probing the complex reaction kinetics of respiratory proteins. In an attempt to determine the structural parameters which govern their reactivity, synthetic heme model compounds capable of simulating particular aspects of the reactivity of the active site of hemoproteins have been successively proposed. Laser photolysis of heme compounds merely induces a reversible photodissociation of one ligand at a time. This is equivalent to performing a fast concentration jump "in situ" and provides a powerful, fast and "clean" chemical relaxation technique. To gather association and dissociation rate constants of various ligands (O2, CO, nitrogenous bases) special methods have been developed or adapted. The problem of comparing and classifying a large number of collected data has been greatly simplified by introducing a Linear Free Energy Relationships formalism. In the first part of this paper, some of the methods and concepts which have emerged from several years of investigations of heme proteins and heme models and which are of a sufficient generality to be useful in other fields of chemical kinetics are reviewed. In the second part of the paper we present the application of the preceding methods to a kinetic study of a series of heme models which were specifically designed to investigate the important problem of H-bonding as a stabilizing factor of the oxygenated heme model and hemoprotein complexes.