A Nonconventional Approach to Supramolecular Formation Dynamics. The Kinetics of Assembly of DNA-Bound Porphyrins

The kinetics of formation of organized assemblies of trans -bis( N -methylpyridinium-4-yl)diphenylporphine (t-H 2 P agg ) on the surface of calf thymus DNA has been studied via stopped-flow techniques. The reactions show a complicated kinetic profile at both 422 (reactant peak) and 450 nm (product peak), beginning with an apparent induction period followed by a rapid color change whose rate depends on the initial conditions of concentration and ionic strength. The kinetic data can be fit with a closed-form integrated rate law involving four kinetic parameters. A theoretical basis for the form of the integrated rate law is offered in which the formation of an aggregation nucleus is rate determining, a step that is catalyzed by the fractal array of porphyrins produced through the reaction. The process is thus considered to be autocatalytic with two of the parameters ( k o and k c ) representing the rate constants for the noncatalytic and catalytic pathways, respectively. The remaining parameters are related to the size of the aggregation nucleus ( m ) and the growth rate of the catalytic array ( n ). The dependence of each of these kinetic terms on drug load and salt concentration is described.