Cooperativity in the self-assembly of porphyrin ladders

Abstract

Cooperativity is a general feature of intermolecular interactions in biomolecular systems, but there are many different facets of the phenomenon that are not well understood. Positive cooperativity stabilizes a system as progressively more interactions are added, and the origin of the beneficial free energy may be entropic or enthalpic in origin. An "enthalpic chelate effect" has been proposed to operate through structural tightening that improves all of the functional group interactions in a complex, when it is more strongly bound. Here, we present direct calorimetric evidence that no such enthalpic effects exist in the cooperative assembly of supramolecular ladder complexes composed of metalloporphyrin oligomers coordinated to bipyridine ligands. The enthalpic contributions of the individual coordination interactions are 35 kJ.mol(-1) and constant over a range of free energies of self-assembly of -35 to -111 kJ.mol(-1). In rigid well defined systems of this type, the enthalpies of individual interactions are additive, and no enthalpic cooperative effects are apparent. The implication is that in more flexible, less well defined systems such as biomolecular assemblies, the enthalpy contributions available from specific functional group interactions are well defined and constant parameters.

Fig. 1.

Cartoon representation of the porphyrin ladder–bipyridine assembly.

Fig. 2.

Synthesis of the porphyrin oligomers.

Fig. 3.

Titration experiments used to characterize the speciations and thermodynamics of assembly. (a) ¹H NMR titration of bipyridine (B) into 2 (Left) and 3 (Right). The downfield shift of the porphyrin β protons reaches a maximum at the ladder stochiometry and then shifts back as excess B induces opening. (b) ITC addition of B to 2 (Left) and 3 (Right). Addition of B results in heat release when the ladder is formed. Further addition of B opens the ladder but does not have a detectable enthalpic effect. (c) Speciation for the ITC experiments in b, calculated from the values of K_ladder and K_open determined by ¹H NMR titration.

Fig. 4.

ΔH per interaction as function of the overall stability of the ladder complex.