Supramolecular assemblies, stability constants

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

As a reference, the NA hydrolysis catalyzed by Zn2+—cyclen 2b has been determined in the presence of 10 mM Triton X-100 by the same method in 10% (v/v) CH3CN aqueous solution (pH 9.2 with 20 mM CHES buffer) with / = 0.10 (NaN03) at 25°C. The rate constant of 4.6 X 10-2 M-1 sec-1 is smaller than that of 0.10 M-1 sec-1 for the same reaction in the absence of Triton X-100. Provided that only the NA partitioned in aqueous phase was subjected to attack by the hydrophilic 2b, it can be estimated that approximately 50% of NA is partitioned in the micellar phase, since the NA value in the presence of Triton X-100 was decreased by half. The comparison of these NA values (5.0 vs. 4.6 X 10 2M-1 sec-1) clearly shows that the lipophilic cyclen complex lib is a 100-fold better catalyst in a micellar system. In the micellar phase, the lipophil-icity of both the substrate NA and catalyst lib would contribute to stabilization of the supramolecular assembly, leading to more frequent collisions between them. 

Pyridyl ligands are certainly candidates for forming stable self-complementary coordination assemblies and are conventionally employed for supramolecular structure formation. Zn and Mg complexes of meso- and b-pyr-role-substituted 2-pyridylporphyrins (4a-c) were therefore prepared [37,38]. Crystallographic analysis was used to verify that the required slipped cofacial arrangement had been generated, and demonstrated the overlapping of one of the pyrrolic rings in each porphyrin. This confirmed the structural similarity between the sample and the special pair. The stability constant was reported to be 5 x 10 for the Mg dimer of the meso-pyridyl case 4b. 

A particularly intriguing kind of supramolecular self-assembly by axial coordination to metaUoporphyrins, namely self-complementary coordination, is reviewed in the second chapter by Yoshiaki Kobuke. Self-complementarity affords large stability constants. The fascinating systems described in Kobuke s contribution were prepared and investigated as models for components of photosynthetic natural systems self-complementary dimers of porphyrins mimic the special pair of the photosynthetic reaction centers, while macrocychc and three-dimensional porphyrin supramolecules were prepared as photosynthetic antenna models. 

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