posted on 2020-07-17, 21:29authored byPernille
S. Bols, Michel Rickhaus, Lara Tejerina, Henrik Gotfredsen, Kristina Eriksen, Michael Jirasek, Harry L. Anderson
The link between allosteric cooperativity
and template-directed
synthesis has been investigated by studying complexes in which two
oligopyridine ligands bind inside a zinc porphyrin nanoring in a stacked
arrangement. The binding of a 6-porphyrin nanoring to two tridentate
ligands (with s-triazine or benzene cores) occurs
with high negative allosteric cooperativity (α ≈ 10–3–10–4). Formation constants
for 1:1 and 1:2 complexes were determined by UV–vis–NIR
denaturation titration, using pyridine as a competing ligand, and
cooperativity factors were confirmed by NMR spectroscopy. The rate
constants for formation of the 1:1 and 1:2 complexes are approximately
equal, and the negative cooperativity can be attributed to faster
dissociation of the 1:2 complex. These tridentate ligands are not
effective templates for directing the synthesis of the 6-porphyrin
nanoring, in keeping with their negative cooperativity of binding.
In contrast, the binding of a 12-porphyrin nanoring to two hexadentate
ligands occurs with high positive allosteric cooperativity (α
> 40), and the ligand is an effective Vernier template for directing
the synthesis of the 12-porphyrin nanoring. This stacked Vernier template
approach creates the product in an open circular conformation, which
is advantageous for preparing macrocycles that do not easily adopt
a figure-of-eight geometry.