Development of cryptand molecules of multiple ligation with allosteric binding behavior

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This work aimed at understanding the mechanistic aspect in the novel binding mode of cryptand-like porphyrinoids and also the structural factors that enables allosteric binding. A number of the cryptand-like porphyrinoids made of six pyrroles and three six-membered(hetero) aromatics were prepared and their multiple binding behavior of guest molecules was investigated. The cryptands containing three and two pyridines bind three and two carboxylic acids, respectively, where ideal positive homotropic allosteric binding behavior was observed through the titration experiments in CH_2Cl_2. The intermediate complexes of a one-to-one or a one-to-two host-guest ratio were observed in acetonitrile to show that allosteric binding based on multiple hydrogen bondings is unfavorable in polar solvents. Three cryptands containing alkyl substituents of a different steric bulk at the pyrrole.-positions were used to study binding behavior of alcohols. VT-NMR experiments on the dry CDCl_3 solution of a mixture of cryptand and alcohol revealed positive allosteric binding behavior. A bulky iso-butyl substituent next to the pyridine increased equilibrium constants. Cryptand with the cyano substituents at the bridgehead greatly improved the stability of the cryptand-like porphyrinoid, which expands the scope of allosteric binding.