Investigation of supramolecular stoichiometry and dynamic for inclusion complex of water soluble porphyrin with cucurbit[7]uril by fluorescence correlation spectroscopy

• Tuning of stoichiometry is achieved in fluorescence correlation spectroscopic titration.
• CB[7] and cationic porphyrin forms 1:1 to 4:1 stoichiometric complexes sequentially.
• Equilibrium binding constant and dynamic parameters are measured for each step.
• Higher order complexation is anti-cooperative in nature for the present host–guest system.

Detection and identification of supramolecular binding stoichiometry are trivial by traditional spectroscopic methods such as, UV–vis spectroscopy, fluorescence spectroscopy and NMR spectroscopy for polytopic system like meso-substituted porphyrin where multiple identical binding sites are present. We use fluorescence correlation spectroscopy (FCS) to understand the interaction and supramolecular binding stoichiometry of inclusion (host–guest) complexes between cucurbit[7]urils (CB[7]) as host and water soluble 5,10,15,20 meso-tetrakis (N-methyl4-pyridyl)-21,23H-porphyrin tetratosylate (TMPyP) as guest. Tuning of binding stoichiometry is achieved in FCS titration for these systems by changing concentration of CB[7] from nano-molar (nM) to milli-molar (mM) keeping TMPyP concentration fixed at ∼3 nM. Stepwise formation of 1:1, 2:1, 3:1 and finally 4:1 (CB[7]:TMPyP) complexes are determined. Inclusion dynamic at each step of complex formation was analyzed by monitoring the diffusion properties and the molecular brightness of free TMPyP as well as respective complexes of different order of complexities. Global target analysis of correlation curves for each step yields association equilibrium constant (K) and dissociation rate constant (k−) unveiling the complexation dynamics of the systems in each step. Observed results reveal that the value of association rate constant (k+) remains more or less same on going from 1:1 to 4:1 complexes while k− successively increases on going from 1:1 to 4:1 complexes. Hence, k− is found to be determining factor for evaluating K value in each step of inclusion complex formation for the studied host–guest system. Formation of higher order complexes is found to be anti-cooperative in nature for the present host–guest system.

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