Abstract
The host-guest complexation between an Alexa 488 labelled adamantane derivative and β-cyclodextrin is studied by Fluorescence Correlation Spectroscopy (FCS). A 1:1 complex stoichiometry and a high association equilibrium constant of K = 5.2 × 104 M−1 are obtained in aqueous solution at 25 °C and pH = 6. The necessary experimental conditions are discussed. FCS proves to be an excellent method for the determination of stoichiometry and association equilibrium constant of this type of complexes, where both host and guest are nonfluorescent and which are therefore not easily amenable to standard fluorescence spectroscopic methods.
Highlights
Supramolecular host-guest chemistry describes the formation of molecular complexes composed of small molecules noncovalently bound to larger molecules in a unique structural relationship [1]
Most technologically interesting host-guest complexes are themselves nonfluorescent and the attachment of a fluorescent label in order to use them in standard fluorescence spectroscopy leads to a dilemma: on one hand, the fluorophore should not interfere in the host-guest complexation under study, but on the other hand a sufficiently strong interaction between fluorophore and host or guest is necessary in order to detect a change in the spectral properties upon complexation
In this contribution we will study by Fluorescence Correlation Spectroscopy (FCS) the stoichiometry and the stability of the inclusion complex formed between the host β-cyclodextrin and the nonfluorescent guest adamantane labelled with Alexa 488 as fluorescent probe
Summary
Supramolecular host-guest chemistry describes the formation of molecular complexes composed of small molecules (guests) noncovalently bound to larger molecules (hosts) in a unique structural relationship [1]. Instead of the change in the spectral properties FCS analyses the variation in the diffusion coefficient of a fluorophore attached to guest or host due to the increase in the molecular weight upon complexation (see Figures 1b,c) The fluorophore itself need not to interact directly with the host-guest complex except for a common diffusive movement This relaxes the conditions imposed on the. We studied recently by FCS hostguest dynamics and determined the fast entry/exit rate constants of fluorescent dyes within cyclodextrins [4,5] In this contribution we will study by FCS the stoichiometry and the stability of the inclusion complex formed between the host β-cyclodextrin (βCD) and the nonfluorescent guest adamantane labelled with Alexa 488 as fluorescent probe (see Figure 2). We discuss the necessary experimental conditions, determine the stoichiometry and the equilibrium constant and compare the results with those published for similar guests obtained with other methods
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