Contrasting complexation behaviour of zwitterionic amyloid probe, SYPRO orange with β-cyclodextrin and captisol

Abstract

The differential complexation studies with cyclodextrins and their charged derivatives have been well studied using cationic, anionic, as well as neutral guest molecules. However, supramolecular complexation with zwitterionic guest molecules is quite limited. Zwitterions are overall neutral molecules with localized charges that can offer additional site-specific electrostatic interaction with cyclodextrins as compared to neutral guest molecules. There is no information on how the different intra- and intermolecular interactions can modulate the binding of a zwitterion with a macrocyclic host. Herein, the supramolecular interactions of biologically important zwitterionic amyloid probe, SYPRO Orange (SO) with β-cyclodextrin (β-CD) and sulfobutylether β-cyclodextrin (SBE-β-CD, Captisol®) have been evaluated and compared using different spectroscopic methods. SO forms inclusion complexes with both β-CD and SBE-β-CD which led to the enhanced fluorescence along with a hypsochromic shift in its emission spectra. It has been shown that the presence of segmental rotation in a guest molecule with charge transfer property favours its association with the macromolecules. The fluorescence enhancement and the binding affinities show contrasting behavior in these two types of host–guest complexes. Time-resolved emission measurements could disentangle the contribution from different inclusion complexes. Ionic strength and temperature variation in the photophysical properties helped to understand the molecular interactions involved in such contrasting behavior.