pH assisted modulation in the binding affinity for BODIPY-benzimidazole conjugate with anionic cyclodextrin

Modulation in the properties of chromophoric guest dyes involving non-covalent supramolecular interactions of various macrocyclic hosts are the intense topics of current research. In this study we report interaction of a sulfobutylether-beta-cyclodextrin (SBE beta CD) host in modulating the photophysical as well as prototropic properties of an inhouse synthesized BODIPY-benzimidazole conjugate dye, abbreviated as BDZ. The SBE beta CD host displays substantially different binding strengths for the protonated and unprotonated forms of the prototropic BDZ dye, inducing a large upward pKa shift for the dye. Furthermore, structural confinement of the dye upon binding with the host leads to substantial enhancement in the fluorescence intensity and lifetime for both the prototropic forms of the dye. This can be attributted to the reduction in the non-radiative deexcitation process for the bound dye. Fluorescence enhancements are also accompanied with large increase in fluorescence anisotropy decay times, substantiating dye-host binding. Since unprotonated BDZ in free state is extremely weak in emission due to its intramolecular charge transfer (ICT) character, its fluorescence enhancement on binding to SBE beta CD is found to be more pronounced than protonated BDZ. Importantly, emission of BDZ-SBE beta CD system appears in the green region of visible spectrum, implying the immense prospect of the system for fluorescence-based sensor and imaging applications in different biological studies.

Automated summary

This study investigates the modulation of photophysical and prototropic properties of the BDZ dye by the sulfobutylether-beta-cyclodextrin (SBE beta CD) host. The SBE beta CD host demonstrates different binding strengths for the protonated and unprotonated forms of the BDZ dye, resulting in a significant upward pKa shift. Furthermore, the dye's structure is constrained upon binding with the host, leading to a substantial enhancement in fluorescence intensity and lifetime, as well as an increase in fluorescence anisotropy decay times, confirming the binding between the dye and host.