Dynamics under confinement: torsional dynamics of Auramine O in a nanocavity

The effect of confinement on the ultrafast torsional relaxation dynamics of a well-known ultrafast molecular rotor (UMR) and a recently reported amyloid fibril sensor, Auramine O (AuO), is investigated inside the nanocavity of a novel cyclodextrin derivative, sulphobutylether beta-cyclodextrin (SBE-beta-CD), using sub-pisosecond fluorescence upconversion spectroscopy. The nanocavity of SBE-beta-CD induces a significant increase in the emission intensity and results in a slower transient decay trace of Auramine O when compared to the native beta-CD. Detailed analysis of the time-resolved emission spectral features shows that the time-dependent changes in both the mean frequency and the width of the emission spectra are considerably slower as compared to bulk water and native beta-CD, which suggests that the excited state torsional dynamics of AuO has been significantly affected in the nanocavity of SBE-beta-CD. This effect on the torsional dynamics has been attributed to the perturbation of the water structure inside the nanocavity of SBE-beta-CD which suppresses the ability of fast collective solvent reorientation motion to promote the excited-state torsional relaxation of Auramine O. The effect of ionic strength of the medium is invoked to analyze the contribution of electrostatic interactions towards the binding of AuO with SBE-beta-CD and is corroborated well by computation of electrostatic potentials for the host molecules. The results also suggest that the hydrophobic interaction provided by the SBE-beta-CD is larger than the native beta-CD.