Zinc sensing by a Janus-faced fluorophore and its application in the cellular microenvironment.

In this work, we have investigated the metal sensing capabilities of dimeric fused heterocycles, PQ-BCN and PQN-BCN, bearing a pyrrole-pyridine chelating unit. PQ-BCN displayed selectivity towards Zn2+ in micromolar levels by steady-state absorbance and fluorescence-assisted screening experiments. The binding stoichiometry of the complex was determined to be a 2:1 metal-to-probe ratio, with an association constant (Ka) of 1.7 × 10⁴ M⁻2. PQN-BCN, however, did not show any preferences towards any metal. Computational experiments revealed the preference for PQ-BCN when compared to PQN-BCN, possibly originating from the difference in the Mulliken charges of the respective chelating nitrogens. A lifetime study on the PQ-BCN-Zn2+ complex gave a bi-exponential fit with an enhancement of its slower component upon interaction with Zn2+ when compared to the free probe. It is the slower component that significantly contributes to the enhancement of fluorescence. Cellular studies were further performed with PQ-BCN using MOC-2 (oral cancer cells), which revealed the non-toxic nature of the probe and its capability to monitor zinc in cells in the presence/absence of zinc transporter ZnPT. Visualization of zinc was also successful without its external supplementation when cells were under peroxide-induced oxidative stress, thereby demonstrating the innate capability of the probe to detect zinc.