Ratiometric Cu2+ Binding, Cell Imaging, Mitochondrial Targeting, and Anticancer Activity with Nanomolar IC50 by Spiro-Indoline-Conjugated Calix[4]arene

A triazole-derivatized, spiro-indoline-linked, 1,3-di-derivative of calix[4]arene (L) has been synthesized to take advantage of its ion-binding capability in the ring-open form. Indeed, the spiro-indoline moiety is well known for its photochromic, acidochromic, and metallochromic properties. Therefore, the L has been explored for Cu2+ binding, cell imaging, and anticancer activity of the corresponding complex since Cu2+ complexes are known for such activity. The conversion from the closed to open form of L is expedited by light or proton, while the metal ion can open as well as stabilize it. The open form of L showed binding of Cu2+ ratiometrically as demonstrated by absorption and fluorescence spectroscopy. This leads to the formation of 1:1 complex with a binding constant of (6.9 +/- 2.3) x 10(5) M-1, with the lowest detection limit being 1.9 nM. In the complex, the Cu2+ is bound by two triazole-N and two phenolic-O groups resulting in a distorted tetrahedral coordination core of CuN2O2 as demonstrated based on density functional theory studies. To form such coordination core, the arms underwent considerable changes in some of the dihedral angles. The binding of Cu2+ to L induces self-assembly of L by varying from simple particles to rodlike structures when bound to Cu2+. The on-off fluorescence intensity of L and its Cu2+-bound species are responsible for imaging cancer cells. The L shows red fluorescence in MDA-MB-231 cancer cells by targeting mitochondria as proved based on the colocalization study carried out using MitoTracker Green. While the L alone is nontoxic to cancer cells, the presence of Cu2+ brings cell death to an extent of 90% with an IC50 value of 165 nM by bringing a substantial quench in the fluorescence of L. A shift of population from G(0)/G(1) and G(2)M phases to the Sub-G(1) phase was observed as the concentration of the complex was increased, indicating cell death as studied by fluorescence-activated cell sorting. Thus, the present work clearly proved that a calix[4]arene functionalized at the lower rim with spiro-indoline moieities when complexed with Cu2+ acts as an efficient anticancer agent and is capable of imaging cancer cells.