Specific Reagent for Cr(III): Imaging Cellular Uptake
of Cr(III) in Hct116 Cells and Theoretical Rationalization
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Abstract
A new rhodamine-based reagent (<b>L</b><sub><b>1</b></sub>), trapped inside the micellar structure
of biologically benign
Triton-X 100, could be used for specific recognition of Cr(III) in
aqueous buffer medium having physiological pH. This visible light
excitable reagent on selective binding to Cr(III) resulted in a strong
fluorescence <i>turn-on</i> response with a maximum at ∼583
nm and tail of that luminescence band extended until 650 nm, an optical
response that is desired for avoiding the cellular autofluorescence.
Interference studies confirm that other metal ions do not interfere
with the detection process of Cr(III) in aqueous buffer medium having
pH 7.2. To examine the nature of binding of Cr(III) to <b>L</b><sub><b>1</b></sub>, various spectroscopic studies are performed
with the model reagent <b>L</b><sub><b>2</b></sub>, which
tend to support Cr(III)-η<sup>2</sup>-olefin π-interactions
involving two olefin bonds in molecular probe <b>L</b><sub><b>1</b></sub>. Computational studies are also performed with another
model reagent <b>L</b><sub><b>M</b></sub> to examine the
possibility of such Cr(III)-η<sup>2</sup>-olefin π-interactions.
Presumably, polar functional groups of the model reagent <b>L</b><sub><b>M</b></sub> upon coordination to the Cr(III) center
effectively reduce the formal charge on the metal ion and this is
further substantiated by results of the theoretical studies. This
assembly is found to be cell membrane permeable and shows insignificant
toxicity toward live colon cancer cells (Hct116). Confocal laser scanning
microscopic studies further revealed that the reagent <b>L</b><sub><b>1</b></sub> could be used as an imaging reagent for
detection of cellular uptake of Cr(III) in pure aqueous buffer medium
by Hct116 cells. Examples of a specific reagent for paramagnetic Cr(III)
with luminescence <i>ON</i> response are scanty in the contemporary
literature. This ligand design helped us in achieving the turn on
response by utilizing the conversion from spirolactam to an acyclic
xanthene form on coordination to Cr(III)