Deep-Red-Fluorescent Zinc Probe with a Membrane-Targeting Cholesterol Unit

Area of Science:

• Chemical Biology
• Biophysical Chemistry
• Cell Biology

Background:

• Organelle-targeting fluorescence probes are crucial for studying analyte trafficking with spatiotemporal resolution.
• Low-energy emission signals minimize autofluorescence, improving signal clarity.
• Deep-red fluorescent probes offer advantages for biological imaging due to reduced background noise.

Purpose of the Study:

• To design and synthesize a novel deep-red fluorescent zinc probe (JJ) with a membrane-targeting cholesterol unit.
• To investigate the probe's fluorescence response to zinc ions and its mechanism of action.
• To evaluate the probe's utility in cellular imaging and its effect on cell membrane integrity.

Main Methods:

• Synthesis of a boron-azadipyrromethene (aza-BODIPY) based zinc probe (JJ) incorporating a tri(ethylene glycol)-cholesterol chain.
• Spectroscopic analysis (fluorescence emission, excitation) in aqueous solutions at pH 7.4.
• Electrochemical and picosecond transient photoluminescence studies to elucidate the fluorescence turn-on mechanism.
• Cellular imaging studies using HeLa cells, colocalization with organelle-specific stains, and membrane integrity assays.

Main Results:

• The synthesized probe JJ exhibits weak fluorescence in aqueous solution, with a significant (approx. 5-fold) fluorescence enhancement upon addition of ZnCl2.
• The fluorescence turn-on mechanism involves the zinc-induced abrogation of a nonemissive intramolecularly charge-separated species.
• The probe demonstrates high selectivity and sensitivity for zinc ions (dissociation constant of 4 pM), with a reversible response unaffected by pH or other biological metal ions.
• JJ is non-toxic to HeLa cells at submicromolar concentrations and rapidly internalizes into lysosomes and endoplasmic reticula.
• Unexpectedly, JJ internalization permeabilizes the cell membrane, facilitating exogenous ion influx, an effect not observed with a control probe lacking the cholesterol unit.

Conclusions:

• The developed deep-red fluorescent zinc probe (JJ) is effective for sensing intracellular zinc ions.
• The membrane-targeting cholesterol unit in JJ disrupts cell membrane integrity, a novel observation with implications for drug delivery and cellular studies.
• JJ's ability to permeabilize membranes warrants further investigation for its potential applications and underlying mechanisms.