Long-wavelength chloride-sensitive fluorescent indicators

Area of Science:

• Biochemistry
• Analytical Chemistry
• Organic Chemistry

Background:

• Quinolinium compounds are established fluorescent indicators for chloride ions (Cl).
• There is a need for indicators with improved optical properties and halide sensitivity.
• Long-wavelength fluorescent indicators offer advantages in biological imaging.

Purpose of the Study:

• To develop novel long-wavelength fluorescent Cl indicators with enhanced optical properties.
• To screen quaternized tricyclic heterocycles for Cl indicator potential.
• To evaluate structure-activity relationships of acridinium-based compounds.

Main Methods:

• Screening of quaternized tricyclic heterocycles.
• Synthesis of five 9-substituted N-methylacridinium compounds.
• Measurement of halide sensitivity using Stern-Volmer constants.
• Determination of optical properties including molar absorbance, excitation/emission maxima, and quantum yields.

Main Results:

• N,N-dimethyl-9,9-bisacridinium (lucigenin) demonstrated high halide sensitivity (Stern-Volmer constants up to 750 M-1 for iodide) and was insensitive to other anions.
• N-methylacridinium-9-carboxamide (MACA) showed excellent optical properties with excitation/emission maxima at 424/500 nm and good halide sensitivity.
• Both lucigenin and MACA possess high quantum yields (0.6-0.7).

Conclusions:

• Acridinium compounds, including lucigenin and MACA, are effective fluorescent Cl indicators.
• These indicators show promise for use in liposomes and membrane vesicles.
• Stability in cell cytoplasm remains a limitation for these acridinium-based indicators.