Rational Design of a Near-infrared Fluorescence Probe for Ca²⁺ Based on Phosphorus-substituted Rhodamines Utilizing...

Area of Science:

• Biomedical imaging
• Organic chemistry
• Photophysics

Background:

• Near-infrared (NIR) fluorescence imaging (650-900 nm) offers low background autofluorescence and high tissue penetration.
• NIR fluorescence is valuable for multicolor bioimaging, complementing green and red channels.
• Photoinduced electron transfer (PeT) is a key mechanism influencing fluorescence quenching in rhodamine dyes.

Purpose of the Study:

• To compare the PeT-mediated fluorescence quenching of silicon-substituted rhodamines (SiRs) and phosphorus-substituted rhodamines (PRs).
• To guide the development of advanced far-red to NIR fluorescent dyes.
• To design and synthesize novel NIR fluorescence probes for calcium ion (Ca2+) imaging.

Main Methods:

• Density functional theory (DFT) calculations to understand electronic properties.
• Photophysical evaluation of newly synthesized PRs.
• Synthesis and characterization of a new Ca2+ probe (CaPR-1) and its acetoxymethyl (AM) derivative (CaPR-1 AM).

Main Results:

• PRs demonstrated higher susceptibility to PeT quenching compared to SiRs.
• CaPR-1 AM exhibited distinct cellular distribution (cytosol) compared to the SiR-based probe (CaSiR-1 AM, localized in lysosomes and cytosol).
• CaPR-1 displayed longer-wavelength absorption and emission (up to 712 nm) than CaSiR-1, enabling Ca2+ imaging in neuronal dendrites and spines.

Conclusions:

• Phosphorus-substituted rhodamines offer advantages over silicon-substituted rhodamines for NIR fluorescence applications due to PeT quenching characteristics.
• The developed CaPR-1 probe is a valuable tool for high-resolution Ca2+ imaging in neuroscience, particularly in brain slices.
• This work provides a foundation for creating next-generation NIR fluorescent probes with tailored photophysical properties.