Fura-2 is a synthetic, ratiometric fluorescent dye used for the quantitative measurement of intracellular calcium ion (Ca²⁺) concentrations. It belongs to the family of benzofuran-based calcium indicators developed by Roger Y. Tsien and colleagues in the early 1980s. The compound is commonly employed in cell biology, neurobiology, and pharmacology to monitor calcium signaling dynamics in living cells and tissues.
Chemical structure and properties
- IUPAC name: 5-(6-methoxy-8-quinolyl)-1,2-bis(2‑amino‑5‑carboxypyridyl)oxadiazole.
- Molecular formula: C₂₆H₂₂N₆O₆.
- Molecular weight: 576.58 g·mol⁻¹.
- Spectral characteristics: In its calcium‑free form, Fura-2 exhibits an excitation maximum near 380 nm and emits fluorescence around 510 nm. Upon binding Ca²⁺, the excitation peak shifts to approximately 340 nm while the emission maximum remains near 510 nm. The ratio of fluorescence intensities at the two excitation wavelengths (F₃₄₀/F₃₈₀) provides a calibration‑independent measure of intracellular Ca²⁺ concentration.
Mechanism of action
Fura-2 contains a high‑affinity calcium‑binding chelating moiety (BAPTA derivative). Binding of a calcium ion induces a conformational change that alters the electronic environment of the fluorophore, resulting in the characteristic shift in excitation spectra. The dissociation constant (K_d) for Ca²⁺ is approximately 145 nM at physiological temperature and ionic strength, allowing detection of Ca²⁺ fluctuations in the low nanomolar to micromolar range.
Cellular loading
The dye is membrane‑impermeant in its free acid form. To introduce Fura-2 into cells, the acetoxymethyl ester derivative (Fura-2 AM) is used. Fura-2 AM diffuses across the plasma membrane; intracellular esterases cleave the AM groups, trapping the charged Fura-2 dye within the cytosol. Hydrolysis efficiency and compartmentalization can vary among cell types, and residual AM‑derived fluorescence may affect background signals.
Applications
| Field | Typical use |
|---|---|
| Neurophysiology | Monitoring Ca²⁺ transients during synaptic transmission, action potential firing, and calcium store release. |
| Cardiology | Assessing Ca²⁺ handling in cardiomyocytes and heart tissue slices. |
| Pharmacology | Evaluating the effect of drugs on Ca²⁺ channels, pumps, and receptors. |
| Cell signaling research | Mapping spatial and temporal patterns of calcium signaling in various cell lines and primary cultures. |
| High‑throughput screening | Adapted to plate‑reader formats for compound libraries affecting calcium pathways. |
Advantages
- Ratiometric measurement reduces artifacts from dye concentration, photobleaching, and uneven loading.
- High sensitivity to physiologically relevant Ca²⁺ changes.
- Well‑characterized calibration protocols exist for converting fluorescence ratios to absolute Ca²⁺ concentrations.
Limitations
- Requires UV excitation (340/380 nm), which can cause phototoxicity and necessitates specialized optical equipment.
- The dye may be sequestered into intracellular organelles (e.g., endoplasmic reticulum, mitochondria), complicating cytosolic measurements.
- Esterase activity variability can affect loading efficiency.
- Fluorescence can be quenched by high concentrations of protein or certain metal ions (e.g., Mg²⁺, Zn²⁺) that partially interfere with calcium binding.
Historical context
Fura-2 was first reported in a 1985 publication by Tsien and co‑workers as a novel calcium indicator that offered ratiometric capabilities superior to earlier single‑wavelength dyes such as Quin-2 and fura‑1. Its development contributed to the widespread adoption of fluorescence imaging for calcium dynamics and earned Tsien a share of the 2008 Nobel Prize in Chemistry.
Safety and handling
Fura-2 and its AM ester are handled as hazardous chemicals. Standard laboratory safety procedures include the use of gloves, protective eyewear, and a fume hood when preparing stock solutions. The dye is light‑sensitive; stock solutions are typically stored in amber vials at –20 °C, protected from repeated freeze‑thaw cycles.
References
- Tsien, R. Y.; Pozzan, T. (1997). "Measurement of cytosolic free calcium with quin2 and fura-2". Cold Spring Harbor Symposia on Quantitative Biology. 62: 65‑71.
- Grynkiewicz, G.; Poenie, M.; Tsien, R. Y. (1985). "A new generation of Ca²⁺ indicators with greatly improved fluorescence properties". Journal of Biological Chemistry. 260 (6): 3440‑3450.
- Haugland, R. P. (2005). Handbook of Fluorescent Probes and Imaging. 10th ed. Molecular Probes.
This article provides an overview of the chemical, functional, and experimental aspects of Fura-2 as a widely used calcium indicator.
