Quin-2 binds calcium tightly and resembles calcium chelator EGTA in ability to bind calcium much more tightly than magnesium. Binding of calcium causes large changes in ultraviolet absorption and fluorescence. The wavelengths of light that cause fluorescence when calcium is bound are longer than the wavelengths that cause fluorescence when it is not bound. When excited at two different wavelengths, the ratio of the fluorescence intensities at the two wavelengths gives the ratio of the concentrations of bound to free calcium. Free Quin-2 concentration can be measured precisely, so free calcium concentration can be calculated precisely. Quin-2 may be injected into cells to measure moment-to-moment changes in intracellular calcium concentration. Quin-2 AM is permeable to cells, and used for studying live cells.
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