calcein

Calcein, also known as fluorexon, fluorescein complex, is a fluorescent dye with excitation and emission wavelengths of 495 and 515 nm, respectively, and has the appearance of orange crystals. Calcein self-quenches at concentrations above 70 mM and is commonly used as an indicator of lipid vesicle leakage. It has also been traditionally used as a complexometric indicator for titration of calcium ions with EDTA, and for fluorometric determination of calcium.

==Applications== thumbnail|After Calcein-AM is taken up into the cell, it is converted by esterases into calcein (below). This is capable of complexing calcium ions, resulting in a green fluorescence. Since only living cells possess sufficient esterases, only live cells fluoresce green after excitation The non-fluorescent acetomethoxy derivate of calcein (calcein AM, AM = acetoxymethyl) is used in biology as it can be transported through the cellular membrane into live cells, which makes it useful for testing of cell viability and for short-term labeling of cells. Alternatively, Fura-2, Furaptra, Indo-1 and aequorin may be used. An acetomethoxy group obscures the part of the molecule that chelates Ca2+, Mg2+, Zn2+ and other ions. After transport into the cells, intracellular esterases remove the acetomethoxy group, the molecule gets trapped inside and gives out strong green fluorescence. As dead cells lack active esterases, only live cells are labeled and counted by flow cytometry.