Fura-FF, pentapotassium salt [Fura-2FF, pentapotassium salt] *CAS 192140-58-2*

Among the ratiometric calcium indicators, Fura-2 and Indo-1 are most commonly used. Fura-2 is excitation-ratioable while Indo-1 is emission-ratioable. Fura-2 is preferred for ratio-imaging microscopy, in which it is more practical to change excitation wavelengths than emission wavelengths. Upon binding Ca2+, Fura-2 exhibits an absorption shift that can be observed by scanning the excitation spectrum between 300 and 400 nm, while monitoring the emission at ~510 nm. The cell-impermeant Fura-2FF is an analog of Fura-2 with much lower calcium binding affinity, Kd ~10 µM.