Fluo-4, Pentapotassium Salt

Fluo-4 is an analog of Fluo-3 with the two chlorine substituents replaced by fluorines, which results in increased fluorescence excitation at 488 nm that gives higher fluorescence signal. Fluo-4 has its absorption maximum at 494 nm, thus making it excitable by the argon-ion laser. Fluo-4 is essentially nonfluorescent without Ca² present, but the fluorescence increases at least 100 times on Ca² binding. Also, because Fluo-4 binds Ca² more weakly (higher Kd) than do fura-2 and indo-1, it is more useful for measuring high transient Ca² concentration during Ca2 spikes. Fluo-4, pentapotassium is membrane-impermeant, but can be loaded into cells via microinjection or scrape loading. We also cell a membrane-permeant version, Fluo-4 AM Ester.

• K_d: ca.335 nM
• &lamba; _{Ex/&lamba; Em (low or high [Ca² ]) = 494nm/516nm}
• ε (494 nm) = 82, 000 M^-1cm^-1
• Orange solid soluble in DMSO and water (pH > 6)
• Store at 4C. Protect from light, especially when in solution
• C₃₆H_25F2K₅N₂O₁₃
• MW: 927.09