Oxyquinoline-based cation sensors

M-5 (Quin-2) was the first practical fluorescent indicator for cytosolic calcium with a simple 6-methoxyquinoline as its fluorophore. Ca2+-binding increases the fluorescence intensity about six-fold (without spectral displacement, in contrast to Fura 2 see Section 10.3.3). The fluorescence lifetime of Quin-2 is highly sensitive to calcium concentration Quin-2 can thus be used as a probe in the technique of fluorescence lifetime imaging. 

Diaza-18-crown-6 substituted with 5-chloro-8-hydroxyquinoline exhibits very interesting complexing properties. For instance, M-6 is very selective for Ba2+ over other alkaline earth cations and for K+ over Na+ in methanol. Unfortunately, investigation of fluorogenic effects with other cations has not been reported. On the other hand, the fluorescence intensity of M-7 was shown to increase by a factor of 1000 in the presence of Mg2+ (in a mixture of methanol/water 1 1 v/v) whereas other alkaline earth ions have no effect. 

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