A NEW F-19 NMR INDICATOR FOR INTRACELLULAR SODIUM

The effect of increased benzo substitution on the cation-binding stren gth of diaza-18-crown-6 has been investigated. The planarity constrain t imposed by increased substitution reduces the effective cavity size thus giving rise to an increased selectivity for sodium over potassium , magnesium and calcium that is sufficient to provide a class of indic ators suitable for measuring intracellular free sodium. The cation bin ding parameters and the associated spectral changes of a selection of compounds with varied aryl fluorination patterns have shown that there is very little electronic interaction between the aromatic rings and the bound cation through the heteroatoms of the crown ether. Only smal l adjustments to the cation affinity can therefore be achieved via alt eration of the aryl substitution patterns. Aryl substitution adjacent to the hetero atoms gives a reduction in the cation affinity of the tr ibenzo-crown ether, similar to that found on fluorination of the calci um chelator BAPTA ortho to the oxyethylene bridge. These observations have led to the preparation of a new indicator for intracellular sodiu m that we have called FCrown-1. The new indicator has a dissociation c onstant for sodium of 11 mmol dm-3 and a selectivity for sodium over p otassium of 1 73-fold. The sodium concentration is reported by the che mical shift (fast exchange) of one of the two types of fluorine presen t with a maximum sodium-induced shift of 4.6 ppm downfield. The resona nce arising from the other fluorine substituents is insensitive to sod ium and acts as an internal chemical shift reference at 8.8 to 13.4 pp m upfield from the reporting signal.