Potentiometric responses of solvent polymeric membrane electrodes for nucleotides based on neutral derivatives of cytosine and thymine have been investigated. Nernstian responses to the 5'monophosphates of guanosine (5'GMP) and adenosine (5'AMP) in their divalent forms are obtained when these membranes contain an ionophore and lipophilic cationic sites. Independent interaction of the ionophore and ionic sites with two different functional groups of the analyte ion allows ditopic recognition based on hydrogen bonds and charge-charge interactions. Selectivity for 5'GMP over 5'AMP is obtained with electrodes based on cytosine derivatives while an electrode based on a thymine derivative shows no such discrimination. These electrodes represent the first cases of ion-selective electrodes (ISEs) with Nernstian slopes in which potentiometric selectivities are due to the formation of hydrogen bonds between a neutral ionophore and a neutral moiety of the analyte. Because the emf response slopes are Nernstian, it is possible to interpret the emf responses on the basis of phase boundary equilibria. The potentiometric selectivities seem to be affected by ionophore solvation and self-assembly involving hydrogen bonds as well as by formation of analyte-ionophore complexes of higher stoichiometry, as shown by 13C NMR spectroscopy of PVC membranes and by variation of the ratio between ionophore and added cationic sites.
- Ion-selective electrodes