HYDROGEN-BOND BASED RECOGNITION OF NUCLEOTIDES BY NEUTRAL-CARRIER ION-SELECTIVE ELECTRODES

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 guan osine (5'GMP) and adenosine (5'AMP) in their divalent forms are obtain ed when these membranes contain an ionophore and lipophilic cationic s ites. Independent interaction of the ionophore and ionic sites with tw o different functional groups of the analyte ion allows ditopic recogn ition based on hydrogen bonds and charge-charge interactions. Selectiv ity 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 io n-selective electrodes (ISEs) with Nernstian slopes in which potentiom etric 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 potentiometr ic selectivities seem to be affected by ionophore solvation and self-a ssembly involving hydrogen bonds as well as by formation of analyte-io nophore complexes of higher stoichiometry, as shown by C-13 NMR spectr oscopy of PVC membranes and by variation of the ratio between ionophor e and added cationic sites.