Three new principles for discrimination of organic guests by the membrane potential change are described, which are based on the host-guest recognition of charged groups, hydrogen bonding groups, or steric shapes of nonpolar moieties, (i) Potentiometric discrimination by the recognition of charged groups has been attained by liquid membrane type electrodes containing lipophilic macrocyclic polyamines as the sensory elements. These electrodes displayed potentiometric selectivities for organic anions according to the amount or proximity of negative charges. Thus, the strongest responses were observed for such guests as ATP4- having a large number of negative charges as well as maleate2- and phthalate2-having negative charges at the closest distance within the molecule, (ii) Potentiometric discrimination by the recognition of hydrogen bonding groups has been attained by an electrode containing a lipophilic cytosine-pendant triamine, which functions as a ditopic receptor for guanine nucleotides by an electrostatic interaction with the phosphate group and a complementary base pairing with the guanine base. This electrode displayed nearly all-or-nothing selectivities between guanine and adenine nucleotides. Selective responses to guanine nucleotides were also obtained by double receptor type electrodes containing a macrocyclic tetraamine and a neutral cytosine derivative despite the fact that both of these hosts, when existed separately, showed only weak responses to these guests, (iii) Potentiometric discrimination by the recognition of steric shapes of nonpolar moieties has been attained by an electrode containing a lipophilic hexaester of calixarene having an inclusion cavity for organic guests. At pH 7.0, the strongest responses were obtained for primary amine guests such as 2-phenylethyiamine and 1-octylamine having no bulky substituent at the a-carbon that could sterically hinder the formation of tripodal hydrogen bonds between the NH3 + group of the guest and the ester carbonyl groups of the host. A comparison of potentiometric selectivities with those of the electrode containing dibenzo-18-crown-6, as well as !H-NMR results on the geometry of the host-guest complex, strongly supported a potentiometric shape discrimination effect based on the inclusion of guests within the calixarene cavity having a well-defined structure.
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