Concentration of Dopamine by Proton-Driven Uphill Transport Using Lasalocid A as the Carrier

Molecular recognition by receptors, followed by signal transduction, leading to transmembrane signaling such as active transport of a target compound, membrane permeability changes and membrane potential changes, has been found to be useful new principles for producing chemical sensors. 1,2 Our latest examples of such sensors include a Ca2+ ion sensor based on a Ca2+ signaling pathway via calmodulin3 and potentiometric bilayer lipid membrane (BLM) sensors based on a variety of synthetic receptors.4 The uphill (active) transport of target compounds is analytically interesting as a new mode of mass transfer for designing separation and sensing systems with signal amplification. 1256 One of the most interesting features of uphill tansport is, as pointed out earlier6'', that if the volume of the receiving solution is purposely made smaller than that of the feed solution, the concentration of an analyte transported in the receiving solution is significantly augmented. Since the first uphill transport membrane electrode was proposed in 1986', this method has already been demonstrated for several liquid membrane systems'-12 in which synthetic receptors, such as methyltrioctylammonium chloride (Cd2+)5,6, dicyclohexyl-l8-crown-6 (K+)9, dibenzoylmethane (Cu2+)9 and a lipophilic derivative of 14-crown-4 (Li+)12, were used. In the present study, an uphill transport system for dopamine using a naturally occurring ionophore lasalocid A is described in which complete (100%) transfer of dopamine from the feed to receiving solutions is achieved. The results are discussed in terms of factors influencing the extent of uphill transport and its chemical selectivity. Lasalocid A (Fig. 1) is one of naturally occurring ionophores known as polyether antibiotics