Facilitated catecholamine transport through bulk and polymer-supported liquid membranes

A series of crown boronic acids, 1-4, were synthesized and studied as carriers for catecholamine transport through bulk liquid membranes (BLMs) and supported liquid membranes (SLMs). Carrier 1 greatly facilitated the transport of primary catecholamines through BLMs; whereas, the more lipophilic analogues 3 and 4 were less effective. A combination of kinetic, mass spectral, and NMR evidence suggests that the transported species in BLMs is the cyclic, zwitterionic, 1:1 complex 7. The SLM transport studies used a liquid membrane of 2-nitrophenyl octyl ether supported by a thin, flat sheet of porous polypropylene. In the absence of carrier there was neglible dopamine transport (<5 × 10^-9 mol/m²·s) at pH 7.2. When the membrane contained carrier 3 (33 mM or about 2% wt), facilitated catecholamine transport was observed in the order of dopamine (5 × 10^-7 mol/m²·s) > epinephrine (1.5 × 10-7 mol/m2·s) > norepinephrine (0.8 × 10^-7 mol/m²·s). SLMs containing carrier 3 were stable, implying that carrier 3 is a very good candidate for transport mechanism studies. Crown boronic acid 4 was an even better transport carrier of primary catecholamines with a transport order of norepinephrine (4.7 × 10^-6 mol/m²·s) > dopamine (3.5 × 10^-6 mol/m²·s) epinephrine (3 × 10^-8 mol/m²·s). It is 10 times more effective than an equimolar mixture of boronic acid 5 and crown 6, which is one of best examples of ditopic cooperativity yet observed in SLM transport. SLMs containing 4, however, did not exibit long-term stability. Overall, it is possible that a device based on SLMs containing crown boronic acid carriers can be developed to selectively extract catecholamines from clinical samples.