Biomimetic Membranes as a Tool to Study Competitive Ion-Exchange Processes on Biologically Active Sites

The change in membrane potential with time is of fundamental importance in cell biology. From the biological point of view we are interested in the mechanism of voltage dependent channel block and related ionic antagonism that happens on the ion-binding sites forming channel necks (Migdalski at al., 2003; Paczosa at al., 2004; Paczosa-Bator at al., 2006). We argue that by applying biomimetic approach, the processes invisible in routine membrane research could be “amplified” and exposed for further scientific exploration. In our case, this argument refers to electrical potential transients and/or local concentration redistributions provoked a competitive calcium/magnesium or potassium/sodium/lithium ions exchange on the biological sites. Voltage-activation of the N-methyl-d-aspartate (NMDA) receptor channel, allowing for calcium ion influx by relieving the block by magnesium ion (Nowak at al., 1984; McBain at al., 1994), or monovalent ion effects such as potassium-sodium/ lithium/TEA(tetraethylammonium) in the case of potassium and sodium channels (Hille, 1992) is used to illustrate the value of biomimetic methodology. From the electrochemical point of view, our strategy means an interest in the timedependent (dynamic) characteristics of a membrane potential resulting from competitive ion-exchange processes. The membranes used in our studies are in electrochemistry known as the electroactive parts of ion-selective sensors sensitive for magnesium, calcium, potassium, sodium and lithium, which are the ions of our interest. To bridge mentioned above biological and electrochemical interests we use biomimetic membranes. The novelty of our approach is in applying conductive polymers (CPs) as with purposely dispersed bioactive sites. This allows observation of a competitive (antagonistic) ion exchange and its coupling with a membrane potential formation process on biologically active sites (BL). The sites in focus of our research, adenosinotriphosphate (ATP), adenosinodiphosphate (ADP), heparin (Hep) and two amino acids – asparagine (Asn) and glutamine (Gln), competitively bind calcium, magnesium, lithium, sodium and potassium ions and thus play an important role in ion-dependent biological membrane processes (Saris