Ion transport mechanisms of bola-amphiphiles in planar bilayer membranes
Date
1997
Authors
Loock, Daniela
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Abstract
The bilayer clamp technique was used as an investigative tool to explore the ion transport activity of bola-amphiphiles across planar lipid bilayers. It was demonstrated that 'small' synthetic molecules can transport ions across lipid bilayer membranes with mechanisms that have not been reported for natural protection channels. In addition to the step conductance changes that are typical for natural ion transporters a variety of signal shapes were observed. The overall transport behaviour could be controlled by influencing specific steps in the mechanism via small structural alterations of the bola-amphiphiles. This approach was analogous to the introduction of point mutations in proteins.
Transporter aggregates were assumed as the ion conducting structure of most bola-amphiphiles investigated. It has been demonstrated that controlling the forces that lead to the stabilization and destabilization of aggregates will lead to control of the observed transport mechanism which was reflected in the signal shapes observed. Head group repulsion, the hydrophilic/hydrophobic balance in the wall units, and the flexibility of the molecule offer possibilities for the regulation of ion transport.
Charge selectivity could be controlled via the introduction of charged head groups and via the degree of hydration of the wall units. 'Dryer' pores led to perfect cation:anion selectivity which in turn gave rise to an unusual signal shape. The development of a local Donnan-potential was invoked in the transport mechanism of these bola-amphiphiles.
Rectifying current-voltage responses were achieved by the introduction cl asymmetry into the mechanism. Two different classes of transporters displayed voltagegated behaviour in planar bilayer membranes. In one case the asymmetry was introduced into the structure of the bola-amphiphile itself. In a second case the unequal distribution of symmetric transporter molecules in the two bilayer leaflets led to the observed non-ohmic current-voltage relationship.
The activity of eleven transporter molecules was studied in planar lipid bilayers and was compared to results previously obtained for these compounds in vesicles. In some cases different conclusions were reached from the two experiments.