Dissipative Assembly of an Ion Transport System




Vu, Paul

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This thesis describes the development of an ion channel system exhibiting dissipative assembly characteristics. In this system an active transporter based on an oligoester fragment terminated in a thioester of 6-aminohexanoic acid (HO2C-Hex-Adip-OctS-Hex-NH2) undergoes thioester cleavage to form a thiol terminated oligoester (HO2C-Hex-ADip-Oct-SH). This fragment was expected to be inactive for ion transport but previous work showed high activity in planar bilayer experiments. In this work, the high activity was shown to be due to the oxidized form of the thiol, the disulfide HO2C-ADip-Oct-SS-Oct-ADip-Hex-CO2H. Air oxidation was found to be quite rapid for the thiol based on ESI-MS (negative ion) and HPLC analysis. Under anaerobic conditions, the thiol fragment was an inactive species for ion transport. In situ air oxidation initiated transport activity associated with the disulfide. The transporter HO2C-Hex-Adip-Oct-Hex-NH2 was active in planar bilayer experiments and was compared to the disulfide via activity grids. The activity of these two compounds was shown to be distinct from each other by conductance and channel duration differences. The activity of HO2C-Hex-Adip-Oct-Hex-NH2 was shown to die off in a period of 30 minutes at pH 8.2. Techniques were developed to stimulate and monitor activity and bilayer quality so that an inactive condition could be confirmed. The addition of Pr-S-Hex-NH3+-Cl as a fuel was shown to extend the activity of HO2C-Hex-Adip-Oct-Hex-NH2 by eight-fold in 1M CsCl electrolyte. Previous work had established the capability of thioester exchange reactions by a reaction between Pr-S-Hex-NH3+-Cl and benzyl thiol in a homogenous solution. The extended activity indicated that the same process may occur in a heterogeneous bilayer system. An inactive system created by the die-off in activity of HO2C-Hex-Adip-Oct-S-Hex-NH2 was treated with Pr-S-Hex-NH3+-Cl to regenerate activity. This cycle could be repeated once the activity died off again. All these findings are consistent with the dissipative assembly of a membrane transport system.



Dissipative, Assembly, Ion Transport, Ion Channel, Synthetic Channel