Towards ionic signal propagation




Sutherland, Todd

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The components necessary to propagate a synthetic ionic signal are described, and experiments leading to the required experimental system are the focus of this work. Two thiol-derivatized fluorescent probe molecules were synthesized that balanced both electrochemical and fluorescent properties necessary for trace analysis. Self-assembled monolayers (SAMs) of 11-(1-1’-biphenyl-4-yloxy)-1-undecanethiol were formed on Au/glass slides by open-circuit incubation and potential-assisted adsorption methods. A potentiostat was built capable of producing current responses on the microsecond time-scale. Monolayer integrity was established by two methods: cyclic voltammetry and chronoamperometry. Monolayers formed under potential-assisted adsorption conditions showed attenuation of the peak current due to Fe(CN)6 3−/4− redox probe in cyclic voltammetry, indicating a tightly packed monolayer. Chronoamperometric studies also confirmed the monolayer integrity by fitting the current response of a potential-step to an equivalent circuit. The chronoamperometric study was dependent on solvent and electrolyte. In water, the difference between bare Au and monolayer protected Au was large, whereas in DMF, the difference was negligible. Likewise, the use of tetra-butyl ammonium hexafluorophosphate as the electrolyte showed little difference between bare Au and monolayer protected Au. The electrochemical reduction of the SAMs was done in various solvents and electrolytes and the products were analysed by HPLC with fluorescent detection. Along the series of solvents from water to MeCN to DMF the current efficiencies for release increased but still were very low. In water and MeCN, the thiol was the sole detectable product, while in DMF, the sole detected product was the disulfide. Reproducibility of release was poor in MeCN and water, probably due to the low solubility of the thiol. Single-channel analysis of two acyclic bola-amphiphiles (diester and diamide) was done to establish their feasibility as components of a synthetic signal propagation system. Channels from the diester derivative have a Na+ conductance of 10.2 pS and a Cs+ conductance of 39.3 pS. Channels from the diester have a Cs+/Na+ permeability ratio of 4.7, Cs+/Cl- permeability ratio of 7.5 and a Na+/Cl- permeability ratio of 3.1. Channels of the diester bola-amphiphile have two lifetimes; 117 ms and 842 ms at -100 mV, 1 M CsCl electrolyte and DiPhyPC lipid at 25 °C. Similarly, Channels from the diamide derivative have a Na+ conductance of 10.3 pS and a Cs+ conductance of 38.9 pS. Chaimels of the diamide have a Cs+/Na+ permeability ratio of 5.2, Cs+/Cl- permeability ratio of 7.2 and a Na+/Cl- permeability ratio of 2.1. The diamide bola-amphiphile channels have a lifetime of 277 ms at +100 mV, 1 M CsCl electrolyte and DiPhyPC lipid at 25 °C. Both channels show a regular non-uniform step-conductance pattern. The sublevel openings, when graphically represented with lifetime data, show the trend that the lower conductance states of one-level openings are also the shorter-lived channels. A traceless linker to release alcohols from a gold surface was developed. Thiobutyric acid was found to undergo intramolecular thiolactone formation after electrochemical reduction from an Au-electrode to liberate the alcohol. A thiobutyric ester at the C-terminus of gramicidin was synthesized. This compound released gramicidin by chemical reduction with DTT as seen by HPLC analysis and MALDI TOF MS. The electrochemical release of the Au-immobilized thiobutyric ester of gramicidin adjacent to a lipid bilayer, as monitored by bilayer clamp technique, produced an increase in channel activity that is consistent with incorporation of gramicidin.



Ion channels, Signal propagation, Self-assembled monolayers, Ionic signal