Derivatization of polyaniline with thiolated phospholipids and biomimetic applications.

摘要

The known intractability of polyaniline (PANI) have presented multiple challenges to improve its solubility and processibility through derivatization. Multiple functionalized PANI products have been made to facilitate the processing of this intrinsic conductive polymer. Such intractability is eased by deprotonation of the PANI emeraldine salt (ES), to the less conductive PANI emeraldine base (EB), which has been proven soluble in several organic solvents. It is precisely the EB form that is the subject of this research, as the chemical structure of this PANI form presents a unique opportunity for derivatization though a concurrent substitution and reduction of its diiminoquinoid rings to diaminobenzenoid rings forming the PANI leucoemeraldine base (LEB) form. The conduction mechanism of PANI is very unique among other intrinsic conductive polymers, as its highly conductive form ES, besides acid doping, it can also be achieved through a concurrent oxidation and protonation of the LEB form that results after substitution on the diiminoquinoid ring.Alkylthiols and alkylamine nucleophiles have been used to derivatize EB, obtaining the LEB reduction product with assembled hydrophobic layer on it, in a similar fashion as thiols self-assemble on gold surfaces. This research started with the derivatization of chemically synthesized EB with several in-house made thiolated phospholipids which were synthesized through multiple and extensive steps. Finally a commercially available thiolated phospholipid product named 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol sodium salt (PTE) was used to speed up the experiments. The completion of the substitution reaction was monitored by using UV-VIS, and FTIR, also a solvent mixture was optimized resulting in the selection of dimethylacetamide(DMAC) as the best solvent for EB, and chloroform as the best for PTE. PANI films supported on glassy carbon (GC) and Platinum (Pt) electrodes were potentiodynamically derivatized with PTE resulting in covalently attached layer of phospholipid. Cyclic voltammetry (CV) results indicated an increase in oxidation potential for the peaks associated with the PTE substitution, and a decrease in oxidation potential for the ES-PNS and PNS-ES transitions indicating that charge became more localized. A change in the oxidation potential values is also linked to the higher electronic density states that occurs due to the substitution in the aromatic ring, and that facilitates the protonation and oxidation of the amine group. Potentiodynamic substitution on PANI films found that 1,2-Dipalmitoyl-sn-Glycero-3-Phosphoethanolamine (DPPE) provided a --NH2 group that was as good of a nucleophile as the --SH in PTE. This was a surprising difference compared to the behavior of alkylamines and alkylthiols as reported by other researchers. PTE substituted PANI films supported on GC electrodes were used to assemble K+ ion specific electrodes based on immobilization of the ionophore valinomycin and the pentadecapeptide gramicidin on the assembled phospholipid layers, thus creating biomimetic lipid membranes. Titration with K+ ions indicated that the assembled biomimetic membrane showed a saturation curve consistent with a facilitated diffusion mechanism. Such mechanism is characterized by following a single substrate binding path in an analogy to Michaelis-Menten enzyme kinetics. Cell membrane has been described as the primary barrier separating the body of living beings from their environment, many researchers are taking on the task of fabricating lipid bilayer membranes based on self-assembled monolayer (SAMS) on conductive polymer that mimic natural cell membranes. This is the first time phospholipids are covalently attached to PANI making assemblies that are promising tools for the electrochemical investigation of membrane proteins in quasi-natural environment.