title: L-tryptophan-induced electron transport across supported lipid bilayers: an alkyl-chain tilt-angle, and bilayer-symmetry dependence.
abstract: Molecular orientation-dependent electron transport across supported 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers (SLBs) on semiconducting indium tin oxide (ITO) is reported with an aim towards potential nanobiotechnological applications. A bifunctional strategy is adopted to form symmetric and asymmetric bilayers of DPPC that interact with L-tryptophan, and are analyzed by surface manometry and atomic force microscopy. Polarization-dependent real-time Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) analysis of these SLBs reveals electrostatic, hydrogen-bonding, and cation-π interactions between the polar head groups of the lipid and the indole side chains. Consequently, a molecular tilt arises from the effective interface dipole, facilitating electron transport across the ITO-anchored SLBs in the presence of an internal Fe(CN)(6)(4-/3-) redox probe. The incorporation of tryptophan enhances the voltammetric features of the SLBs. The estimated electron-transfer rate constants for symmetric and asymmetric bilayers (k(s) = 2.0×10(-2) and 2.8×10(-2) s(-1)) across the two-dimensional (2D) ordered DPPC/tryptophan SLBs are higher compared to pure DPPC SLBs (k(s) = 3.2×10(-3) and 3.9×10(-3) s(-1)). In addition, they are molecular tilt-dependent, as it is the case with the standard apparent rate constants k(app)(0), estimated from electrochemical impedance spectroscopy and bipotentiostatic experiments with a Pt ultramicroelectrode. Lower magnitudes of k(s) and k(app)(0) imply that electrochemical reactions across the ITO-SLB electrodes are kinetically limited and consequently governed by electron tunneling across the SLBs. Standard theoretical rate constants (k(th)(0)) accrued upon electron tunneling comply with the potential-independent electron-tunneling coefficient β = 0.15 Å(-1). Insulator-semiconductor transitions moving from a liquid-expanded to a condensed 2D-phase state of the SLBs are noted, adding a new dimension to their transport behavior. These results highlight the role of tryptophan in expediting electron transfer across lipid bilayer membranes in a cellular environment and can provide potential clues towards patterned lipid nanocomposites and devices.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India.
authors.forename: Nirod Kumar
authors.initials: NK
authors.lastname: Sarangi
authors.forename: Archita
authors.initials: A
authors.lastname: Patnaik
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D004579:Electron Transport; D004583:Electrons; D006860:Hydrogen Bonding; D007211:Indoles / Q000737:chemistry; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D008055:Lipids / Q000737:chemistry; D018625:Microscopy, Atomic Force / Q000379:methods; D053761:Nanocomposites / Q000737:chemistry; D036103:Nanotechnology / Q000379:methods; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D002412:Cations; D007211:Indoles; D008051:Lipid Bilayers; D008055:Lipids; D015060:1,2-Dipalmitoylphosphatidylcholine; C030374:indole; D014364:Tryptophan