L-tryptophan-induced electron transport across supported lipid bilayers: An alkyl-chain tilt-angle, and bilayer-symmetry dependence

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)64-/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 (ks=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 (ks=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 kapp0, estimated from electrochemical impedance spectroscopy and bipotentiostatic experiments with a Pt ultramicroelectrode. Lower magnitudes of ks and kapp0 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 th0 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. Turn up the transport: Molecular orientation-dependent electron transport across symmetric and asymmetric supported lipid bilayers (SLBs) in the presence of transport-active L-tryptophan is reported (see picture). In spite of tryptophan expediting electron-transfer rates across the SLBs, kinetic limitations effectuate electron tunnelling with a potential-independent tunnelling coefficient β=0.15 Å-1. A novel two-dimensional phase-dependent transport with an insulator- semiconductor transition is highlighted. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.