DOI: 10.1002/cphc.201200655 l-Tryptophan-Induced Electron Transport across Supported Lipid Bilayers: an Alkyl-Chain Tilt-Angle, and Bilayer- Symmetry Dependence Nirod Kumar Sarangi and Archita Patnaik* [a] 1. Introduction Modification of solid surfaces using biomolecules in controlled processes could serve as a means of constructing model bio- logical reaction media and developing new nanodevices and biosensor platforms. [1] Lipid bilayer membranes deposited on solid supports are artificial cell membranes connoted as sup- ported lipid bilayers (SLBs) that could be effective cell-mem- brane-mimicking model systems in vitro. The lipid membranes retain their fluidity on solid substrates due to the presence of a ~ 1 nm thick water layer between them and serve as model- cell membrane systems. [2] The adsorption of lipid vesicles on hydrophilic solid substrates leads to the formation of planar extended bilayers through fusion and rupture processes. [3–6] Langmuir—Blodgett (LB) [7–9] and Langmuir–Schaefer [10] meth- ods have been proved to be invaluable towards the prepara- tion of SLBs. Besides these, alternative SLBs could be achieved from spin-coated [11] films in organic solvents followed by the hydration with appropriate buffers. The stepwise formation process was interpreted in terms of physical parameters, such as the membrane curvature, surface tension, and interfacial energy. [12] Asymmetric hybrid bilayers could be achieved on hy- drophobic self-assembled monolayers, for example, alkane- thiol/Au or octadecyltricholorosilane/SiO 2 substrates. [13, 14] Vari- ous oxide surfaces, such as glass, [15] silica, [16] mica, [17] and tita- nia [18] were used to form SLBs through the spontaneous rup- ture of liposomes. Gold and platinum have been shown to form vesicular layers instead of SLBs. [19] Phase transitions of upper and lower leaflets of 1,2-dipalmitoyl-sn-glycero-3-phos- phocholine (DPPC) monolayers in SLBs have been found to be decoupled due to the substrate support. [20] Tero et al. [21] report- ed the influence of surface hydroxyl ( OH) groups on the SLB formation and found that the fluidity of SLBs is affected by the density of the OH groups on the silicon dioxide surfaces. Indium tin oxide (ITO) being electrically conducting and opti- cally transparent, has been proved promising for direct SLB for- mation. [22–25] Immobilization of lipid vesicles on solid substrates for oligonuclide recognition, [26] layer-by-layer assembly, [27] steric entrapment through hydrophobic interaction, [28] chemical binding, [29] and attractive surface-polarizable binding [30] have Molecular orientation-dependent electron transport across sup- ported 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers (SLBs) on semiconducting indium tin oxide (ITO) is reported with an aim towards potential nanobiotechnologi- cal 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, hydro- gen-bonding, and cation–p interactions between the polar head groups of the lipid and the indole side chains. Conse- quently, 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 con- stants 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 mo- lecular tilt-dependent, as it is the case with the standard appar- ent rate constants k 0 app , estimated from electrochemical impe- dance spectroscopy and bipotentiostatic experiments with a Pt ultramicroelectrode. Lower magnitudes of k s and k 0 app 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 0 th   accrued upon electron tunneling comply with the poten- tial-independent electron-tunneling coefficient b = 0.15  1 . In- sulator–semiconductor transitions moving from a liquid-ex- panded to a condensed 2D-phase state of the SLBs are noted, adding a new dimension to their transport behavior. These re- sults highlight the role of tryptophan in expediting electron transfer across lipid bilayer membranes in a cellular environ- ment and can provide potential clues towards patterned lipid nanocomposites and devices. [a] N. K. Sarangi, Prof. A. Patnaik Department of Chemistry Indian Institute of Technology Madras Chennai 600 036 (India) E-mail : archita59@yahoo.com Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201200655. 4258 # 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13, 4258 – 4270