Interpreting Ion Fluxes to Channel Arrays in Monolayers Josep Monne ´, † Yolanda Dı ´ez, † Jaume Puy, † Josep Galceran,* ,† and Andrew Nelson ‡ Departament de Quı ´mica, UniVersitat de Lleida, RoVira Roure 191, 25198 Lleida, Spain, and Self Organising Molecular Systems (SOMS) Centre, UniVersity of Leeds, Leeds, United Kingdom ReceiVed May 17, 2007. In Final Form: July 20, 2007 The exponentially decaying permeability model interprets the chronoamperometric currents arising from Tl + reduction at a Hg electrode covered with a phospholipid monolayer (DOPC) containing gramicidin monomer by combining three processes: (i) the diffusion of an ion to a membrane surface with an array of channels, (ii) the conformational dynamics of the individual channels, and (iii) the passage of the ion through the channels. The introduction of a variable permeability allows us to uncouple the diffusion from the heterogeneous processes, given that the concentration of a species at the active surface can be obtained by semi-integration of the currents. Consideration of a reverse step for the dehydration process at the mouth of the channel allows the analysis of potential steps away from diffusion- limited conditions where a Nernstian-like behavior of the relevant parameter is observed. The model has been successfully applied to data with all trans retinol or benzo-R-pyrene as additive to the phospholipid monolayer and to monolayers without any additive at all. 1. Introduction The phospholipid monolayer on a mercury electrode is a classical electrochemical system for modeling biological mem- brane processes. 1-5 In spite of the fact that only a monolayer is being investigated, as opposed to a bilayer which forms the back- bone of biological membranes, the sheer simplicity and ma- nipulability of the model makes it a perfect vehicle for under- standing the fundamental principles of biological membrane structure and function. This model system has also been shown to have a particular useful application to study the fundamental physical chemical mechanisms of ion channel function with incorporated monomolecular gramicidin in the monolayer. 5-9 Since Nelson’s pioneering work, many other people (see, for instance, refs 10-13) have developed supported membrane systems with incorporated ion channels of increasing complexity. The use of permeant electroactive probes to characterize these channel systems has not been extensively used in spite of its sensitivity. This is mainly due to the lack of a model to analyze the data. There remains, therefore, a requirement for a generic analysis which can relate the currents observed in these channel arrays to channel function. In previous work, 14 we have analyzed the currents and the permeation when potential steps (in diffusion-limited conditions) are applied to a system where the phospholipid monolayer contained monomolecular gramicidin channels. The use of additives on such systems is a typical experimental procedure to understand the effect of drugs, anaesthetics, or other bioactive compounds on the properties of biological membranes. It is known that hydrophobic bioactive compounds influence gramicidin activity in phospholipid membranes, because of their interaction with the peptide and/or the lipid monolayer structure, affecting the stability and the ion transport characteristics of gramicidin leading to changes in the permeability of the membrane. 15 In the present work, we extend, in a more quantitative way, the study carried out previously 6 considering the impact of additives all trans retinol and benzo-R-pyrene (polyconjugated systems) on the permeability of gramicidin-modified phospholipid monolayers to Tl + . We have used retinol because of its significant biological function and its presence in biological membranes, 16 and we have used benzo-R-pyrene to look at the effects of a potential pollutant 17 on ion channel function, so that the system could be used in later applications as a sensor if there is a response. Mechanistically, the effect of both retinol and benzo-R-pyrene could be of interest because retinol is polyconjugated and benzo- R-pyrene is polyaromatic, and thus, the molecules have the potential to interact with the aromatic tryptophan residues of gramicidin and affect its structure and ion channel function. We also refine the EDP (exponentially decaying permeability) model 14 to improve the fitting of currents and permeability at some potentials with experimental values obtained when these additives are used. The refinement tackles an important factor. Previously, the application of the EDP was restricted to potentials defining the limiting current region of Tl + reduction. This is somewhat restrictive, since the permeability of the gramicidin modified monolayer at less negative potentials approaching the PZC of mercury is also of interest. We describe how this is done in this paper by introducing an additional treatment which includes the redox behavior of Tl + /Tl(Hg) and allows the estimation of the * Corresponding author. † Universitat de Lleida. ‡ University of Leeds. (1) Nelson, A.; Auffret, N. Mar. EnViron. Res. 1988, 24, 51-56. (2) Nelson, A.; Auffret, N. J. Electroanal. Chem. 1988, 244, 99-113. (3) Nelson, A.; Auffret, N.; Readman, J. Anal. Chim. Acta 1988, 207, 47-57. (4) Nelson, A.; Auffret, N. J. Electroanal. Chem. 1988, 248, 167-180. (5) Moncelli, M. R.; Becucci, L.; Nelson, A.; Guidelli, R. Biophys. J. 1996, 70, 2716-2726. (6) Nelson, A. Biophys. J. 2001, 80, 2694-2703. (7) Becucci, L.; Moncelli, M. R.; Guidelli, R. Biophys. J. 2002, 82, 852-864. (8) Moncelli, M. R.; Becucci, L.; Guidelli, R. Biophys. J. 1994, 66, 1969- 1980. (9) Moncelli, M. R.; Becucci, L.; Buoninsegni, F. T.; Guidelli, R. Biophys. J. 1998, 74, 2388-2397. (10) Quist, A. P.; Chand, A.; Ramachandran, S.; Daraio, C.; Jin, S.; Lal, R. Langmuir 2007, 23, 1375-1380. (11) Terrettaz, S.; Mayer, M.; Vogel, H. Langmuir 2003, 19, 5567-5569. (12) Becucci, L.; Moncelli, M. R.; Guidelli, R. Langmuir 2003, 19, 3386- 3392. (13) Gritsch, S.; Nollert, P.; Jahnig, F.; Sackmann, E. Langmuir 1998, 14, 3118-3125. (14) Monne ´, J.; Galceran, J.; Puy, J.; Nelson, A. Langmuir 2003, 19, 4694- 4700. (15) Suchyna, T. M.; Tape, S. E.; Koeppe, R. E.; Andersen, O. S.; Sachs, F.; Gottlieb, P. A. Nature 2004, 430, 235-240. (16) N’soukpoe-Kossi, C. N.; Sedaghat-Herati, R.; Ragi, C.; Hotchandani, S.; Tajmir-Riahi, H. A. Int. J. Biol. Macromol. 2007, 40, 484-490. (17) Thiele-Bruhn, S.; Brummer, G. W. Plant Soil 2005, 275, 31-42. 10581 Langmuir 2007, 23, 10581-10588 10.1021/la701447g CCC: $37.00 © 2007 American Chemical Society Published on Web 09/18/2007