10332 Phys. Chem. Chem. Phys., 2012, 14, 10332–10344 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 10332–10344 Intermolecular interactions in electron transfer through stretched helical peptidesw Daniel E. Lo´pez-Pe´rez, a Guillermo Revilla-Lo´pez, a Denis Jacquemin, b David Zanuy, a Barbara Palys, c Slawomir Sek* c and Carlos Alema´n* ad Received 9th March 2012, Accepted 21st May 2012 DOI: 10.1039/c2cp40761b The helical peptide Cys-Ala-Lys-(Glu-Ala-Ala-Ala-Lys) 2 -Ala-NH-(CH 2 ) 2 -SH has been organized forming a self-assembled monolayer on gold (0.602 peptides per nm 2 ), its conductance behavior under stretching conditions being studied using scanning tunnelling microscopy and current sensing atomic force microscopy. The helical conformation of the peptide has been found to play a fundamental role in the conductance. Moreover, variation of the current upon molecular stretching indicates that peptides can be significantly elongated before the conductance drops to zero, the critical elongation being 1.22  0.47 nm. Molecular dynamics simulations of a single peptide in the free state and of a variable number of peptides tethered to a gold surface (i.e. densities ranging from 0.026 to 1.295 peptides per nm 2 ) have indicated that the helical conformation is intrinsically favored in solvated environments while in desolvated environments it is retained because of the fundamental role played by peptide–peptide intermolecular interactions. The structure obtained for the system with 24 tethered peptides, with a density of 0.634 peptides per nm 2 closest to the experimental one, is in excellent agreement with experimental observations. On the other hand, simulations in which a single molecule is submitted to different compression and stretching processes while the rest remain in the equilibrium have been used to mimic the variation of the tip–substrate distance in experimental measures. Results allowed us to identify the existence, and in some cases coexistence, of intermolecular and intramolecular ionic ladders, suggesting that peptide-mediated electron transfer occurs through the hopping mechanism. Finally, quantum mechanical calculations have been used to investigate the variation of the electronic structure upon compression and stretching deformations. Introduction Understanding charge transfer processes in peptides and proteins is of fundamental importance not only to unravel key biological processes involving energy conversion and mass transduction but also to understand molecular based electronics. 1–7 In particular, electron transfer through a-helices have received much attention because this secondary structure is frequently found in proteins playing a crucial role in long range electron transfer processes. Consequently, model helical peptides have been widely studied both in solution 1,8–10 and on metal surfaces. 4–6,11–18 These contributions allowed us to conclude that, beyond a critical length, the electron transfer mechanism changes from electron tunnelling to a more complex process characterized by a shallow distance dependence with the helix dipole accelerating the electron transfer. For this latter aspect, the direction of the molecular dipole moment has been found to be an important variable. 15–17 For example, self-assembled monolayers of poly- alanine on gold surfaces were used by Sek et al. to show that electron transfer occurs more rapidly in the C-terminus-to- N-terminus direction than in the reverse one. 15 Moreover, these studies demonstrated that hydrogen bonding inter- actions increase the rate of electron transfer. For example, Toniolo and co-workers examined the influence of the number of hydrogen bonds on electron transfer in a-aminoisobutyric acid (Aib) homoligomers with a 3 10 helix conformation. 19 In 2005 Kraatz and co-workers reported a review 20 entitled ‘‘Peptide Electron Transfer: More Questions than Answers’’, in which the approaches to the investigation of electron a Departament d’Enginyeria Quı´mica, ETSEIB, Universitat Polite `cnica de Catalunya, Av. Diagonal 647, 08028, Barcelona, Spain. E-mail: carlos.aleman@upc.edu b CEISAM, UMR CNRS 6230, Faculte ´ des Sciences et des Techniques, BP 92208, Universite´ de Nantes, 2, rue de la Houssinie `re, 44322 Nantes Cedex, France c Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. E-mail: slasek@chem.uw.edu.pl d Center for Research in Nano-Engineering, Universitat Polite`cnica de Catalunya, Campus Sud, Edifici C’, C/Pasqual i Vila s/n, Barcelona E-08028, Spain w Electronic supplementary information (ESI) available: See DOI: 10.1039/c2cp40761b PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Universitat Politecnica de Catalunya on 03 July 2012 Published on 27 June 2012 on http://pubs.rsc.org | doi:10.1039/C2CP40761B View Online / Journal Homepage / Table of Contents for this issue