Peptide molecular junctions: Electron transmission through individual amino acid residues Joanna Juhaniewicz, Slawomir Sek * Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland article info Article history: Received 1 October 2009 Received in revised form 25 January 2010 Accepted 27 January 2010 Available online 2 February 2010 Dedicated to professor Jacek Lipkowski on the occasion of his 65th birthday Keywords: Peptide Electron transfer Molecular junction Scanning tunneling microscopy Monolayers Gold electrodes abstract Three short chain peptides of general formula Cys–AA–CSA, where CSA is cystamine, AA is Gly, Ala or Pro, were synthesized and used for the determination of the electron transmission through individual amino acid residues. The conductance measurements were carried out using STM-based molecular junction method. The results of our comparative study indicate that the ability of these molecules to mediate elec- tron transfer is not significantly affected by the presence of the methyl side chain in alanine or the cyclic structure of proline. Thus the efficiency of electron transmission is determined mostly by the peptide backbone structure. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Amino acids are fundamental building blocks of peptides and proteins. Their chemical properties and the particular sequence of the residues determine the three dimensional structure and the biological activity of the proteins. Therefore, amino acids seem to be good candidates for the design and synthesis of artificial functional molecules which display desired properties. By careful selection of the amino acid sequence within the peptide, it is potentially possible to obtain the molecule with specific secondary structure, polarity, reactivity, etc. This way, peptides can be suit- ably tailored for the applications in electrochemical (bio)sensors, nanodevices and molecular electronics [1–5]. In all aforemen- tioned cases, the efficient action of such systems involves the use of the molecules with specific, well-defined electronic conduc- tance. It is known that peptides can act as mediating bridges for long range electron transfer and the efficiency of this process may be affected by the details of the amino acid sequence, the sec- ondary structure of the peptide and the presence of the hydrogen bonds [6–11]. Thus the fundamental problem is how to design the suitable functional molecule to attain desired structure and the electronic conductance. In order to solve this problem, first we need to gain our knowledge about the electronic conductance through individual amino acid residues. This may help to differen- tiate between amino acids which are good ‘‘conductors” and those acting mostly as a structural foundation for peptide. Therefore, in this paper, we report the results of our comparative studies on electron transfer through individual amino acid residues, i.e. Gly, Ala and Pro, incorporated into the short peptides. The electron transmission through single molecules was investigated using STM-based molecular junction method. Our approach involved the entrapment of peptide molecules between the substrate and the tip of a scanning tunneling microscope. Several groups have demonstrated the use of similar methods for the conductance mea- surements of alkanedithiols, viologens, diamines, dicarboxylic acids, diisonitryles, peptides and DNA [12–20]. In this study, we used self-assembled monolayers of peptides of general formula Cys–AA–CSA where CSA is a cystamine linker, AA is either glycine (Gly), alanine (Ala) or proline (Pro), and Cys is a cysteine residue (see Scheme 1). By changing the amino acid in the middle section of the molecule, we were able to compare the efficiency of electron transmission through each residue, i.e. Gly, Ala or Pro. This way, it was possible to verify how the presence of the side chain in alanine or the cyclic system of proline affects the electron transfer properties of the peptides. 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.01.029 * Corresponding author. Tel.: +48 228220211. E-mail address: slasek@chem.uw.edu.pl (S. Sek). Journal of Electroanalytical Chemistry 649 (2010) 83–88 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem