Unraveling the Interplay of Backbone Rigidity and Electron Rich Side-Chains on Electron Transfer in Peptides: The Realization of Tunable Molecular Wires

Electrochemical studies are reported on a series of peptides constrained into either a 3₁₀-helix (<b>1</b>–<b>6</b>) or β-strand (<b>7</b>–<b>9</b>) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (<b>1</b> and <b>2</b>) and (<b>7</b> and <b>8</b>) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides <b>1</b>, <b>4</b> and <b>5</b>, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides <b>3</b>–<b>6</b> confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (<b>1</b> and <b>7</b>) and saturated tethered peptides (<b>2</b> and <b>8</b>) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to <b>7</b>, <b>8</b> and <b>9</b> provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a “stepping stone” for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices