A Ruthenium(II) Tris(bipyridyl) Amino Acid:  Synthesis and Direct Incorporation into an α-Helical Peptide by Solid-Phase Synthesis

The synthesis of a ruthenium(II) tris(bipyridyl) amino acid is described along with its incorporation into a helix-forming peptide using standard solid-phase peptide synthesis methods. The physical properties of the metalloamino acid separately and in the context of the peptide are reported. The electrochemical potential, absorption spectrum, emission and excitation spectra, and emission lifetime data for the ruthenium(II) tris(bipyridyl) amino acid are similar to those of ruthenium(II) tris(bipyridine), indicating that the amino acid functionality introduced on one of the bipyridine rings does not strongly perturb the properties of the metal complex. The ruthenium(II) tris(bipyridyl) amino acid is amenable to direct incorporation into a synthetic peptide using solid-phase methods and BOC/benzyl chemistry. The 22 amino acid alanine-based peptide produced is 67% helical at 0 °C in aqueous buffer, as measured by circular dichroism spectroscopy. The 2,2,2-trifluorethanol helix induction curve and the temperature dependence of the helicity are typical for alanine-based peptides. The Lifson−Roig helix propagation parameter, w, derived from circular dichroism and NMR-monitored kinetic hydrogen−deuterium exchange is 0.5 ± 0.1 at 0 °C, indicating a moderate helix propensity for this metalloamino acid.