Electron transfer across polypeptides. 3. Oligoproline bridging ligands

A series of cobalt(III)-L-ruthenium(III) binuclear (I-IV), with a bridging oligoproline peptide derivatized with an isonicotinoyl (iso) group at the N-terminal, has been synthesized and purified by HPLC. These complexes provide a peptide spacer that separates the metal ions at distances determined by the peptide conformation and structure. Reduction of the above complexes to the precursor complex, the Co super(III)-L-Ru super(II) species, was followed by a slow rate of intramolecular electron transfer with unimolecular rate constants of 1.2 x 10 super(-2), 1.04 x 10 super(-4), 0.64 x 10 super(-5), 5.6 x 10 super(-5), and 1.4 x 10 super(-4) S super(-1) for n = 0, 1, 2, 3, and 4. Over 2000 times variation in rate is seen for this series of binuclear complexes, which have identical reduction potential, inner-sphere reorganization energies, and charge type. A decrease in rate by introducing (Pro) sub(1) and (Pro) sub(2) reflects an increase in the separation between the donor and acceptor. For the (Pro) sub(3) and (Pro) sub(4) compounds, the slow rate of electron transfer allows enough time for the conformation change of the proline to bring the two metal ions into close proximity, resulting in a more rapid rate of electron transfer.