Electron Transfer across Polypeptides. 5. Rapid Rates of Electron Transfer between Os(II) and Co(III) in Complexes with Bridging Oligoprolines and Other Polypeptides

Article abstract of DOI:10.1021/ja00311a035

A series of Os(III)-L-Co(III) binuclear complexes with a number of peptide bridges as in <(NH3)5Os(III)-L-Co(III)(NH3)5>(BF4)5, where L = iso(Pro)_0-4, iso(Phe)2, iso(Gly)2, iso=isonicotinyl, have been synthetized and characterized.The peptide bridges are spacers that separate the Os(III) center from the Co(III) center.Upon one electron reduction using pulse radiolysis techniques, the <(NH3)5Os(II)-L-Co(III)(NH3)5>⁴⁺ complexes are formed.These precursor complexes undergo intramolecular electron transfer on time scales varying from microseconds to many seconds.For L = iso(Pro)_n, n = 0-4, the rates of electron transfer, k_u(25 deg C) (s^-1), and the activation parameters, ΔH^* (kcal/mol) and ΔS^* (eu), were, for n = 0,1,2,3,4,, respectively, 1.9 x 1E5, 10.2, 0; 2.7 x 1E2, 11.7, -8; 0.74, 12.7, -16; 0.9 x 1E-1, 12.4, -22; and 0.9 x 1E-1, 11.5, -25.For the more flexible iso(Gly)2 and iso(Phe)2 bridges, the rates of electron transfer, k_u (25 deg C)(s^-1), and the activation parameters, ΔH^* (kcal/mol) and ΔS^* (eu), were 4.8, 11.4, -17; and 6.9, 12.2, -14, respectively.The Os(II) site in all of these complexes has approximately the same redox potential ( -0.26 V vs.NHE).Because the driving force and reorganization energy in these complexes are very similar, variation in the rate of electron transfer can be related directly to the properties of the peptide bridge moiety.The rates of intramolecular electron transfer are more rapid than the rate of trans to cis proline isomerization, and therefore electron transfer takes place while the proline is in the extended trans configuration, the predominant isomer under these conditions