Ion Atmosphere Relaxation and Percolative Electron Transfer in Co Bipyridine DNA Molten Salts

• Published in: Journal of the American Chemical Society Vol. 125; no. 22; pp. 6784 - 6790
• Main Authors: Leone, Anthony M, Tibodeau, Jennifer D, Bull, Steven H, Feldberg, Stephen W, Thorp, H. Holden, Murray, Royce W
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 04.06.2003
• Subjects: 2,2'-Dipyridyl - chemistry; Analytical chemistry; Chemistry; Cobalt - chemistry; DNA - chemistry; Electrochemistry - methods; Electrons; Exact sciences and technology; Miscellaneous; Oligonucleotides - chemistry; Organometallic Compounds - chemistry; Oxidation-Reduction; Analytical chemistry; Electron transfer; Relaxation; Biochemistry; DNA; Molten salt
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0348795

Polypyridyl complexes of Co decorated with 350-Da polyether chains (Co350 2+) form molten phases of nucleic acids when paired with DNA counterions (Co350DNA) or 25-mer oligonucleotides. Analysis of voltammetry and chronoamperometry of mixtures of these phases with complexes having ClO4 - counterions (Co350(ClO4)2) and no other diluent provides charge transport rates from the oxidation and reduction currents for the complexes. As the mole fraction of the Co350(ClO4)2 complex in the mixture is varied from ca. 0.25 to 1, the physical diffusion constants derived from the CoIII/II wave increase from 1 × 10-11 cm2/s to 5 × 10-10 cm2/s, and apparent diffusion constants dominated by the CoII/I electron self-exchange increase from 1 × 10-10 cm2/s to 2 × 10-8 cm2/s. Pure Co350DNA melts, containing no Co350(ClO4)2 complex, do not exhibit recognizable voltammetric waves; DNA suppresses the CoII/I electron transfer reactions of Co complexes for which it is the counterion. There are therefore two microscopically distinct kinds of Co350 complexes, those with DNA and those with ClO4 - counterions, with respect to their CoII/I electron-transfer dynamics, leading to percolative behavior in their mixtures. The electron-transfer rates of the CoII/I couple are controlled by the diffusive relaxation of the ionic atmosphere around the reaction pair, and the inactivity of the bound Co complexes can be attributed to the very low mobility of the anionic phosphate groups in the DNA counterion. Substitution of sulfonated polystyrene for DNA produced similar results, suggesting that this phenomenon is general to other polymer counterions of low mobility. We conclude that the measured CoII/I charge transport and electron-transfer rate constants reflect more the diffusive mobility of the perchlorate counterion than the intrinsic CoII/I electron hopping rate.