| Several ruthenium (Ru) complexes were systematically studied to investigate their ability to electrostatically bind and transfer electrons to cytochromes. Once binding was established and optimized, the complex with the strongest association was used to study the detailed mechanism of electron transfer (ET) between the Ru complex and cytochrome (Cb5). In this study, Cb5 was used as a model protein to gauge the affinity of positively charged ruthenium complexes to cytochromes with negatively charged surface residues. It was determined that complexes with 4+ charges bind to Cb 5 with KD values in the 7–15 μM range. The results for binuclear and mononuclear complexes showed no significant difference, even though the mononuclear complexes contained lysine-type functional groups (LysGrp) similar to the biological partners of Cb5. The length of the lysine-type group was also explored and determined not to have a considerable effect on the affinity of Ru complexes for Cb5. Mononuclear, LysGrp-containing complexes with 6+ charges associated to Cb5 with KD values in the 0.5–0.6 μM range. Again the length of the chain had no impact on the results. The association of Ru complexes to Cb5 followed a charge-based model that was independent of all other parameters studied (number of Ru nuclei as well as number and length of the LysGrp). The 6+ charged complex (6pn) with the highest affinity was able to transfer electrons to Cb 5 at a rate of 5 × 106 s−1. However, the back reaction was fast as well, presumably due to the close proximity of the two metal centers (Ru and heme iron). The behavior of the 6pn complex was noticeably affected by the presence of aniline (AN), and was found to be air sensitive. The excited state of the 6pn complex was quenched by AN at a rate of 1.1 and 2.2 × 108 M−1*s −1 in the presence and absence of Cb5, respectively. The rate constant for scavenging of 6pn(III) by AN was measured at 1.7 × 108 M−1*s−1. From this information, the path of ET for 6pn and Cb5 in the presence of AN was determined not to include excited state quenching by AN. |
