| Intracomplex electron transfer (ET) occurs most often in intrinsically transient, low affinity complexes. As a result, the means by which adequate specificity and reactivity is obtained to support effective ET still is poorly understood. This thesis focuses on two such ET complexes: cytochrome b5 (cyt b5) in reaction with physiological partners, myoglobin (Mb) and hemoglobin (Hb). The [Mb/Hb, cyt b5] complexes fall into the Dynamic Docking (DD) paradigm, where ET reactivity occurs within a rapid exchange limit; a large ensemble of weakly bound protein-protein configurations contribute to binding, but only a few are reactive. We've developed an approach using the separately and severally heme-neutralized Mb/Hb and/or cyt b5 complexes to probe the elusive reactive conformations, which are too few to be observed directly. Reactive complex formation is dependent upon not only the 'global' protein charges, but also upon the microenvironments on the reactive interface of both proteins. In addition, we find the free energies of interaction for the repulsive heme-heme interactions are not nearly as strong as predicted. We thus propose new details about this protein complex interface and offer a reactive model that includes differential, intracomplex dynamics.; In attempts to simplify ET kinetics and second-order dynamics, we've also studied ET of [Mb/Hb, cyt b5] encapsulated within a sol-gel (SG) pore. The observed intermediate formed by photoinitiated intracomplex ET proves kinetic studies are possible within the SG; however, these studies show little triplet quenching, suggesting few reactive complexes. To probe the environment within the SG, we have performed the first NMR studies of solute molecules within the SG pores. Small molecule probe studies have shown that the 'bulk' of the pore is much like solution; however, there is a strong electrostatic effect of the negatively charged silicate walls, slowing a minority of positively charged probes. Preliminary NMR studies of encapsulated paramagnetic proteins also show electrostatic effects from the SG and only minor structural effects of encapsulation. Perhaps most promising, the combination of SG-encapsulation and NMR spectroscopy can provide detailed structural data on intermediates formed by ligand photolysis, and give snapshots of conformational changes during ligand rebinding. |
