Cytochrome c (cyt c) acts as an electron shuttle in biological respiration and photosynthesis. Although the understanding of this protein and its electron transfer (ET) reaction is relatively highly developed, many of its aspects remain unclear. We use supramolecular assemblies of cytochrome c to understand its ET at an interface in terms of how the ET depends on solution composition, SAM composition, temperature, immobilization methods and the nature of the electron tunneling pathway. From these studies we have learned: (1) the formal potential and the surface charge of cytochrome c are modified when binding to a negatively charged surface; (2) cytochrome c molecules assume a wide distribution of geometries when electrostatically binding to a negatively charged surface, which leads to a wide distribution of ET rate constants; (3) when the protein is within 14 Å of the electrode, the ET rate is controlled by local frictional motions rather than by electron tunneling; (4) the binding-inactive sites, the diluent molecules on the SAM can provide alternative electron tunneling pathways from electrode surface to the heme of cytochrome c, and in suitable conditions the diluent molecules dominate in the electron tunneling pathway.

Poly (phenylethynylene) (PPE) based conjugated polyelectrolytes are a class of polyions having rigid backbones. We present a fluorescence correlation spectroscopy (FCS) study on the hydrodynamic properties of complexes formed by two PPE-SO₃^- polymers, having different charge density, with octadecyl trimethylammoniumbromide (OTAB) below the critical micelle concentration. The concentration ratio CO_TAB/C_monomer ranges from 0.2 to 1800. The hydrodynamic radius of the complexes as a function of OTAB concentration has three regimes. In the low concentration regime, the complex has a comparable size with the polymer in deionized water. In the intermediate concentration regime the complexes have the largest size and substantial heterogeneity. In the high concentration regime, the complexes have a size that is about three times larger than that in the low concentration regime. The results significantly extend the understanding of the interaction between polyelectrolyte and ionic surfactant, and indicate that the rigidity of polymer backbone and CO_TAB/C_monomer concentration ratio act to determine the composition of polyelectrolyte/surfactant complexes.