ABSTRACT
Biological electron transfer (ET) reactions often occur over long distances (> 10 A) between cofactors that are embedded within a complicated protein matrix [14]. It has therefore become a goal of current research to investigate the putative role of proteins, and specific protein structures, in propagating long-range elec tron donor-acceptor interactions [5-12]. Recent studies of both native and chem ically modified ET proteins have produced conflicting information concerning the ability of the protein matrix to mediate long-range electronic coupling. For example, Dutton and co-workers [13,14] have compared the available ET data obtained from the bacterial photosynthetic reaction centers to conclude that the native protein matrix behaves much as an isotropic solvent that presents a homo geneous barrier to electron tunneling. This conclusion was based on the observa tion that the rates of activationless ET reactions occurring in the reaction cen ter follow a uniform distance dependence, as measured by their through-space
edge-to-edge separations. The similarity of such behavior obtained for reactions that traverse different regions of this protein complex suggests that the donoracceptor coupling strengths are not strongly affected by their specific structural environments. Quantitation of these observations further indicates that the photosynthetic protein medium is a relatively inefficient conduit for mediating long-range donor-acceptor interactions and must therefore play a minor role in promoting the high efficiency of these reactions. An opposing view of proteinbased ET mechanisms is held by Gray and co-workers [15,16] who have con ducted numerous investigations of ruthenium-modified metalloproteins to show that these proteins do indeed display considerable heterogeneity in their ET prop erties. Thus, several situations have been reported in which protein-based ET reactions can occur across comparable through-space distances, but with rate constants that vary by as much as several orders of magnitude [15-17]. Such observations have been explained in terms of a tunneling pathway model in which long-range donor-acceptor interactions are mediated by optimal combina tions of covalent, hydrogen bond, and through-space interactions that exist in a particular region of a given protein [18,19]. In contrast to the conclusions of Dutton and co-workers [13,14], this work appears to demonstrate that different protein structures can indeed display different barriers to electron tunneling. If true, these observations could have important implications for the future design of synthetic peptide systems for use in bioelectronics, therapeutics, and energy conversion schemes. Thus, the ongoing debate concerning the fundamental nature of protein-based electron transfer has caused numerous studies to be performed in an effort to learn more about the properties of native, chemically modified, and de novo designed ET systems. The latter approach to this problem offers a controlled manner in which to investigate this problem through the synthesis of well-defined, peptide-based donor-acceptor systems that can adopt the various structural motifs found in native proteins. After a brief introduction to electron transfer theory and experimental methods, this chapter will review some of the recent work conducted on model ET systems whose range of structural complex ity extends from relatively simple peptide-bridged donor-acceptor compounds to organized peptide assemblies having well-defined tertiary and quaternary protein structures.
