| alt bridges occur in a variety of natural structural motifs including proteins, protein-protein complexes, protein-substrate complexes, and protein-DNA or protein-RNA assemblies. In addition to their role as natural structural building blocks, salt bridges also control reaction chemistry by modulating redox activity at the protein active site. One way this occurs is when salt bridges mediate redox processes, as in sulfite and nitrite reductases (SiRs and NiRs). Salt bridges are also proposed pathways for electron transfer in cytochrome c oxidase. We are interested in examining the affect of proton motion or the proton position within a salt bridge on fixed distance electron transfer (ET) rates. There are an array of chromophore/quencher complexes where an electron donor is held at some fixed distance relative to an electron acceptor. Upon excitation of either the donor or acceptor, depending on the design of the particular molecule, it is possible to time the electron as it makes its journey from one point to another, that is to measure the rate of photoinduced ET at some fixed distance. This strategy has been employed to characterize the distance dependence of ET reactions, and also assess the role of solvation in fixed distance ET reactions. Alternatively, pulse radiolysis experiments have been employed to measure the rates of electron transfer at fixed distances. From these classical experiments the existence of the Marcus inverted region has been established experimentally, nearly 30 years after it was proposed by theory. In the inverted region for ET, increasing the exergisity of an ET reaction results in a decrease in the reaction rate, in sharp contrast to the prediction of classical chemical reaction theory.;Presented here is a series of donor/acceptor complexes assembled via the non-covalent amidinium-carboxylate salt bridge. A series of donor/acceptor complexes have been chemically modified with the amidine functional group. It is shown that the amidine group is readily transformed into a 1:1 amidinium-carboxylate salt bridge with concomitant protonation by a carboxylic acid. This salt bridge structural motif parallels Asp-Arg salt bridges occurring in proteins, yet has the advantage of having only one specific two hydrogen bond binding mode, where the guanidine group of arginine has multiple binding modes. The synthetic details of preparing and characterizing amidines and their salt bridge complexes are described. The association constants for various amidinium-carboxylate salt bridge complexes have been determined by either titration or dilution... |
