Photoinduced electron transfer within donor-(salt bridge)-acceptor complexes has been investigated. We now report the first comparative study of electron transfer through an asymmetric salt bridge interface formed from the 1:1 association of an amidinium to a carboxylate via two hydrogen bonds. Laser flash excitation prompts an electron to transfer from a highly reducing excited state of a derivatized Ru(II) bipyridine donor complex to a dinitrobenzene acceptor juxtaposed by the salt bridge interface. The rate of electron transfer through the D-(amidinium-carboxylate)-A salt bridge is similar to 10(2) times slower than that for the pair when the interface is switched, D-(carboxylate-amidinium)-A. This large difference shows that a salt bridge can significantly influence the kinetics of-electron transfer and, accordingly, bears considerably on electron transport within the biological milieu of proteins and enzymes.