Cytochrome c (cyt c) and cytochrome f (cyt f) molecular recognition characters and their structural changes due to complex formation with negatively charged aspartic acid peptides (Aspptd＇s) have been studied. Changes in the absorption spectrum of cyt c in the Soret region were detected when Aspptd＇s, up to penta-Asp, were added to the cyt c solution. These changes were the same as those observed when cyt c interacted with plastocyanin (PC), indicating that Aspptd＇s interacted with cyt c in the same way as PC. Conformational changes of cyt c due to interaction with Aspptd＇s observed by resonance Raman spectroscopy were similar to those reported for cyt c when bound with its native partner, cytochrome c oxidase. Electrochemical measurements showed that the redox potential of cyt c and cyt f shifted to lower potentials by 7-20 mV upon Aspptd binding, showing the enhancement in the electron donor ability of both cyt c and cyt f upon complex formation with Aspptd. The changes in the absorption spectrum and redox potential increased with the length and concentration of Aspptd. The observed structural and redox changes of cyt c and cyt f are attributed to adduct formations with Aspptd＇s by electrostatic interactions and suggest that similar changes would occur for cyt c and cyt f when interacting with proteins. Aspptd＇s, tetra and penta-aspartic acid, served as competitive inhibitors of the electron transfer from cyt c or cyt f to PC, which was ascribable to the same adduct formation.