Using absorbance, fluorescence, resonance light scattering, and circular dichroism spectroscopy, we studied the self-assembly of the anionic meso-tetra(4-sulfonatophenyl)porphine (TPPS42-/4-) and a cationic 22-residue polypeptide. We found that three TPPS42-/4-molecules bind to the peptide, which contains nine lysine residues in the primary sequence. In acidic solutions, when the peptide is in the random-coil conformation, TPPS42- bound to the peptide forms. excitonically coupled J-aggregates. In pH 7.6 solutions, when the peptide secondary structure is partially a-helical, the porphyrin-to-peptide binding constants are approximately the same as in acidic solutions (similar to 3 x 10(6) M-1), but excitonic interactions between the porphyrins are insignificant. The binding of TPPS42-/4- to lysine-containing peptides is cooperative and can be described by the Hill model. Our results show that porphyrin binding can be used to change the secondary structure of peptide-based biomaterials. In addition, binding to peptides could be used to optimize porphyrin intermolecular electronic interactions (exciton coupling), which is relevant for the design of light-harvesting antennas for artificial photosynthesis.