Self-assembly of peptide-porphyrin complexes leads to pH-dependent excitonic coupling.
Using absorbance, fluorescence, resonance light scattering, and circular dichroism spectroscopy, we studied the self-assembly of the anionic meso-tetra(4-sulfonatophenyl)porphine (TPPS(4)(2-/4-)) and a cationic 22-residue polypeptide. We found that three TPPS(4)(2-/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, TPPS(4)(2-) bound to the peptide forms excitonically coupled J-aggregates. In pH 7.6 solutions, when the peptide secondary structure is partially alpha-helical, the porphyrin-to-peptide binding constants are approximately the same as in acidic solutions (approximately 3 x 10(6) M(-1)), but excitonic interactions between the porphyrins are insignificant. The binding of TPPS(4)(2-/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.
Department of Chemistry and Biochemistry, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey 08028, USA. dkuciauskas@gmail.com
Darius
D
Kuciauskas
Gregory A
GA
Caputo
D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D010455:Peptides / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*; D011485:Protein Binding; D017433:Protein Structure, Secondary; D013056:Spectrophotometry, Ultraviolet
D010455:Peptides; D011166:Porphyrins; C016136:tetraphenylporphine sulfonate