Characterization of Self Assembled Peptide Porphyrin Complexes as Light Harvesting Antennas

Abstract

Following a biomimetic approach, synthetic polypeptides designed to induce porphyrin self-assembly into nanostructures were created. Because of their stability and favorable spectroscopic properties, peptide-porphyrin aggregates could be applied as light harvesting antennas in dye-sensitized nanocrystalline solar cells. Using a designed porphyrin binding motif (PBM) consisting of 3 cationic lysine residues, we have developed and characterized a short, amphiphilic peptide that can efficiently bind three meso-tetra (4-sulfonatophenyl) porphine (TPPS) molecules and facilitate three-dimensional orientation into an excitonically coupled J-aggregate. The ability of the peptide to act as a scaffold for TPPS interactions was shown to be dependent on the sample pH and the resulting impacts on peptide secondary structure conformation using a combination of fluorescence, absorbance and circular dichroism spectroscopy. Under all conditions tested the peptide was shown to cooperatively bind three TPPS molecules per peptide with little change in Kd from pH 1.8 through 7.6. At low pH (3.6 and below) the peptide adopted a random coil conformation and allowed for the TPPS molecules to form the J-aggregate structure. Upon neutralization of the sample pH, the peptide adopts a predominantly alpha-helical structure which is concomitant with the loss of J-aggregate formation within the TPPS molecules. Using a series of peptides that contained Trp at different positions with respect to the PBMs we have initial measures of the sequence specificity of the cooperative binding event in terms of which PBM sequence is more likely to be the FIRST in the peptide to bind a TPPS molecule.