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The ultimate goal of this project is to develop new, porphyrin-based nanostructures capable of harvesting solar energy and transferring this energy within the framework of a third generation solar cell.  The initial direction toward achieving this goal is focused on the development of peptide based scaffolds which will promote the self-assembly of the porphyrin (Meso-tetra (4-sulfonatophenyl) porphine; TPPS₄) into absorptive and conductive J-aggregates.  These TPPS₄ J-aggregates are ideal in their applicability to the area of dye-sensitized solar cells in that they exhibit a very high molar absorptivity (making them good absorbers of visible light) as well as being capable of excitonic coupling under proper conditions (making them good conductors of the absorbed energy).  This scenario couples the light collection and the energy transfer steps within a solar cell in one structure.

The TPPS molecule has been previously shown to spontaneously form excitonically coupled J-aggregate structures at very low pH (below pH 2.0).  Ideally, our scaffolding model would allow for this coupling to take place at higher, more tractable pH conditions as well as provide a stable structure that would not dissociate upon environmental pH differences that may occur during the manufacture of the dye sensitized solar cell.  The first attempts at the design of a scaffold-peptide to promote TPPS₄ self assembly was based on naturally occurring host defense peptides.  These molecules are small, highly soluble, and strongly cationic; all properties that should promote the interaction with TPPS₄ monomers. This design consisted of three clusters of cationic Lysine amino acids in an i, i+3, i+4 spacing in the peptide primary sequence.  These clusters, or Porphyrin Binding Regions (PBRs) were designed such that all residues would reside on the same face of the peptide if it adopts an alpha-helical conformation (a known property of many host defense peptides).  Our first design was successful in promoting the self-assembly of three TPPS monomers into a J-aggregate structure at pH 3.6, well above the acidic solution pH previously required to promote TPPS₄ self assembly in the absence of peptide.  Using both absorption and fluorescence spectroscopy the binding was shown to be both specific (one TPPS₄ per PBR in the peptide or one TPPS₄ per three lysine residues) and cooperative (Hill coefficients greater than 1 in all binding events) at all pH values tested.  We also noted that while the peptide-TPPS₄ interaction exhibited similar Kd values at all pH values tested, the porphyrins did not exhibit excitonic coupling at neutral pH.  This indicates that the peptide-TPPS interaction is decoupled from the TPPS₄ J-aggregate formation.  When examined structurally it was found that the peptide scaffold adopted a random-coil secondary structure at sample pH values where excitonic coupling was observed but adopted the alpha-helical conformation under conditions where no coupling was observed.  This is indicative that the peptide secondary structure is in some way linked to the coupled aggregate formation.  Specifically it implies that the helical conformation in some way inhibits the coupling or, conversely, the coupling in some way prevents the peptide from adopting the helical conformation.

Our next attempts at characterizing the peptide-TPPS₄ aggregates was focused on the electron transfer dynamics in the peptide-TPPS₄ complexes.  As previously described, the peptide-TPPS₄ binding event occurs with similar affinity at all pH values examined, although excitonic coupling only occurs at pH 3.6 or below.  Using fluorescence lifetime analysis, transient absorption spectroscopy using a pump-probe design, and polarized transient absorption spectroscopy we determined the energy transfer dynamics in peptide-TPPS complexes that did and did not exhibit excitonic coupling (pH 3.6 and pH 7.6).  These results showed a greater than 30-fold increase in electron transfer times at pH 7.6.  The rate of transfer was found to be ~9ps in the low pH conditions in which the peptide was in the random coil conformation and the TPPS molecules formed the coupled J-aggregate.  Alternatively, the transfer rate was shown to be ~279ps at neutral pH where the J-aggregate is not formed and the peptide adopts the helical secondary structure.  These results clearly show that the ability to couple TPPS₄ molecules into a J-aggregate is critical for the energy transfer requirements in a dye-sensitized solar cell and this coupling is linked to the secondary structure of the peptide scaffold that is promoting the assembly.

Our next steps in the characterization of peptide-TPPS₄ self assembly is to (A) characterize the pH dependence of the interaction, (B) further characterize the binding event between the peptide and TPPS₄ to develop stronger PBRs, and (C) develop and characterize second-generation peptide designs based on the initial results.  Aims A and B are currently underway utilizing a series of peptides in which the position of the naturally fluorescent tryptophan residue in the primary sequence was varied.  All peptide variants have already been shown to bind and couple TPPS molecules with similar affinity compared to the original peptide design.  The order of binding to the individual PBRs in the peptide will be examined using fluorescence quenching methods which are highly sensitive to fluorophore-quencher distances.  Aim C will build on the principles of helix-propensity in the amino acid sequence to design peptides which are less helix-favoring in the hopes of creating a more stable peptide-TPPS₄ aggregate at higher pH values by inherently disfavoring the helical conformation.