Gregory A. Caputo, PhD, Rowan University
The overall goal of the funded work is develop novel porphyrin-based materials that will be implemented as both light harvesting and electron conductive structures in next generation dye sensitized solar cells (DSSCs). The need for these novel approaches stems from the growing energy demands throughout the developed and developing world, an increased focus on sustainable and renewable energy sources, inefficiency and high cost associated with traditional silicon-wafer photovoltaic cells. The approach to this development has focused on the well characterized porphyrin (TPPS) and a series of peptidic scaffolds.
TPPS is known to spontaneously self-assemble into large, excitonically coupled nano- and meso-scale structures under very low pH conditions (below pH 1). These structures, commonly referred to as J-aggregates due to the signature red-shifted absorption peak of the aggregate, have significant potential for application to DSSCs in that they both conduct electrons efficiently and can absorb electromagnetic radiation in the UV and visible ranges. The three dimensional arrangement of individual TPPS monomers within the self assembled nanostructure is highly structured, requiring proper alignment between monomers to allow excitonic coupling and formation of the J-aggregate, not simply an aggregate. The drawback to direct application of this material is primarily related to the extremely low pH conditions required for the aggregates to spontaneously self assemble ( < pH 1.5 ) which provides logistic hurdles to the widespread production and manufacturing of a TPPS based DSSC as well as potential restrictions on material lifetime. In an attempt to circumvent this restriction, we have developed a series of peptide scaffolds to bind and orient the TPPS monomers with the goal of inducing a J-aggregate structure at higher pH conditions closer to neutral. Previous results on our initial designs were successful in promoting scaffolded J-aggregate formation up to pH 3.6, a significant enhancement over spontaneous aggregation conditions. Our work over the past year has focused on three distinct areas of peptide-TPPS aggregates: (1) binding order within the peptide-TPPS aggregate, (2) reversibility of J-aggregate formation, and (3) alternate peptide sequences to promote J-aggregate formation.
The majority of effort was focused on second generation sequences that were designed to either (A) disfavor helical conformations at pH conditions where peptide-moderated J-aggregates do not form or (B) modulate the binding moieties that interact between the TPPS monomers and the peptide itself. The first series of designs was critical in deciphering the relationship between peptide scaffold structure and the ability to form the J-aggregate structure. The question arose wether the peptide structure was preventing aggregate formation due to steric and spatial constraints placed on the amino acid-PPS interaction. Previous results showed that under conditions where J-aggregates formed, no peptide structure was formed. Alternatively, the same experiments showed that when the peptide adopted a helical conformation, J-aggregates were absent. This structural relationship was investigated by design of a new scaffold using helix-disfavoring components as intervening amino acids between the porphyrin binding regions. The study was successful in that the new design was not helical under identical conditions to those which induced helix formation in the parent template. Additionally, spectroscopic investigations of these systems showed that the porphyrin was still capable of binding to the new template, but did not form J-aggregates. This answers the question relating to peptide structure preventing J-aggregate formation at higher pH conditions.
The second area of focus was to determine the structure activity relationship between the moieties binding the TPPS molecules and the formation of J-aggregates. This utilized altered scaffold sequences in which the specific cationic amino acids which contact and position the TPPS groups were altered. Initial work focused on the di-amino-propionic acid group which is a shortened version of the lysine groups that were in the original scaffold. These amino acids effectively bound the TPPS groups which was expected considering that the cationic moiety was identical. However, J-aggregate formation was reduced on this scaffold, leading to the conclusion that the added flexibility of the tetramethylene side chain of lysine allows a greater degree of flexibility in the bound TPPS and facilitating the aggregated geometry. Ongoing work is investigating arginine as a cationic group and preliminary spectroscopic studies indicate binding and some degree of J-aggregate formation.
Our ongoing experiments is using a significantly shortened peptide scaffold for J-aggregate formation. These sequences are designed based on the N-terminal, high affinity binding region of the original template molecule. This allows for ease of synthesis and purification, but prevents the inclusion of multiple porphyrin binding sites on a scaffold. In an attempt to overcome this limitation, we have incorporated the redox-sensitive amino acid cysteine at the termini of the peptide to allow for controlled polymerization. By modulating the pH and redox potential in the samples, we aim to polymerize the individual peptide-TPPS pairs into a much larger aggregate. This peptide was only 9 amino acids in length but initial spectroscopic characterizations indicate a distinct J-aggregate peak was induced by peptide binding. This initial work indicates a peptide:TPPS ratio that is different than the 1-peptide 1-TPPS model designed, indicating potential nucleation of larger aggregates. This is a key goal for development of peptide scaffolded aggregates since the scaffolds will become unwieldy and cost prohibitive as sizes increase.