UC Santa Barbara

What happens if you introduce a chiral side chain to a conjugated polymer backbone? The conjugated polymer is forced to answer the question: to twist or not to twist? The ability of conjugated polymers to form helical nanostructures depends on the molecular structure of the conjugated backbone. Two conjugated polymers, PCPDTBT* and PCDTPT*, containing chiral side chains, were synthesized in order to study their secondary structures using circular dichroism (CD) spectroscopy. PCPDTBT* provides a strong chiral response. However, the single structural difference of the substitution of a nitrogen atom on the PCDTPT* monomer for a –CH group on the PCPDTBT* monomer decreases the CD signal, indicating a less chiral aggregation. Theoretical calculations suggest the CDT-PT monomer has a higher rotational barrier compared to the CDT-BT monomer. This higher barrier to rotation provides an explanation for the planar conformation of the PCDTPT backbone, its promising transistor performance, and the inability of PCDTPT* to form a helical nanostructure.

Where did that chiral side chain come from anyway? Biocatalysts, such as Baker’s yeast, contain enzymes particularly suited to synthesizing chiral molecules such as chiral side chains. Biocatalysis is advantageous for chemical production as reaction conditions tend to be mild and environmentally-friendly. However, the cell membrane can often act as a barrier; preventing the starting material from entering the cell and the product from leaving. Just as conjugated polymers can be forced into unnatural structures, yeast can be forced to perform biocatalysis. Conjugated oligoelectrolytes (COEs) provide a potential solution based on their ability to intercalate into membranes, resulting in cell membrane permeabilization. Yeast cells were stained with these molecules and used in a model biocatalysis reaction (the conversion of fumaric acid to L-malic acid) where membrane diffusion is known to be the limiting step. As the number of phenylenevinylene units in the conjugated backbone is increased, the permeabilization ability of the COE decreases, along with the biocatalytic yield. The highest improvement utilized DSBN+, containing the smallest number of phenylenevinylene units, to achieve 4.5 times greater yield compared to a control reaction with unpermeabilized cells. If this result proves to be generalizable, a variety of reactions, with applications from pharmaceuticals to nanoparticles, could benefit from a simple and easy-to-use method of biocatalysis acceleration.