Masson: Building Blocks Self-assemble into Polymers with Help of Pumpkin-shaped Molecule

Dr. Eric Masson and his research group recently showed that Cucurbit[8]uril (CB[8]), a molecule that resembles a hollow pumpkin, can be used to connect building blocks to one another in a well-defined sequence.

Masson, the Roenigk Chair and Associate Professor of Chemistry & Biochemistry at Ohio University, took inspiration from nature’s building blocks that make up DNA and proteins. DNA strands have well-defined sequences of nucleobases (adenine, thymine, cytosine and guanine), and proteins have well-defined sequences of amino acids (glycine, alanine and 18 more natural ones). Those nucleobase or amino acid building blocks are linked to one another via backbones made of carbon, hydrogen and oxygen atoms, as well as nitrogen or phosphorus.

CB[8] has a large cavity, and can thus encapsulate – and connect – two guest molecules together. Masson and his group showed that CB[8], negative Iron complexes and positive iridium complexes assemble into dynamic polymers with a well-defined sequence, not unlike DNA strands and proteins. Very surprisingly, the prevalent sequence was (Iridium-Iridium-Iron-)n.

This study, “Sequence-Specific Self-Assembly of Positive and Negative Monomers with Cucurbit[8]uril Linkers,” was published in the Journal of the American Chemical Society, the academic journal with the highest impact factor for general chemistry topics in the United States.

The co-authors include OHIO graduate students Mersad Raeisi and Kondalarao Kotturi, and Ohio University alum Ian del Valle, who earned a B.S. in Chemistry Pre-Medicine and an M.S. in Chemistry from the College of Arts & Sciences and is now a medical student in the OHIO Heritage College of Osteopathic Medicine, according to his LinkedIn and Twitter.

Co-authors Jan Schulz, Paulina Dornblut were master’s students from Leipzig University, Germany. They spent a semester at OHIO as exchange students and carried out research in Masson’s group while taking classes in the department. This exchange stems from the very fruitful collaboration OHIO has had with Leipzig University for the past 25 years.

Abstract: The self-assembly into dynamic oligomers of Cucurbit[8]uril (CB[8]), a positive ditopic Ir(III) bis-terpyridine complex, and a negative ditopic Fe(II) bis-terpyridine complex flanked by four butyrate side chains was assessed to answer a seemingly straightforward question: does CB[8] adopt a social self-sorting pattern by encapsulating both positive and negative units into a heteroternary complex? We showed that this is indeed the case, with CB[8] linking a positive Ir unit to a neighboring negative Fe unit whenever possible. Furthermore, the solubility of the dynamic oligomers was significantly affected by their sequence; upon addition of 0.6–1.2 equiv of positive Ir oligomer to its negative Fe counterpart, the predominant assembly present in solution was a mixed oligomer with a (Fe–Ir–Ir−)_n sequence. Weak interactions between the negative butyrate side chains and the partially positive outer wall of CB[7] were also identified by two-dimensional nuclear magnetic resonance techniques, and resulted in a negative pK_a shift (0.10 pK_a unit) for the terminal carboxylic groups.