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42220-GB4
High-Affinity Reversible Complexes in Water
The initial proposal for this project was to create and study high-affinity, multivalent complexes in water. The specific system was to be based on the synthetic, water-soluble host compound, cucurbit[8]uril (Q8). This host can bind simultaneously and specifically to two different guests: methyl viologen (MV) and an electron-rich aromatic guest such as naphthalene or indole. Our initial studies probed the basic properties of this system (solubilities, binding thermodynamics) and explored alternative guests for Q8 in aqueous solution. This work resulted in five important discoveries and two full articles in J. Am. Chem. Soc.1,2: 1) the Q8•MV complex can selectively recognize the amino acid tryptophan over all others;1 2) Q8•MV recognizes peptides containing N-terminal tryptophan over tryptophan at other positions;1 3) the system has a built-in optical sensor, allowing us to follow complex formation quantitatively by UV-visible and fluorescence spectroscopy;1 4) Q8 can recognize and cooperatively dimerize peptides containing N-terminal phenylalanine with unprecedented selectivitly;2 and 5) based on high-resolution crystallographic studies, the mechanism of selective recognition is based on the simultaneous inclusion of indole in the cavity of Q8 and chelation of the proximal N-terminal ammonium group by three carbonyl groups of Q8.2 This work was completed by three undergraduate students—Meghan Bush (class of 2005, now a PhD candidate at University of Chicago), Nicole Bouley (class of 2007, now in graduate school at Caltech), and Lisa Heitmann (class of 2008, and a 2007 Goldwater Scholar); the X-ray studies were carried out in collaboration with Prof. P. John Hart at UT Health Science Center, San Antonio. This work was completed in year 1 of this grant.
In year 2, and with the initial project goals still in mind, our initial work made possible the construction of high-affinity, reversible complexes. The idea is to make use of the unique properties of Q8 to construct a multivalent peptide receptor by self assembly. Specifically, a scaffold presenting two MV groups was constructed by solid-phase synthesis. This scaffold efficiently recruits two equivalents of Q8, and the resulting self-assembled receptor binds to a target peptide containing two tryptophan residues with a 10-fold increase in affinity due to multivalency. This system provides the first example of a self-assembled multivalent host for biomolecular recognition, and it offers a major synthetic advantage by circumventing the huge technical challenge of covalently linking synthetic hosts. This advantage will allow us to efficiently optimize the system for high-affinity by exploring structural rigidity and higher valency using our synthetically friendly approach. This project had contributions from three undergraduate students—Aimee Kennedy (class of 2009), Brian Halbert (class of 2007, now at Tulane Medical School), and Raymond Skunda (class of 2009)—and a postdoctoral research fellow (Dr. Joseph Reczek), and has been submitted recently to Angewandte Chemie.
Overall, this grant has been instrumental in jump-starting my research program. It gave initial direction, and it has supported the discovery and exploration of a new, important, and, productive avenue of research.
