Association of Protein Helices and Assembly of Foldamers: Stories in Membrane and Aqueous Environments
Date of Award
Doctor of Philosophy (PhD)
Physics & Astronomy
William F. DeGrado
Solvents play an important role in association and assembly of molecules. Here we studied solvent effects on proteins and organic chemicals in different contexts. First, X-ray crystal structures show that helix dimers in membrane- and water-soluble proteins have distinct behaviors in packing and sequence selection. Transmembrane dimers are stabilized by compact packing and hydrogen bonding between small residues. Meanwhile, water-soluble dimers utilize hydrophobic residues for packing irrespective of the size of the interface and tight dimers are rare. Secondly, we apply the results learned above to a complex system in which a designed protein binds to single-walled carbon-nanotube in aqueous environments. Previous designs of the hexameric helical bundles utilized leucine and alanine residues to make two distinct helix-helix interfaces. Our molecular dynamics simulations showed that the alanine-comprising interface is much more labile than the leucine-comprising one. This result can be interpreted by the scarcity of tight soluble helix dimers as mentioned above. Thus more stable modular helix-helix interfaces have to be employed to design peptides binding to carbon-nanotubes with higher affinities. Lastly, we describe a serendipitous discovery of the crystalline framework structure by an amphiphilic triarylamide foldamer. Foldamers are peptide-like polymers of non-natural monomers arranged in defined sequence and chain length that are able to adopt protein-like secondary and tertiary structures. In contrast with traditional metal-organic and organic frameworks, which exploit strong directional coordination and hydrogen bonding for assembly in organic solvents, the crystal herein is built up from a combination of noncovalent hydrophobic, hydrogen-bonded, and electrostatic interactions in aqueous solution. The structure is in honeycomb geometry with each cubicle as a truncated octahedron. A new supramolecular synthon, in which hydrogen bonding and π-π stacking are encompassed, was discovered in the crystal structure. Through NMR experiments we probed the oligomeric states of the foldamer in the early stages prior to crystallization. The hierarchic crystal structure was discussed in terms of supramolecular synthons in crystal engineering.
Zhang, Shaoqing, "Association of Protein Helices and Assembly of Foldamers: Stories in Membrane and Aqueous Environments" (2013). Publicly Accessible Penn Dissertations. 823.
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