In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division, Jianjun Cheng of the University of Illinois at Urbana-Champaign will synthesize and study water-soluble, alpha-helical polypeptides with charged side chains. Polypeptides are important materials with broad applications and characteristic secondary structures. Water-soluble polypeptides bearing charged side chains typically adopt random coil conformations due to side-chain charge repulsion. In this study, we aim to develop water-soluble, alpha-helical polypeptides by elongating charge-containing amino-acid chains to position the charges distally from the polypeptide backbone and then to study their biophysical properties. Preliminary studies will be performed to test these polymers for use in cell penetration and bacterial inhibition. The broader impacts involve training undergraduate and graduate students, participating in summer high school outreach and undergraduate broadening participation activities at the University of Illinois, and disseminating research results through publications in journals and presentations at conferences.
Peptides are chains of chemically linked amino acids and are the components of proteins. This work will expand the repertoire of chemistry that can be used to create new large molecules that mimic natural biomaterials and will lead to a better understanding of how peptides can be engineered to form stable secondary structures, such as an alpha-helix, in water. The results of these studies could have many important long term impacts on applications in which protein-based materials are important, including long term applications in medicine, pharmaceutical processing, and nanotechnology.
Polypeptides can be regarded as synthetic analogs of natural proteins. They share the same backbone as peptide and proteins. Because of their similarity, synthetic polypeptides can adopt protein-like secondary structures such as α-helices and β-sheets, which provide unique properties and functions of the polymers. However, maintaining the secondary structures and increasing water solubility of polypeptide are always contradictory. Synthetic helical polypeptides usually exhibited poor water solubility because of strong hydrophobicity. Hydrophilic polypeptides with charged amino acid residues, on the other hand, disrupted the helical conformation due to side-chain charge repulsion. Although several efforts have been reported to improve polypeptide water-solubility by attaching neutral hydrophilic groups (oligoethylene glycol, hydroxy or sugar groups), the design of water-soluble, helical polypeptides has never been achieved before, in particular those with charged side chains. With the support from NSF, we discovered when the side-chain length was sufficiently long to distally place the terminal charged groups away from the polypeptide backbone, the resulting polypeptides with charged side chains adopt α-helical conformation due to enhanced hydrophobic interactions and reduced side-chain charge repulsion. The helical conformation is stable over a wide range of pH (pH 1 to 11), salt concentration (0-4M), and temperature (0 – 70 oC). We synthesized a large variety of the polypeptides following this strategy and they all adopt similar helical structure. When these helical ionic polypeptides are used as non-viral gene delivery vectors, excellent helix-associated transfection results were obtained in several cell lines (Angewandte Chemie International Edition, 2012, 51, 1143-1147). Another series of helical ionic polypeptides with guanidine side chains, served as helical poly(arginine) mimics, showed much higher cell-penetrating capabilities compared with commercial available cell-penetrating peptides (Chemical Science, 2013, 4, 3839 – 3844). Preliminary studies suggested these helical ionic polypeptides entered cells through non-endocytic pore-formation mechanism. In order to better understand the structure-property relationship and broaden the application in biological field, various helical ionic polypeptides with different side-chain hydrophobicities and charged groups were synthesized and their membrane activities were evaluated. The polypeptides were also incorporated into supramolecular self-assembled nanoparticles to facilitate the oral delivery of therapeutic TNF-α siRNA (Biomaterials, 2013, 34, 2340-2349). In addition, our recent work on trigger-responsive polypeptides provided a new strategy to design smart polypeptide gene delivery vectors, whose membrane activity and cytotoxicity could be easily altered by changing the conformation and charged states of polypeptides (Angew Chem. Int. Ed., 2013, 52, 9182-9186). With more work/publication underway, we believe we have had significant investigations on the water-soluble, helical polypeptides with charged groups for their design, synthesis and preliminary studies on their helix-associated membrane activities, through this NSF support.
