Polypeptides are naturally occurring polymers that are utilized for a variety of different biological processes including structural support, catalysis, and signaling. Composed of repeating amino acid monomeric units, the structure and function of polypeptides is easily modified by the side chain group of the amino acids. Preparation of biomaterials from a variety of α-amino acids is best accomplished through ring opening polymerization of α-amino acid N- carboxyanhydrides (NCAs). Various initiation systems to prepare polymers via this methodology
are outlined in chapter 1. Although classical preparations include the use of primary amines and strong base systems, the field has been greatly expanded to include transition metals, alcohols, and thiols. Each of the systems provides a unique set of criteria for the resulting polymer, which allows the final function of polypeptide biomaterials to be matched to the optimized initiation system. The use of three distinct initiation systems for the preparation of biomaterials is covered in subsequent chapters.
In chapter 2, the preparation of stimuli responsive chemically crosslinked polypeptide biomaterials is outlined. Biologically occurring non-canonical di-α-amino acids were converted into new di-N-carboxyanhydride (di-NCA) monomers in reasonable yields with high purity. Five different di-NCAs were separately copolymerized with tert-butyl-L-glutamate NCA to obtain covalently crosslinked copolypeptides capable of forming hydrogels with varying crosslinker densities. Comparison of hydrogel properties with residue structure revealed that different di-α- amino acids were not equivalent in crosslink formation. Notably, L-cystine was found to produce significantly weaker hydrogels compared to L-homocystine, L-cystathionine, and L-lanthionine, suggesting that L-cystine may be a sub-optimal choice of di-α-amino acid for preparation of copolypeptide networks. The di-α-amino acid crosslinkers also provided different chemical stability, where disulfide crosslinks were readily degraded by reduction, and thioether crosslinks were stable against reduction. This difference in response may provide a means to fine tune the reduction sensitivity of polypeptide biomaterial networks.
In chapter 3, an approach to the preparation of poly(dehydroalanine) (ADH) is discussed. Examination of bulky side chain modified α-amino acid N-carboxyanhydrides based off of serine and cysteine is performed, including their ability to undergo fast living polymerization utilizing Co(PMe3)4. The lead candidate, tBu-MA Cys NCA, displayed unique properties similar to that of Mn-MA Cys NCA, which allowed for the preparation of long soluble polymer chains of a variety of architectures. Subsequent modification of poly(S-carbo-tert-butoxymethyl-L-cysteine) under mild conditions with iodomethane leads to selective and near quantitative conversion to ADH. Preliminary studies into the modification of these residues with small molecule nucleophiles are discussed.
Finally, in chapter 4, the potential of α-amino acid N-thiocarboxyanhydrides for the preparation of polypeptides via transition metal mediated ring opening polymerization is examined. The preparation of poly(L-methionine) as a precursor to functionalizeable biomaterials from Met NTA is reported. Optimization of the polymerization is explored through systematic variation of polymerization conditions. Furthermore, examination of the polymerization mechanisms through the generation of a thioalloc α-amino acid amide ligand demonstrates that the presence of the carbonyl sulfide byproduct in the polymerization can lead to the formation of nickel carbonyl species, which may lead to poisoning of the initiator. This demonstrates that the end chain active metallocycle species is not stabile during this polymerization. Additional work will need to be performed to optimize the transition metal based polymerization of NTAs.