Synthesis of Well-defined Poly(amino acids): Polytyrosine Derivatives

Jamie M. Messman¹, Deanna L. Pickel¹, Apostolos Avgeropoulos², and Nikolaos Politakos^2
1Macromolecular Nanomaterials Group, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831
²Department of Materials Science and Engineering, University of Ioannina, Greece

Achievement
In collaboration with CNMS users from the University of Ioannina, Greece, we developed a synthesis route for the monomer, O-benzyl-L-tyrosine NCA that minimized impurities to control the polymerization of the amino acid monomer with predictable molecular weights. To our knowledge, this is the first report of the controlled polymerization of the monomer, O-benzyl-L-tyrosine-N-carboxyanhydride (NCA). Purification of the monomer generated long, needle-like crystals (see picture below). We believe that the rigorous purification of the monomer as well as solvent and initiator are essential to the successful polymerization of well-defined polymers having predictable molecular weights and low polydispersity (polydispersity index, PDI < 1.05), which is a measure of molecular homogeneity. Nature produces polypeptides with complete molecular homogeneity (i.e., PDI = 1.0), and it is our goal to mimic nature’s control of molecular design. The man-made systems described herein are essentially the most analogous natural materials that can be synthesized. The figure below demonstrates the exquisite control over molecular homogeneity as shown by a Gaussian distribution of the size exclusion chromatogram as well as the molecular weight distribution determined by matrix-assisted laser desorption time-of-flight (MALDI TOF) mass spectroscopy. We believe the materials synthesized are strong candidates for drug carrier precursors. Furthermore, the synthetic routes developed in this study provide the tools to prepare and study a variety of bio-inspired polymers to examine the fundamental interactions and assembly of atoms and molecules into functional structures as well as to learn from nature (bio-mimetic). As such, these synthetic and characterization tools are available to users of the CNMS to address fundamental issues in nanoscience and nanotechnology.

Sigificance
Nature creates an incredibly complex and diverse range of structures and functions through precisely tailored synthesis of macromolecules composed of twenty amino acid building blocks. The biological activity of proteins arises from nature’s ability to precisely arrange the sequence of amino acids, which controls conformation (coils, a-helix, b-sheet) and governs the creation of well-defined hierarchical structures (tertiary and quaternary structures) through self-assembly. Although we can design and synthesize an almost endless variety of macromolecular structures, a major scientific challenge is to understand how to design and synthesize a macromolecule for a specific function. Understanding the synergistic interplay between the synthesis, structure, and properties of macromolecules will have a profound impact on our fundamental understanding of the structure and function of biomaterials, and it will accelerate the design of new biomimetic materials with precisely engineered properties. In this project, we describe our recent success in the synthesis of well-defined poly(O-benzyl-L-tyrosine), a homopolymer analog of the amino acid tryosine. Control of molecular weight is imperative as it relates to final polymer properties. The synthesis of poly(amino acids), or polypeptides, functionalized with hydroxyl groups, which can be generated by the post-polymerization modification of poly(O-benzyl-L-tyrosine), is of interest to our users because it provides a handle to couple cancer drugs to a potentially biocompatible polymer for targeted delivery to tumors. The self-assembly of these polymers and their derivatives as a function of molecular weight is of interest since it provides insight into the fundamental structure-property relationships in these synthetic poly(amino acids). Additional information can be gained on the impact of copolymer composition on the secondary and tertiary structure of the polypeptides and their solution phase aggregates through copolymerization using other NCA monomers.

This research at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.