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-N-carboxyanhydride (NCA) that minimized impurities to control the polymerization of this amino acid monomer with predictable molecular weights. To our knowledge, this is the first report of the controlled polymerization of this NCA and the first irrefutable proof of living polymerization of any NCA by the normal amine mechanism in the absence of the activated monomer mechanism. Rigorous purification of the monomer as well as solvent and initiator are essential to the successful synthesis of well-defined polymers having predictable molecular weights and low polydispersity (polydispersity index, PDI < 1.05), which are indicators of termination free living polymerization. Size-exclusion chromatography (SEC) combined with matrix-assisted laser desorption time-of-flight mass spectroscopy (MALDI TOF MS) confirmed that molecular weights reflect reaction stoichiometry and are extremely narrow (near Poisson molecular weight distributions). The materials synthesized may be 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.

Significance
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. Nature produces polypeptides with complete molecular homogeneity (i.e., PDI = 1), and it is our goal to mimic nature’s control of molecular design. The man-made systems described herein are the most analogous to natural materials that can be synthesized. For over 50 years synthetic chemists have searched for means to produce precisely tailored “living polypeptides”, which allows for control of chain length, the ability to create well defined block copolymers incorporating polypeptide segments, and the ability to manipulate polypeptide architectures (branching). As recently as two years ago, in a definitive review, the leader in this field, Kritcheldorf (Angew Chem, 45, 5752 (2006)), questioned the existence of methods for living polypeptide synthesis. This research demonstrates success in the synthesis of a well-defined poly(O-benzyl-L-tyrosine), a homopolymer analog of the amino acid tyrosine, where the polymerization is without question living and proceeds via initiation by the normal amine mechanism in the absence of a detectable contribution to polymerization by the activated monomer, or other mechanisms.. This work provides iron-clad proof of prior controversial claims of the Hadjichristidis group (Biomacromolecules, 5, 1653 (2004)) on the living nature of amine initiated polymerization of NCAs under high purity conditions. Control of molecular weight is imperative as it relates to final polymer properties and to the achievement of successful living polymerization is the key to tuning polypeptide architectures. 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 because it provides a vehicle 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.