Design and Development of β-Sheet Polypeptides for Biomedical Applications

Abstract

Synthetic polypeptides are continued to be one of the most challenging area to mimic the properties of the natural biomolecules (i.e, proteins) in macromolecular systems for both fundamental understanding and new technology development. -Sheet forming polypeptides are one of the least explored synthetic systems and this main restriction is associated with the uncontrollable-precipitation of the polymer chains during their synthesis. As a result, the -sheet polymerization was found to be “non-living” with dead propagation chains and they were inefficient for the synthesis of block copolymer architectures. This important problem in polypeptide area is addressed in this thesis work by developing new steric-hindrance ring opening polymerization strategy by controlling the helicity of the propagating polymer chains. New bulky N-carboxyanhydride (NCA) monomers were designed having t-butylbenzene pendant by multi-step organic synthesis and N-heterocyclic carbene was explored as a catalyst to make high molecular weight and narrow polydisperse soluble polypeptides. This ROP process was successfully demonstrated for two -sheet forming polypeptides such as poly(ʟ-serine) and poly(ʟ-cysteine). These new t-butylbenzene functionalized polypeptides were found to be readily soluble in tetrahydrofuran and chloroform, etc, and they were produced in high molecular weights having Mn = 32 kDa with dispersity Đ  1.3. ROP kinetics were studied by real-time FT-IR and 1H-NMR to determine the actual content of the secondary structures in the propagating chains. These studies established that -helical conformational front in the propagation chain was speeding up the polymerization kinetics with good degree of control in the ROP process. Reversible-conformational transitions in the post polymerization deprotection was found to restore the -sheet secondary structures in poly(ʟ-serine)s. Furthermore, the scope of this approach was expanded for the building inaccessible block copolymer macromolecular architectures. The serine-glutamate di-block copolymer was synthesized for the first time using polyserine as macroinitiators. The diblock copolymer was anionic in nature and self-assembled in to tiny (~30 nm) spherical nano particles. The diblock copolymer was able to encapsulate doxorubicin as well as near IR imaging probe—IR-780 and the drug/dye loaded nanoparticles were explored for in vitro cellular uptake and IVIS mediated in vivo biodistribution studies. The newly designed bulky monomer of serine was employed for the first time in aqueous ring opening polymerization induced self-assembly (ROPISA) to obtain polypeptide emulsions. The in-situ encapsulation of various dyes was carried out by adding dye molecules in the beginning of ROPISA process and the dye loaded polypeptide nano formulations were obtained which were explored in cellular imaging applications. The newly developed t-butylbenzene substituted steric-hindrance approach is valuable in yielding soluble polymers and this approach could be useful for exploring new polypeptide architectures for long-term impact.