ΔTt-Mechanism in the Design of Self-Assembling Structures

Abstract

Protein-based polymers can be designed in which self-assembly occurs as the temperature is raised above the onset temperature, Tt, of an inverse temperature transition for hydrophobic folding and assembly. Instead of changing the temperature, however, by many means the value of Tt can be lowered from above to below an operating temperature to drive hydrophobic folding and assembly. This is the ΔTt- mechanism. Modulation of charges on the polymer provides the most dramatic means of controlling Tt and therefore becomes the most effective means for controlling self-assembly. The formation of charge raises the value of Tt and causes disassembly, whereas neutralization of charge by lowering degree of ionization or by increasing ion-pairing drives self-assembly.For example, a polymer with one Asp(COO−) or Glu(COO−) per 30 residues can be in solution with its Tt above 100°C. Titration with a cationic drug lowers Tt to below 25°C and results in self-assembly into a drug delivery vehicle capable of a constant release profile for a constant surface area, and the vehicle simply disperses as the drug is released. Similarly an anionic drug can induce self-assemby of a cationic, e.g., Lys(NH3+)-containing, protein-based polymer.Two solutions of protein-based polymers, one polymer with negative charges, e.g., COO−, and the other with positive charges, e.g., NH3+ and both with hydrophobic residues sufficient to shift the pKa values of their respective functional groups, can exhibit their individual inverse temperature transitions at temperatures much higher than body temperature, even greater than 100°C. On combining the two solutions, the polymers self-assemble with a Tt below room temperature. Each polymer by ion pairing with the other dramatically lowers the temperature of the inverse temperature transition for polymer self-assembly. The effectiveness of this self-assembly of two oppositely charged protein-based polymers increases as the individual hydrophobic-induced pKa shifts are larger and as steric matching occurs in the ion-pairing between the pair of polymers. Furthermore, the same protein-based polymer can contain both positive and negative charges with hydrophobically shifted pKa values to become locked in self-assembled structures. This bears analogy to certain globular proteins, which on unfolding would similarly self-assemble to form the neurofibrillary tangles and amyloid plaques of Alzheimers and related diseases.In summary, ion-pairing within properly designed protein-based polymers results in self-assembling materials and structures by means of the ΔTt-mechanism.