Five axioms for the functional design of peptide-based polymers as molecular machines and materials: Principle for macromolecular assemblies

Abstract

Peptide-based polymers can be transformable hydrogels, elastomers, regular thermoplastics, or inverse thermoplastics, and can function as diverse molecular machines. When of the appropriate composition, these polymers exhibit hydrophobic folding and assembly transitions in the accessible aqueous range as the temperature is raised from below to above a critical value referred to as Tt, the temperature for the onset of an inverse temperature transition. In this article, the design of these Tt-type molecular machines and materials is systematized in terms of five axioms for peptide-based polymer engineering: The first axiom is represented by a hydrophobicity scale that provides the information with which to design peptide-based polymers with a particular value of Tt. The second axiom concerns thermomechanical transduction wherein raising the temperature from below to above Tt results in hydrophobic folding and assembly with the performance of mechanical work. The third axiom states that at constant temperature, diverse energy inputs lower Tt from above to below an operating temperature by acting on a functional moiety within the polymer to drive hydrophobic folding and assembly with the isothermal performance of mechanical work. The fourth axiom concerns polymer compositions having two different functional groups, with each responsive to a different energy input and with each part of a common hydrophobic folding domain. The two functional groups become coupled one to the other such that an energy input acting on one functional group becomes an energy output by having changed the property of the second functional group. The fifth axiom states that the efficiency of conversion from one form of energy to another by means of the fourth axiom increases nonlinearly with hydrophobicity of the common hydrophobic folding and assembly domain. These phenomenological axioms allow design of peptide-based polymers capable of diverse energy conversions involving the intensive variables of mechanical force, temperature, pressure, chemical potential, electrochemical potential, and light. Knowledge of the physical process underlying the five axioms enhances their use in the design of materials and devices for medical and nonmedical applications. The physical process is demonstrated by the use of cartoons. © 1998 John Wiley & Sons, Inc. Biopoly 47: 167–178, 1998