Nature's complex copolymers: Engineering design of oligopeptide materials

Proteins evolved in nature can form precise structures exhibiting nanometer-length-scale features that have great potential for industrial application. However, many of these applications remain unrealized because of limitations in combinatorial and directed evolutionary approaches. An alternative approach that might be able to overcome these limitations is rational design of protein (or oligopeptide in the case of <50 amino acid) sequences. This article reviews the advances that have been made in understanding the protein sequence/structure problem by starting with "simple", repetitive sequences and building desired complexity. The review is paralleled by a discussion of micelle, colloid, and block copolymer literature to provide a polymer science context for understanding the protein-folding problem. We note that pentapeptide repeat units provide a useful definition of hydrophobicity, the key determinant of secondary structure, by causing the protein backbone to twist in order to become amphiphilic. Heptad repeating α-helices and hydrophobic-charged/polar-hydrophobic-charged/polar repeating β-sheets show that these amphiphiles can be driven into tertiary structures similarly to surfactants forming micelles. We describe how electrostatic charges can provide specificity for structure and assembly conditions. Finally, we provide several examples of protein materials that have been rationally designed using these principles.