Conformational control of inorganic adhesion in a designer protein engineered for cuprous oxide binding

Combinatorial selection of peptides that bind technological materials has emerged as a valuable tool for room-temperature nucleation and assembly of complex nanostructured materials. At present, the parameters that control peptide-solid binding are poorly understood, but such knowledge is needed to build the next generation of hybrid materials. Here, we use a derivative of the DNA binding protein TraI engineered with a disulfide-bonded cuprous oxide binding sequence called CN225 to probe the influence of sequence composition and conformation on Cu2O binding affinity. We previously reported a statistically significant enrichment in paired arginines (RR) among a family of cuprous oxide binding peptides and hypothesized that this is a key motif for binding. However, systematic alanine (A) substitutions in the CN225 RR motif (creating RA, AR, and AA pairs) do not support the hypothesis that RR is critical for Cu2O binding by CN225. Instead, we find that the presentation of the peptide in a disulfide-constrained loop (i.e., the conformation present during combinatorial selection) is crucial for binding to the metal oxide. Our results suggest that caution should be exerted when extrapolating from statistical data and that, in some cases, conformation is more important than composition in determining peptide-inorganic adhesion.