Functionalization of nanoparticles with biopolymers has yielded a wide range of structured and responsive hybrid materials. DNA provides the ability to program length and recognition using complementary oligonucleotide sequences. Nature more often leverages the versatility of proteins, however, where structure, assembly, and recognition are more subtle to engineer. Herein, a protein was computationally designed to present multiple Zn2+ coordination sites and cooperatively self-associate to form an antiparallel helical homodimer. Each subunit was unstructured in the absence of Zn2+ or when the cation was sequestered with a chelating agent. When bound to the surface of gold nanoparticles via cysteine, the protein provided a reversible molecular linkage between particles. Nanoparticle association and changes in interparticle separation were monitored by redshifts in the surface plasmon resonance (SPR) band and by transmission electron microscopy (TEM). Titrations with Zn2+ revealed sigmoidal transitions at submicromolar concentrations. The metal-ion concentration required to trigger association varied with the loading of the proteins on the nanoparticles, the solution ionic strength, and the cation employed. Specifying the number of helical (heptad) repeat units conferred control over protein length and nanoparticle separation. Two different length proteins were designed via extension of the helical structure. TEM and extinction measurements revealed distributions of nanoparticle separations consistent with the expected protein structures. Nanoparticle association, interparticle separation, and SPR properties can be tuned using computationally designed proteins, where protein structure, folding, length, and response to molecular species such as Zn2+ can be engineered.
Bibliographical noteFunding Information:
The authors acknowledge support from National Science Foundation (NSF) under award CHE 1508318; the authors also acknowledge partial support from the NSF DMREF program (DMR-1234161), CHE-1412496, and CHE-1709518. J.G.S. acknowledges additional support from the Penn Laboratory for Research on the Structure of Matter (NSF DMR-1120901), the National Institutes of Health (R01 GM-071628, R01 HL-085303). S.-J. P. acknowledges the support from the National Research Foundation of Korea grant funded by the Korea government (MSIP) (NRF-2015R1A2A2A01003528). This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575, under grant number TG-CHE110041. We thank Benjamin T. Diroll for assistance with SAXS studies on the citrate-capped AuNPs. We thank Daniel A. Hammer for useful discussions. We thank Ronen Marmorstein and Michael Grasso for assistance in collection of the AUC data.
© 2017 American Chemical Society.