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Jiajun Chen1 2 Harley Pyles3 Enbo Zhu4 Juan Liu5 David Baker3 Yu Huang4 Hendrik Heinz5 James De Yoreo1

1, Pacific Northwest National Laboratory, Richland, Washington, United States
2, University of Washington, Seattle, Washington, United States
3, University of Washington, Seattle, Washington, United States
4, University of California, Los Angeles, Los Angeles, California, United States
5, University of Colorado Boulder, Boulder, Colorado, United States

In Nature, sequence-specific interactions between proteins and inorganic materials lead to formation of hierarchical structures exhibiting complex functions. Achieving the same level of hierarchy and function in synthetic materials requires an understanding of how sequence combines with macromolecular-solvent-surface interactions to control assembly dynamics and materials architecture. Here we report the results of in situ AFM investigations aimed at gaining that understanding for two systems: synthetic proteins designed via Rosetta de novo design software to assemble on mica (001) and synthetic peptides with sequences selected via phage display to bind to MoS2 (0001). The synthetic proteins consisted of nanorods of repeating cross-helical subunits designed to give an epitaxial match between carboxylic side chains and the K+ sub-lattice of mica. As K+ solution concentration was varied from 10 mM to 3M, protein coverage and order increased, going from sparse, non-interacting nanorods to small domains of co-aligned nanorods to highly-ordered 2D smectic crystals extending across many mm. The development of order was highly dynamic and cooperative with ordered domains initially forming and disappearing rapidly (~1-10s). The time for stable, ordered structures to emerge depended on K+ and protein concentrations and the degree of order depended on the number of sub-units in a nanorod. Short repeats (2-4) exhibited poor order and long repeats (18) produced highly ordered arrays. Introduction of end-to-end interactions further modified assembly to produce long, continuous, co-aligned protein nanowires. The results suggest these proteins exhibit many features of colloidal systems due to their rigidity and high surface charge. However, the ability to vary protein-protein, protein-substrate, and solution-protein interactions by design enables the assembly dynamics and order to be tuned. The phage-selected MoS2-binding peptides also assembled into highly-ordered 2D arrays. These arrays consisted of monomer-high rows of dimers exhibiting an epitaxial match to the underlying MoS2 substrate. During assembly, peptides first joined to form dimers as the smallest building blocks. These small units then packed closely to generate rows with a width of 4.1nm and aligning at 30° to the densest S-atom packing direction of the MoS2 lattice. High-resolution structural images and comparison between different sequences indicate the existence and position of phenyl groups play an important role in surface attachment and the inter-molecular interactions leading to assembly. MD simulations predict binding energies are on the order ~100 kcal/mol and that the dimers are further stabilized via hydrogen bonds. Although the final arrays are 2D, due to the 1D nature of the constituent rows, there is no free energy barrier to nucleation and no critical size. Thus, nucleation rates vary linearly with concentration and are finite for all concentrations above the solubility limit

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