Protein-directed self-assembly of a fullerene crystal

Abstract

Learning to engineer self-assembly would enable the precise organization of molecules by design to create matter with tailored properties. Here we demonstrate that proteins can direct the self-assembly of buckminsterfullerene (C60) into ordered superstructures. A previously engineered tetrameric helical bundle binds C60 in solution, rendering it water soluble. Two tetramers associate with one C60, promoting further organization revealed in a 1.67-Å crystal structure. Fullerene groups occupy periodic lattice sites, sandwiched between two Tyr residues from adjacent tetramers. Strikingly, the assembly exhibits high charge conductance, whereas both the protein-alone crystal and amorphous C60 are electrically insulating. The affinity of C60 for its crystal-binding site is estimated to be in the nanomolar range, with lattices of known protein crystals geometrically compatible with incorporating the motif. Taken together, these findings suggest a new means of organizing fullerene molecules into a rich variety of lattices to generate new properties by design.

Figure 1. Protein/C₆₀ super-assembly.

(a) COP, a stable tetramer in isolation, interacts with C₆₀ moieties by means of a surface-binding site that includes Tyr residues (other aromatic side chains also likely admissible), and further self-assembles into a co-crystalline array with fullerene. (b) Ultraviolet absorption spectra of a C₆₀/COP suspension and COP alone demonstrate that primitive fullerene (green) dissolves in the aqueous phase in the presence of protein. (c) SEC traces of COP alone or in association with C₆₀ or C₆₀Sol. Top and bottom plots show absorbances at 340 and 220 nm, respectively. The lower-retention peaks arising due the addition of C₆₀ or C₆₀Sol are consistent with the molecular weight of a COP octamer (for example, dimer of tetramers; Supplementary Fig. 10). (d) Each COP tetramer in the C₆₀Sol–COP crystal is associated with four fullerenes (one per chain), each fullerene being wedged between two adjacent COP tetramers, for an overall stoichiometry of two fullerenes for one COP tetramers. (e) Omit map (2F_o−F_c, contoured at 1.2σ) showing electron density of the C₆₀ group (orange sticks) sandwiched via π–π stacking between Tyr residues from adjacent COPs. (f) Residues involved in C₆₀ coordination are shown with sticks and labelled. (g) Surface representation of the C₆₀ coordination site, coloured by relative in vacuo electrostatic potential (red to blue corresponds to negative-to-positive relative potentials).

Figure 2. COP crystal adjusts to incorporate fullerene.

(a) Superposition of apo COP (green) and C₆₀Sol–COP (cyan) shows no significant structural changes in the helix bundle. Side-chain differences around the fullerene-binding site are highlighted in the box. (b) The distance between aromatic Tyr9 residues adjusts in C₆₀Sol–COP to incorporate the fullerene. (c) Different views of the COP–fullerene lattice. (d) Significant changes in the crystal lattice between COP alone and C₆₀Sol–COP structures. Viewed from the top, C₆₀Sol–COP forms a honeycomb structure (ii), whereas apo COP exhibits a tetrameric cube pattern (i).

Figure 3. Assembly of fullerenes endows crystal with electronic transport capabilities.

(a) Three views of the C₆₀Sol–COP crystal lattice. (b) C₆₀ groups are arranged in a helical manner along parallel inner channels in the assembly. (c) A side view of the channel showing nearest-neighbor inter-C₆₀ distances. (d) Semi-logarithmic current–voltage characteristic of C₆₀Sol–COP supercrystal (red dots) and disordered C₆₀Sol–COP (orange diamonds). Disordered C₆₀ film dried from a bare C₆₀/toluene solution (green squares), crystal buffer solution (blue triangles) and a COP-alone protein crystal (violet open circles) were also characterized as controls.