Free-floating ultrathin two-dimensional crystals from sequence-specific peptoid polymers

Journal name:
Nature Materials
Volume:
9,
Pages:
454–460
Year published:
DOI:
doi:10.1038/nmat2742
Received
Accepted
Published online

Abstract

The design and synthesis of protein-like polymers is a fundamental challenge in materials science. A biomimetic approach is to explore the impact of monomer sequence on non-natural polymer structure and function. We present the aqueous self-assembly of two peptoid polymers into extremely thin two-dimensional (2D) crystalline sheets directed by periodic amphiphilicity, electrostatic recognition and aromatic interactions. Peptoids are sequence-specific, oligo-N-substituted glycine polymers designed to mimic the structure and functionality of proteins. Mixing a 1:1 ratio of two oppositely charged peptoid 36mers of a specific sequence in aqueous solution results in the formation of giant, free-floating sheets with only 2.7nm thickness. Direct visualization of aligned individual peptoid chains in the sheet structure was achieved using aberration-corrected transmission electron microscopy. Specific binding of a protein to ligand-functionalized sheets was also demonstrated. The synthetic flexibility and biocompatibility of peptoids provide a flexible and robust platform for integrating functionality into defined 2D nanostructures.

At a glance

Figures

  1. Two-dimensional crystalline sheets formed from two oppositely charged peptoid polymers.
    Figure 1: Two-dimensional crystalline sheets formed from two oppositely charged peptoid polymers.

    Atomic colour scheme: carbon, yellow; nitrogen, blue; oxygen, red. a, Chemical structure of a negatively charged periodic amphiphilic peptoid, (Nce–Npe)18. b, Chemical structure of a positively charged periodic amphiphilic peptoid, (Nae–Npe)18 c, Molecular model of the sheets assembled from (Nae–Npe)18 and (Nce–Npe)18. The modelled conformation shows that hydrophobic groups face each other in the interior of the sheet and oppositely charged hydrophilic groups are alternating and surface-exposed.

  2. Imaging of 2D crystalline sheets assembled from periodic amphiphilic peptoid polymers.
    Figure 2: Imaging of 2D crystalline sheets assembled from periodic amphiphilic peptoid polymers.

    In typical conditions, 0.1mM of (Nae–Npe)18 and (Nce–Npe)18 were mixed in Tris–HCl buffer (pH 9.0, 100mM). a, Fluorescent optical microscope image of sheets stained with Nile Red (1μM) that are free-floating in aqueous solution. b, Fluorescent optical microscope image of individual sheets. c, SEM images of sheets on Si substrate. d, Height-mode AFM image of a sheet (Z range: 20nm).

  3. Direct observation of peptoid chains using aberration- corrected TEM (TEAM 0.5).
    Figure 3: Direct observation of peptoid chains using aberration- corrected TEM (TEAM 0.5).

    a, High-resolution TEM image of peptoids assembled into a sheet. Inset: Fourier transform of the image showing the alignment of peptoids along the sheet edge. b, Magnified image of the individual peptoids, fully extended along one direction in a sheet. TEAM 0.5 was operated at 80kV with a third-order spherical aberration tuned to Cs=−0.010mm. To decrease the energy spread of the incoming electron beam and consequently achieve sub-ångström resolution, the gun electron monochromator was inserted. As a negative value of the third-order spherical aberration was set in combination with a positive fifth-order spherical aberration, overfocusing yielded white-atom contrast.

  4. Characterization of sheets and sheet-formation kinetics.
    Figure 4: Characterization of sheets and sheet-formation kinetics.

    a, XRD data from the sheets. The three equally spaced peaks marked by arrows indicate that the thickness of the sheet is 27Å. The two peaks at 0.46 and 0.69Å−1 are lamellar harmonics of 27Å. b, Isothermal titration calorimetry curve for the (Nae–Npe)18/(Nce–Npe)18 system. Five-microlitre aliquots of 1.8mM (Nae–Npe)18 were titrated into 0.2mM of (Nce–Npe)18. c, Composition of sheets prepared with different initial mixing ratios of (Nae–Npe)18 and (Nce–Npe)18. All resulting sheets contained a 1:1 ratio as determined by analytical high-performance liquid chromatography. d, The yield of sheet formation depends on the relative concentration of (Nae–Npe)18/(Nce–Npe)18. Sheet population scales with the surface coverage as measured by fluorescence microscopy. X indicates that there is no sheet formation. According to our estimation, for the sample containing 0.1mM of (Nae–Npe)18 and (Nce–Npe)18, the yield of sheet formation is 64%.

  5. Sheet-formation kinetics and mechanism.
    Figure 5: Sheet-formation kinetics and mechanism.

    a, Fluorescence microscopy images of the cloudy solution 1h after mixing of the two peptoids. Only spheres are observed. b, Four hours after mixing. Some spheres begin to organize in a planar sheet-like structure as pointed out by the white arrow. c, Six hours after mixing. Well-defined sheets associated with high concentrations of spheres are observed. d, TEM image of spheres found 30min after mixing. The initial sheets protruding from spheres are pointed out by the black arrows. e, Study of sheet-formation kinetics using FRET efficiency and monitoring the transmittance change over time of 650nm light through the solution. f, Size distribution of sheets at two time points measured by SEM and fluorescence microscopy.

  6. Specific protein binding to functionalized sheets.
    Figure 6: Specific protein binding to functionalized sheets.

    a, The streptavidin-binding peptide sequence cyclo-[CHPQFC]- was incorporated at the N-terminus of the sheet-forming peptoid by means of a short hydrophilic linker. b, Self-assembly with the functionalized peptoid yields a similar population and morphology of nanosheets in comparison to un-functionalized peptoid as indicated by fluorescence microscopy using Nile Red. c, Ligand-bearing sheets show a high level of fluorescence after binding to Cy3–streptavidin. d, Un-functionalized sheets fluoresce very weakly under identical conditions and washes, indicating that binding to the modified sheets is specific.

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Author information

Affiliations

  1. Molecular Foundry; National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Ki Tae Nam,
    • Sarah A. Shelby,
    • Philip H. Choi,
    • Amanda B. Marciel,
    • Ritchie Chen,
    • Li Tan,
    • Tammy K. Chu,
    • Ryan A. Mesch,
    • Byoung-Chul Lee,
    • Michael D. Connolly &
    • Ronald N. Zuckermann

Contributions

K.T.N. and R.N.Z. designed research and wrote the paper. K.T.N., S.A.S., A.B.M., P.H.C., R.C. and L.T. carried out analytical experiments. T.K.C., R.A.M, B-C.L. and M.D.C. carried out chemical synthesis. C.K. carried out the aberration-corrected TEM. All authors discussed the results and commented on the manuscript. R.N.Z. guided all aspects of the work.

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The authors declare no competing financial interests.

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