Controlling the growth and shape of chiral supramolecular polymers in water

Abstract

A challenging target in the noncovalent synthesis of nanostructured functional materials is the formation of uniform features that exhibit well-defined properties, e.g., precise control over the aggregate shape, size, and stability. In particular, for aqueous-based one-dimensional supramolecular polymers, this is a daunting task. Here we disclose a strategy based on self-assembling discotic amphiphiles that leads to the control over stack length and shape of ordered, chiral columnar aggregates. By balancing out attractive noncovalent forces within the hydrophobic core of the polymerizing building blocks with electrostatic repulsive interactions on the hydrophilic rim we managed to switch from elongated, rod-like assemblies to small and discrete objects. Intriguingly this rod-to-sphere transition is expressed in a loss of cooperativity in the temperature-dependent self-assembly mechanism. The aggregates were characterized using circular dichroism, UV and 1H-NMR spectroscopy, small angle X-ray scattering, and cryotransmission electron microscopy. In analogy to many systems found in biology, mechanistic details of the self-assembly pathways emphasize the importance of cooperativity as a key feature that dictates the physical properties of the produced supramolecular polymers.

Fig. 1.

Schematic representation of the self-assembly of discotic amphiphiles: The hydrophobic BTA core (solid circle) directs the self-assembly into a helical architecture; the hydrophobic, chiral amino acid substituents (dashed line) in the second layer determine the handedness and stability of the helix; the peripheral hydrophilic groups introduce an amphiphilic character to the design.

Fig. 2.

Schematic representations of the fluorinated discotic amphiphiles: the BTA core, chiral amino acid (L pentafluorophenylalanine), and aminobenzoate spacer are schematically depicted in green; the peripheral hydrophilic M(III) complexes (depicted in blue) introduce an increasingly pronounced “ionic character” (depicted in red) to the molecular building blocks: overall neutral M(III)-DOTA 1, singly charged M(III)-DTPA-N-MA 2, and doubly charged M(III)-DTPA 3 [for paramagnetic discotics 1a, 2, and 3a M(III) = Gd(III); for diamagnetic discotic 1b and 3b M(III) = Y(III)].

Fig. 3.

SAXS profiles for the fluorinated discotics 1a (A) and 3a (B) in citrate buffer (100 mM, pH 6).

Fig. 4.

Cryo-TEM micrographs for self-assembled discotic amphiphile 1a (0.66 mM) vitrified at 288 K in citrate buffer (100 mM, pH 6); scale bar represents 50 nm.

Fig. 5.

Cryo-TEM micrographs for discotic amphiphile 3a (1 mM) vitrified at 288 K in citrate buffer (100 mM, pH 6); scale bar represents 50 nm.

Fig. 6.

Schematic 3D representation of a stack of 11 monomers of 3a. (Left) Side view of the stack; (Right) top view.

Fig. 7.

Temperature-dependent CD spectra in a 100-mM citrate buffer at pH 6: (A) discotic 1a (5·10^-6 M), (B) discotic 2a (2.5·10^-6 M) and discotic 3a (20·10^-6 M) and the resulting normalized and fitted CD cooling curves monitored at D λ = 282 nm for discotic 1a, (E) λ = 283 nm for discotic 2a, (F) λ = 269 nm for discotic 3a. The lower three graphs show the normalized data as degree of aggregation Φ_n vs. temperature: 0 refers to the molecular dissolved state and 1 to a fully polymerized system; red lines represent the best fits (37, 54).

Fig. 8.

(A) Temperature-dependent CD spectra in a citrate buffer (100 mM, pH 6) and overall NaCl concentration of 0.5 M, of discotic 3a (7·10^-6 M), and (B) the resulting normalized and fitted CD cooling curves monitored at λ = 276 nm.

Fig. 9.

Cryo-TEM micrographs for discotic amphiphile 3a (1 mM) vitrified at 288 K in citrate buffer (100 mM, pH 6) and overall NaCl concentration of 3 M (Left) and 5 M (Right); scale bar represents 50 nm.