Steric Constraints Induced Frustrated Growth of Supramolecular Nanorods in Water

A unique example of supramolecular polymerisation in water based on monomers with nanomolar affinities, which yield rod-like materials with extraordinarily high thermodynamic stability, yet of finite length, is reported. A small library of charge-neutral dendritic peptide amphiphiles was prepared, with a branched nonaphenylalanine-based core that was conjugated to hydrophilic dendrons of variable steric demand. Below a critical size of the dendron, the monomers assemble into nanorod-like polymers, whereas for larger dendritic side chains frustrated growth into near isotropic particles is observed. The supramolecular morphologies observed by electron microscopy, X-ray scattering and diffusion NMR spectroscopy studies are in agreement with the mechanistic insights obtained from fitting polymerisation profiles: non-cooperative isodesmic growth leads to degrees of polymerisation that match the experimentally determined nanorod contour lengths of close to 70 nm. The reported designs for aqueous self-assembly into well-defined anisotropic particles has promising potential for biomedical applications and the development of functional supramolecular biomaterials, with emerging evidence that anisotropic shapes in carrier design outperform conventional isotropic materials for targeted imaging and therapy.

Investigation of the Ice-Binding Site of an Insect Antifreeze Protein Using Sum-Frequency Generation Spectroscopy

We study the ice-binding site (IBS) of a hyperactive antifreeze protein from the beetle Dendroides canadensis (DAFP-1) using vibrational sum-frequency generation spectroscopy. We find that DAFP-1 accumulates at the air-water interface due to the hydrophobic character of its threonine-rich IBS while retaining its highly regular β-helical fold. We observe a narrow band at 3485 cm(-1) that we assign to the O-H stretch vibration of threonine hydroxyl groups of the IBS. The narrow character of the 3485 cm(-1) band suggests that the hydrogen bonds between the threonine residues at the IBS and adjacent water molecules are quite similar in strength, indicating that the IBS of DAFP-1 is extremely well-ordered, with the threonine side chains showing identical rotameric confirmations. The hydrogen-bonded water molecules do not form an ordered ice-like layer, as was recently observed for the moderate antifreeze protein type III. It thus appears that the antifreeze action of DAFP-1 does not require the presence of ordered water but likely results from the direct binding of its highly ordered array of threonine residues to the ice surface.

Liquid crystals of self-assembled DNA bottlebrushes

Early theories for bottlebrush polymers have suggested that the so-called main-chain stiffening effect caused by the presence of a dense corona of side chains along a central main chain should lead to an increased ratio of effective persistence length (lp,eff) over the effective thickness (Deff) and, hence, ultimately to lyotropic liquid crystalline behavior. More recent theories and simulations suggest that lp,eff ∼ Deff, such that no liquid crystalline behavior is induced by bottlebrushes. In this paper we investigate experimentally how lyotropic liquid crystalline behavior of a semiflexible polymer is affected by a dense coating of side chains. We use semiflexible DNA as the main chain. A genetically engineered diblock protein polymer C4K12 is used to physically adsorb long side chains on the DNA. The C4K12 protein polymer consists of a positively charged binding block (12 lysines, K12) and a hydrophilic random coil block of 400 amino acids (C4). From light scattering we find that, at low ionic strength (10 mM Tris-HCl), the thickness of the self-assembled DNA bottlebrushes is on the order of 30 nm and the effective grafting density is 1 side chain per 2.7 nm of DNA main chain. We find these self-assembled DNA bottlebrushes form birefringent lyotropic liquid crystalline phases at DNA concentrations as low as 8 mg/mL, roughly 1 order of magnitude lower than for bare DNA. Using small-angle X-ray scattering (SAXS) we show that, at DNA concentrations of 12 mg/mL, there is a transition to a hexagonal phase. We also show that, while the effective persistence length increases due to the bottlebrush coating, the effective thickness of the bottlebrush increases even more, such that in our case the bottlebrush coating reduces the effective aspect ratio of the DNA. This is in agreement with theoretical estimates that show that, in most cases of practical interest, a bottlebrush coating will lead to a decrease of the effective aspect ratio, whereas, only for bottlebrushes with extremely long side chains at very high grafting densities, a bottlebrush coating may be expected to lead to an increase of the effective aspect ratio.

Supramolecular polymers for organocatalysis in water

An l-proline-functionalised benzene-1,3,5-tricarboxamide derivative self-assembles in water into well-defined, one-dimensional, helical, supramolecular polymers that efficiently catalyse aldol reactions.

Stimuli-responsive colloidal assembly driven by surface-grafted supramolecular moieties

A robust method is described for precisely functionalizing silica colloids with short-chain alkanes and self-associating o-nitrobenzyl protected benzene-1,3,5-tricarboxamides (BTAs). Controlled deprotection affords activation of the latent supramolecular moieties by facilitating short-range hydrogen-bonding interactions between surface-functionalized silica particles. Control of mesoscale assembly of the responsive colloidal suspensions is demonstrated with two different external triggers. First, the amount of active (i.e., deprotected) BTAs is efficiently tuned by varying the exposure time to UV radiation. Controlled activation of the BTAs translates to regulating the valence of the system. After activation, the binding strength of individual BTAs can be modulated with temperature, providing an additional handle with which the assembly behavior is manipulated. This dual-regulation approach is a powerful and sensitive avenue for controlling colloidal assembly processes.

Solution structure of a cucurbit[8]uril induced compact supramolecular protein dimer

Supramolecular assembly of a beta-barrel protein via cucurbit[8]uril results in compact z-shaped protein dimers. SAXS data reveal the formation of a well ordered protein dimer, notwithstanding being connected by a reversible and flexible peptide linker, and highlight the supramolecular induced interplay of the proteins, analogous to covalently linked proteins.

Controlling the number of dendrimers in dendrimicelle nanoconjugates from 1 to more than 100

Herein, we present a facile strategy to controllably build up dendrimicelles by self-assembly of anionic PAMAM dendrimers with cationic-neutral diblock copolymers. We present a systematic study incorporating a full decade (0-9) of dendrimer generations, tracing the gradual variation from aggregates (G0 and G1) to self-assembled micelles (G2-G8), and an unidendrimer micelle structure (G9) by different scattering techniques (light and X-ray). The formed micelles (G2-G9) are spherical in shape with a hydrodynamic radius of about 25 nm. Interestingly, the micellar size, structure and number of incorporated block copolymers are independent of the dendrimer generation (for G2 to G9), while the aggregation number of the dendrimers decreases from 108 to 1, and the stability of the micelles increases upon an increase in the dendrimer generation. Moreover, the micelles with lower generation dendrimers transform from spherical into worm-like structures upon an increase in the positive charge fraction (excess polymers) or ionic strength, while micelles with higher generation dendrimers do not show such a transition. This differential behavior is in-line with a change from a flexible configuration into rigid globular nanoparticles with increasing dendrimer generation. The reported systematic investigation of dendrimicelles comprising a full decade of dendrimer generations provides the basis for versatile strategies focused on building up new (multi)functional materials, e.g. by packing multiple types of dendrimers with different functional groups or encapsulated cargos controllably within one micelle.

Observation of vibrational energy exchange in a type-III antifreeze protein

We performed time- and polarization-resolved pump-probe and two-dimensional infrared (2D-IR) experiments to study the dynamics of the amide I vibration of a 7 kDa type-III antifreeze protein. In the pump-probe experiments, we used femtosecond mid-infrared pulses to investigate the vibrational relaxation dynamics of the amide mode. The transient spectra show the presence of two spectral components that decay with different lifetimes, indicative of the presence of two distinct amide subbands. The 2D-IR experiments reveal the coupling between the two bands in the form of cross-peaks. On the basis of previous work by Demirdöven et al. ( J. Am. Chem. Soc. 2004 , 126 , 7981 - 7990 ), we assign the observed bands to the two infrared-active modes α(-) and α(+) found in protein β-sheets. The amplitudes of the cross-peak were found to increase with delay time, indicating that the cross-peaks originate from population transfer between the coupled amide oscillators. The time constant of the energy transfer was found to be 6-7 ps.

Triple helix formation in amphiphilic discotics: demystifying solvent effects in supramolecular self-assembly

A set of chiral, amphiphilic, self-assembling discotic molecules based on the 3,3'-bis(acylamino)-2,2'-bipyridine-substituted benzene-1,3,5-tricarboxamide motif (BiPy-BTA) was prepared. Amphiphilicity was induced into the discotic molecules by an asymmetrical distribution of alkyl and oligo(ethylene oxide) groups in the periphery of the molecules. Small-angle X-ray scattering, cryogenic transmission electron microscopy, and circular dichroism spectroscopy measurements were performed on the discotic amphiphiles in mixtures of water and alcohol at temperatures between 0 °C an 90 °C. The combined results show that these amphiphilic discotic molecules self-assemble into supramolecular fibers consisting of either one or three discotic molecules in the fiber cross-section and that the presence of water induces the bundling of the supramolecular fibers. The rich phase behavior observed for these molecules proves to be intimately connected to the mixing thermodynamics of the water-alcohol mixtures.

Mechanistic control over morphology: self-assembly of a discotic amphiphile in water

We report on the self-assembly of discotic amphiphiles that contain chelated gadolinium(iii) ions and are based on the C3-symmetrical benzene-1,3,5-tricarboxamide motif. Fluorescence spectroscopy, SAXS and cryo-TEM experiments demonstrate that a bimodal distribution of small and large aggregates is formed in a ratio that is dependent on the ionic strength. The results correlate with the previously reported degree of cooperativity of the polymerization mechanism, which increases with increasing NaCl concentration. Hence, by tuning the electrostatic interactions between the ligands at the periphery we can tune the cooperativity of the self-assembly. This tunability provides a versatile handle to adjust the size and shape of the discotic amphiphiles, which have potential as supramolecular MRI contrast agents.