Spherical supramolecular aggregates of α-amino acids with a typical diameter of 100–200 nm are formed spontaneously after dissolution in water at a concentration of a few mM, i.e. well below the solubility limit. Their presence was shown by nanoparticle tracking analysis (NTA), atomic force microscopy (AFM), and ESI mass spectrometry (ESI-MS). There is a dynamic equilibrium between the aggregates and dissolved individual molecules which allows them to penetrate through dialysis membranes and filters. The same phenomenon was observed for para-amino salicylic acid and two dipeptides. Thermodynamic considerations suggest an entropy-controlled process

The aggregation of beta-cyclodextrin vesicles can be induced by an adamantyl-substituted zwitterionic guanidiniocarbonylpyrrole carboxylate guest molecule (1). Upon addition of 1 to the cyclodextrin vesicles at neutral pH, the vesicles aggregate (but do not fuse), as shown by using UV/Vis and fluorescence spectroscopy, dynamic light scattering, zeta-potential measurements, cryogenic transmission electron microscopy, and atomic force microscopy. Aggregation of the vesicles is induced by a twofold supramolecular interaction. First, the adamantyl group of 1 forms an inclusion complex with beta-cyclodextrin. Second, at neutral pH the guanidiniocarbonylpyrrole carboxylate zwitterion dimerizes through the formation of hydrogen-bonded ion pairs. Because the dimerization of 1 depends on the zwitterionic protonation state of 1, the aggregation of the cyclodextrin vesicles is also pH dependent; the cyclodextrin vesicles do not interact at pH 5 or 9, at which 1 is either cationic or anionic and, therefore, not self-complementary. These observations are consistent with molecular recognition of the vesicles through a combination of two different supramolecular interactions, that is, host-guest inclusion and dimerization of zwitterions, at the bilayer membrane surface.

Supramolecular nanoassemblies are gaining increasing importance as promising new materials with considerable potential for novel and promising applications. Within supramolecular nanoassemblies the connectivity of the monomeric units is based on reversible noncovalent interactions, like van der Waals interactions, hydrogen bonding, or ionic interactions. As the strength of these interactions depends on the molecular surrounding, the formation of nanoassemblies in principle can be controlled externally by changing the environment and/or the molecular shape of the underlying monomer. This way it is not only possible to switch the self-assembly on or off, but also to change between different aggregation states. In this minireview we present some recent selected approaches to supramolecular stimuli-responsive nanoassemblies.

This work describes a new type of vesicles that can be reversibly opened and closed by changing the protonation state of a self-assembling zwitterion. Vesicles are interesting nanomaterials with potential applications such as delivery systems for drug targeting. For this purpose, however, external control of vesicle formation is required. To date, vesicle formation has often been based on the self-assembly of amphiphilic macromolecules such as lipid derivatives or block copolymers. [1,2] Recently, vesicle formation from other types of building blocks, such as shape-persistent macrocyles, cyclodextrins, or small peptides has also been reported. [3] Most of these molecules are still classical amphiphiles and their self-assembly is mainly driven by hydrophobic or aromatic interactions.We have recently introduced a new type of vesicle forming-molecule, namely a self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion derived from alanine. We have shown by NOESY NMR studies in combination with TEM and AFM experiments that, in DMSO, the zwitterion forms ion-paired dimers that then further aggregate into hollow vesicles of approximately 40-50 nm diameter.[4] We have also used this special type of hydrogen-bond-assisted ion-pair formation between the guanidiniocarbonyl pyrrole cation [5] and a carboxylate unit for the construction of other types of nanostructures such as dimers, [6] loops, [7] and supramolecular polymers.[8] As the guanidiniocarbonyl pyrrole moiety has an approximate pK a value of 6-7, ion-pair formation with a carboxylate (pK a ca. 3-5) only occurs within a narrow pH range. This restriction offers the possibility to deliberately turn the ion-pair formation on or off by changing the pH from neutral to slightly acidic or basic.[9] We therefore wanted to explore whether we can also trigger vesicle formation by using pH as an external signal.[10] To test this hypothesis, we have synthesized a serine-derived guanidiniocarbonyl pyrrole carboxylate zwitterion 3 and studied its vesicle formation on surface with AFM as well as in solution by using dynamic light scattering (DLS) and a pulsed field gradient (PFG)-based NMR method. The latter method allows us to selectively detect the solvent molecules encapsulated in the vesicles and also to quantify the permeability of the vesicle membranes.[11] We show herein that vesicle formation of 3 can indeed be switched on and off in a controlled manner by reversible changes of its protonation state. Furthermore, we found that the vesicles are rather impermeable and the measured exchange rate of solvent molecules across the membrane is very small.The synthesis of the serine-derived guanidiniocarbonyl pyrrole carboxylate zwitterion 3 is shown in Scheme 1. l-serine methyl ester hydrochloride 2 was coupled with the Boc-protected guanidinocarbonyl pyrrole carboxylic acid 1 [12] using PyBOP in a mixture of CH 2 Cl 2 and DMF as the coupling reagent (67 % yield). The Boc group was cleaved with TFA, and the methyl ester was subsequently hydrolyzed with LiOH to produce ...

The neutral heteroleptic hexacoordinate silicon(IV) complexes 4 and 5 (SiO(6) skeletons) and the neutral pentacoordinate silicon(IV) complexes 7-9 (SiO(4)N skeletons) were synthesized, starting from the hexacoordinate precursor 2 and the pentacoordinate precursor 6, respectively. In these reactions, two monoanionic cyanato-N ligands are replaced by one dianionic bidentate O,O-chelate ligand. Compounds 4, 5, and 7-9 were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. The chiral silicon(IV) complexes 4, 5, 7, and 8 were obtained as racemic mixtures, whereas 9 was isolated as a cocrystallizate consisting of the two diastereomers, (C,S)-9 and (A,S)-9 (ratio 1:1). The stereodynamics of 5 and 8 were studied by variable-temperature (1)H NMR experiments.