A New Mechanically-Interlocked [Pd2 L4 ] Cage Motif by Dimerization of two Peptide-based Lemniscates.

Most metallo-supramolecular assemblies of low nuclearity adopt simple topologies, with bridging ligands spanning neighboring metal centers in a direct fashion. Here we contribute a new structural motif to the family of host compounds with low metal count (two) that consists of a pair of doubly-interlocked, Figure-eight-shaped subunits, also termed "lemniscates". Each metal is chelated by two chiral bidentate ligands, composed of a peptidic macrocycle that resembles a natural product with two pyridyl-terminated arms. DFT calculation results suggest that dimerization of the mononuclear halves is driven by a combination of 1) Coulomb interaction with a central anion, 2) π-stacking between intertwined ligand arms and 3) dispersive interactions between the structure's compact inner core bedded into an outer shell composed of the cavitand-type macrocycles. The resulting cage-like architecture was characterized by NMR, MS and X-ray structure analyses. This new mechanically bonded system highlights the scope of structural variety accessible in metal-mediated self-assemblies composed of only a few constituents.

An azobenzene container showing a definite folding - synthesis and structural investigation.

The combination of photo-switchable units with macrocycles is a very interesting field in supramolecular chemistry. Here, we present the synthesis of a foldable container consisting of two different types of Lissoclinum macrocyclic peptides which are connected via two azobenzene units. The container is controllable by light: irradiation with UV light causes a switching process to the compact cis,cis-isomer, whereas by the use of visible light the stretched trans,trans-isomer is formed. By means of quantum chemical calculations and CD spectroscopy we could show that the trans→cis isomerization is spatially directed; that means that one of the two different macrocycles performs a definite clockwise rotation to the other, caused by irradiation with UV light. For the cis→trans isomerization counterclockwise rotations are found. Furthermore, quantum chemical calculations reveal that the energy of the cis,cis-isomer is only slightly higher than the energy of the cis,trans-isomer. This effect can be explained by the high dispersion energy in the compact cis,cis-isomer.

Linear Relationship between 13 C†NMR Chemical Shifts and the Bending of sp-Carbon Chains.

Polyynes show a strictly linear relationship between the energy impact and the bending of the polyyne chain. The energy, which is necessary to bend the acetylenic chain, decreases with the increasing number of acetylene units. A deviation from linearity in polyynes can be realized in solution by violation of the mutual-exclusion principle between IR and Raman spectra. However, there is still no possibility to measure the extent of the nonlinearity in solution. Herein, we show that the 13 C NMR spectroscopy represents an appropriate tool for this as we found an almost perfect linear relationship between the bending of the alkyne chain and the change of the chemical shift of the outer acetylenic carbon atoms. By using molecular bows in which the alkyne chain can be bent by switching the azobenzene unit, this correlation can be proved experimentally. In the future, this correlation should enable the determination of the extent of the bending and the strain energy in polyynes. Consequently, polyynes could be employed as probes for measuring further molecular forces.

Switching Process Consisting of Three Isomeric States of an Azobenzene Unit.

Azobenzene and its derivatives are among the most commonly used switching units in organic chemistry. The switching process consists of two states, in which the trans isomer has a stretched and the cis isomer a compact form. Here, we have designed a system in which all isomeric states of an azobenzene moiety (trans → cis-(M) → cis-(P)) are passed step by step. The first step involves a change in the distance between the benzene units, which is common for azobenzene derivatives. In the second step an inversion of the helicity (M→P) of the cis azobenzene unit takes place. The third step leads back to the stretched trans isomer. This switching cycle is achieved by coupling the azobenzene unit with two chiral clamps and with a further azobenzene switching unit.

Twisting of Alkynes towards a Carbon Double Helix.

The carbon allotrope exhibiting only one-dimensional sp-hybridized carbon atoms is called carbyne. However, its existence is very controversial. Studies on model compounds for carbyne revealed that many oligoalkynes show not a straight, but a bent structure of the carbon chain. Here, we question whether it would also be possible to obtain a more complex structure from carbyne, such as a dimeric double helix. Based on quantum chemical calculations, we show that only a small energetic expense is needed for the formation of a double helix starting from oligoalkyne chains. In some cases, the double helix-like conformation is more stable than the corresponding conformation with a parallel arrangement of the acetylene chains. Furthermore, model systems were synthesized in which two diphenyl oligoalkyne chains are fixed and twisted by a chiral imidazole-containing clamp. A structural investigation of these model systems was performed based on UV and CD spectroscopy and quantum chemical calculations. The observed twisting in these model systems can be regarded as the first small step towards an imaginable carbon double helix.

Bio-inspired Herringbone Foldamers: Strategy for Changing the Structure of Helices.

Cyclic oligomers of azole peptides were isolated from a multitude of marine organisms and were used for a large number of molecular machines. As shown previously, oligomers derived from achiral imidazole amino acids fold into canonical helices. Here we show that a minor change, the introduction of a methyl group in the δ position, results in a significant change in the secondary structure of the corresponding oligomers. Instead of a canonical helix, a noncanonical herringbone helix is formed. In the latter, the slope along the helix changes its sign at least twice per turn. This strategy allows a remarkable change of the secondary structure via a small modification. By means of enantiomerically pure amino acids, we were able to control, for the first time, both the helicity of the helix and the form of the herringbone. The investigation of the underlying herringbone basic element and its folding to a noncanonical helix were conducted by NMR and CD spectroscopy, as well as by X-ray crystallography and quantum chemical calculations.

Imidazole-peptide foldamers: parabolic dependence of the folding process on the water content of the solvent.

Oligomers of azole peptides have been isolated from a multitude of marine organisms. Up to now, these azole-containing dipeptide-analogue oligomers have only been found as cyclic n-mers (mostly tri- and tetramers) in nature. Herein, we show that imidazole-containing pseudopeptides form helixlike secondary structures in different solvents. The screw sense of the helix can be determined by attaching a single chiral imidazole unit to the N terminus of the oligomer. Investigation by means of circular dichroism (CD) spectroscopy showed that the folding process of the helix depends on the water content of the solvent in a parabolic way. In a pure organic medium, the helix is stabilized by hydrogen bonds between the hydrogen atoms of the amide groups and the nitrogen atoms of the azole ring. In aqueous solution, the formation of the helix is driven by dispersion interactions. The formation of the helix is more pronounced in aqueous solution than in organic solvents.