Organic & Biomolecular Chemistry Blog

Cyclodextrins are macrocyclic oligomers of α-D-glucose. Their interaction with guest molecules is largely driven by the hydrophobic effect (the expulsion of water from the lipophilic cavity, into bulk, is entropically favourable).

In this HOT paper Cepeda and co-workers explore the solution phase behaviour of β-cyclodextrin/alkyltrimethylammonium bromide mixtures, varying the surfactant alkyl chain length from hexyl (C₆) to octadecyl (C₁₈).

Cepeda et al. employ the solvolysis of methoxybenzenesulfonyl chloride (MBSC) as a UV ‘handle’ to monitor the interaction between β-cyclodextrin and surfactant guests. In the presence of aggregated cationic surfactants, MBSC hydrolysis is hindered, owing to the reduced polarity of the micelle interior. MBSC solvolysis can be used to monitor the integrity of surfactant micelles and to calculate the concentration of unbound cyclodextrin.

In the presence of aggregated surfactants Cepeda et al. detect a significant concentration of ‘free’ β-cyclodextrin; the highest of which with decyl (C₁₀) and dodecyl (C₁₂) surfactants. This reflects the balance between the hydrophobic effect (smaller surfactant chains displace less water from the cyclodextrin cavity) and surfactant self-assembly (longer surfactant chains favour micelle formation).  The new model created to describe these interactions will aid the further development of commercial applications for these systems.

Competition between surfactant micellization and complexation by cyclodextrin
M. Cepeda, R. Daviña, L. García-Río, M. Parajó, P. Rodríguez-Dafonte and M. Pessêgo
DOI: 10.1039/C2OB26318A

Published on behalf of Steve Moore, Organic & Biomolecular Chemistry web science writer.

Researchers in Switzerland have discovered a fluxional molecule that dynamically resolves to a single metastable structural isomer.

Bullvalene is an intriguing molecule. A small polycycle with 10 carbons and 10 hydrogens, and an unusual property: it has no permanent chemical structure.

Its bonds are constantly rearranging through a seemingly endless series of Cope rearrangements. It’s estimated that there are over 1.2 million possible valence tautomers, and due to the rapidity of the conversions, all carbons and protons appear as equivalent on the NMR timescale.

Professor Jeff Bode and his group at ETH Zürich have an interest in these fluxional molecules and their potential uses as chemical sensors.

During a recent investigation into the racemisation of oligo-substituted members of the bullvalene family, they noticed some interesting behaviour in a sample of a tetrasubstituted bullvalene. A single isomer was spontaneously forming, and could be isolated from the mixture. This isomer was surprisingly stable, with a half-life of just over 4 hours at room temperature.

Now, tetrasubstituted bullvalenes have 1680 possible structural isomers and previous work has shown that the mixture is fully dynamic, so the appearance of just one isomer above all the others is pretty remarkable.

Their latest HOT PAPER builds on these results, and seeks to provide further information about the nature of this fascinating molecule.

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