Chemical Science Blog

Scientists from the UK have used a molecular capsule to control the reactivity of an organic compound.

A molecule’s reactivity is commonly controlled using protecting group chemistry, in which a certain functional group is blocked to avoid unwanted reactions at that site. However, this can be costly in terms of time and reagents.

Now, Maarten Smulders and Jonathan Nitschke from the University of Cambridge have used a supramolecular cage as a whole-molecule protecting group. An organic molecule can be encapsulated within the cage, preventing its reaction, and addition of a competing guest can then release the molecule and initiate the reaction when required.

The team constructed the cage from four iron units in a tetrahedral arrangement. External sulfonate groups confer water solubility, while the interior of the cage provides a hydrophobic environment to encapsulate organic molecules. Unlike traditional protecting group chemistry, this method relies not on the functionality of the molecule but on its size and shape, providing a complementary protection strategy to those commonly used.

Encapsulation of furan prevents its reaction, but upon addition of benzene as a competing guest, furan is released from the cage and undergoes a Diels-Alder reaction with maleimide

Read the full story in Chemistry World

Link to journal article
Supramolecular control over Diels–Alder reactivity by encapsulation and competitive displacement
Maarten M. J. Smulders and Jonathan R. Nitschke
Chem. Sci., 2012, Advance Article
DOI: 10.1039/C1SC00847A

New guanosine-based gels for use as tissue engineering scaffolds have been made by scientists in the US. The gels can gel aqueous solutions and cell media at physiological salt concentrations. They are also injectable and non-toxic to cells.

Previous gels have had limited use because of poor lifetime stability and the need for specific salt concentrations or pH values that are not physiological. These new gels form helical assemblies rather than the quartet assemblies that are normally found in guanosine gelators. This allows them to form hydrogels at physiological salt concentrations – as low as 0.5wt% in 100mM NaCl.

The researchers acknowledge that there are issues to be addressed, such as the development of gel systems that allow cell adhesion without added gelatine, and studies into the lifetime of the gels in cell media.

Reference:
Toward Potential Supramolecular Tissue Engineering Scaffolds Based on Guanosine Derivatives
L E Buerkle, H A von Recum and S J Rowan, Chem. Sci., 2011
DOI: 10.1039/c1sc00729g

It is possible to induce a pronounced structural reorganisation of a metallasupramolecular cage complex with a very conservative change in solvent polarity, say researchers from Switzerland and Turkey.

Solvent-switchable nanostructures reported so far operate via solvent-induced reduction or stabilisation of non-covalent interactions. This cage switches from octanuclear prismatic in chloroform to tetranuclear in dichloromethane. The key to success here is the incorporation of metallacrown recognition units into flexible nanostructures, which allowed generation of solvent specific bonding pockets.

Reference:
A solvent-responsive coordination cage
B Kilbas, S Mirtschin, R Scopelliti and K Severin, Chem. Sci., 2011
DOI: 10.1039/c1sc00779c

A molecule that can distinguish between structurally similar carboxylic acids and organoboronic acids – a significant analytical challenge – has been made by US researchers. The molecule is a cruciform (cross-shaped).

Identifying compounds with closely related structures like this is important to identify counterfeited, decomposed or compromised pharmaceuticals, food additives and alcoholic beverages. The compound can distinguish between 12 carboxylic acids and nine organoboronic acids. It works by binding to the compounds and giving a different fluorescent signal for each one.

Reference:
Identification of Carboxylic and Organoboronic Acids and Phenols with a Single Benzobisoxazole Fluorophore
J Lim, D Nam and O S Miljanic, Chem. Sci., 2011
DOI: 10.1039/c1sc00610j

The RSC Macrocylic and Supramolecular Chemistry meeting (MASC-11) is taking place on the 19^th and 20^th December 2011 at the University of Bath, UK.

A number of high profile authors will be speaking at the meeting including Chem Soc Rev Associate Editor, Professor Phil Gale, and Professor Kay Severin who is giving the Chemical Science sponsored lecture. To find out more about Professor Severin’s research, download his latest Chemical Science articles:

• Dative boron–nitrogen bonds in structural supramolecular chemistry: multicomponent assembly of prismatic organic cages
• Constitutionally selective amplification of multicomponent 84-membered macrocyclic hosts for (−)-cytidine•H⁺

Dr Jonathan Nitschke will be delivering the Dalton Transactions lecture as a result of being awarded the 2011 Dalton Transactions European/African Lectureship earlier on this year. Jonathan has also recently been awarded the ChemComm 2012 Cram Lehn Pedersen award.

Read more about Jonathan’s research by downloading his Chemical Science articles: 

• Selective anion binding by a “Chameleon” capsule with a dynamically reconfigurable exterior 
• Reactivity modulation in container molecules.

ChemComm, Chemical Science and Chem Soc Rev Deputy Editor, Joanne Thomson, will be attending the event. If you would like to arrange a meeting with Joanne, please email her at the Editorial Office.

To attend MASC-11, register before the 12^th December 2011. The deadline for poster abstract submissions is 1^st December.

Researchers from the University of Reading have designed an organocatalysed cascade reaction for the construction of nitrocyclohexanes 4.

This elegant domino reaction enables the union of two achiral reagents to generate products containing up to five contiguous stereocentres in excellent levels of enantio- and diastereoselectivity.

André Cobb’s group employed a thiourea catalyst 3 to initiate a Michael-Michael cascade reaction between nitro-esters 1 and nitro-styrenes 2. The catalyst is thought to first coordinate to the nitro-ester prior to intramolecular deprotonation at the α-position to generate a nitronate. Synchronous coordination with nitrostyrene enables the first stereoselective Michael addition to generate a second nitronate primed for cyclisation onto the conjugated ester.

This cascade process demonstrates the power of organocatalysis for the asymmetric assembly of complex molecular architecture from simple starting materials.

Read more – download Cobb’s Edge article.

A DNA-based computational device with variable input has been developed by scientists led by Nadrian Seeman in the US. Previous examples of DNA-based computers have used fixed systems, but here they used the same system with different settings. This means that the system could be reusable, unlike other systems that perform one computation and then can’t be used again.

The readout of the system is obtained by algorithmically assembling gold nanoparticles. Although arranging gold nanoparticles by DNA self-assembly has been reported, this is the first time such an arrangement of nanoparticles has been obtained as the result of a computation.  The system may be considered as a first step in an assembly of a programmed array incorporating nanomechanical devices or other varieties of biochemical, chemical or materials systems that can be organised as the result of a simple computation.

Reference:
A Programmable Transducer Self-Assembled from DNA
B Chakraborty, N Jonoska and N C Seeman, Chem. Sci., 2011
DOI: 10.1039/c1sc00523e