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Novel disulfide reducing agent

07 December 2019 - Orgo

Mthembu et al. report on a new reducing agent for disulfides (DOI). A range of such agents widely used in biochemistry are 2-mercaptoethanol, dithiothreitol (DTT) and TCEP and serve deprotection purpose and can cleave spacers. They all tend to suffer from poor reducing power, low reactivity, limited pH range, limited solubility range and notably bad smell. The South-African / Spanish collective have designed a new agent aimed at removing some of those disadvantages. They did so building on another reducing agent, 2-(dibenzylamino)butane-1,4-dithiol (DABDT) introduced in 2012 by An American group (Lukesh et al. DOI) as an alternative for DTT. DTT works by thiol-disulfide exchange with formation of an internal disulfide. The compound is basic and will therefore only work in acidic environments. In DTBA two hydroxyl groups are replaced by a single amino group making it less basic.
In the new work the molecule was modified to introduce solubility in organic solvents by adding two benzyl groups to the amine moiety. Starting from aspartic acid, a reaction with benzyl chloride formed a new compound with four benzyl groups added. The ester groups were then cleaved with lithium aluminium hydride to the corresponding diol. Converting the alcohol groups to a thiol proved difficult as the reagent of first choice Lawesson's reagent did not work. The alternative sequence that did work was conversion to the di-thioester and sodium borohydride reduction. Lithium aluminum hydride is a stronger reducing agent but was found to oxidize DABDT. DABDT is soluble in solvents such as DMF and dichloromethane but reaction still requires a base such as N,N-siisopropylethylamine. A test reaction was the deprotection of the cysteine terminal of a solid Ac-Cys-(SDMP)-Gly-Phe-Leu-Rink amide resin in DMF / water with DIEA.

A MOF for nitrogen dioxide

03 December 2019 - Clean air

Schröder and Yang of the University of Manchester report on a new way to remove nitrogen dioxide from an exhaust using MOF's (Li et al. DOI). NOx compounds are hazardous to human health and damage eco systems. The diesel emissions scandal made clear new ways to tackle NOx are in need (previous report here). The authors point out that selective catalytic reduction with ammonia requires high temperatures, toxic chemicals and metals, is inefficient and easily poisoned by water or sulfur dioxide.
The use of MOF's in NOx capture was first reported in 2010 (DOI), the MFM-520 metal-organic framework (MOF) used in the study has been around since 2006 (DOI) from the same research group and can be synthesized from zinc chloride and 4,4-bipyridyl-3,3,5,5-tetracarboxylate as ligands. In the framework zinc is pentacoordinated with links to the carboxylate units and the pyridyl units. Key to the innovation is the presence of bowtie-shaped pores with dimensions of 6 by 4 Angstrom. The nitrogen dioxide uptake is 4.2 mmole/g at 0,01 bar and very selective: up to 600 times more nitrogen dioxide is adsorbed than carbon dioxide. X-ray diffraction revealed that the bowtie pores are occupied by dinitrogen tetroxide which is the the dimerization product. Removing the nitrogen dioxide and resetting the MOF is surprisingly simple, all it takes is immersing the MOF in water and the compound is converted to nitric acid. This acid has practical and commercial uses and perhaps somewhere in the future diesel drivers can regularly deposit a bucket of it at recycling stations.

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