Chemical Science Blog

Resiniferatoxin (1) and daphnetoxin (2) are members of the daphnane diterpenoids—a class of diterpenoid orthoester compounds, many of which exhibit fascinating therapeutic activity. Resiniferatoxin, whose total synthesis has been achieved only once, is a strong analgesic possessing a complex and densely functionalised tetracyclic core. Now, the Inoue research group at the University of Tokyo have used a series of radical reactions to construct the tetracyclic skeleton of the daphnane diterpenoids (3).

The first radical process was a 3-component coupling of a cyclopentanone (4), O,Se-acetal 5 and an allylstannane (6), and proceeded via a bridgehead radical generated from the reactive O,Se-acetal species. This impressive coupling assembled the A and C rings of the cyclic skeleton, forming 5 consecutive stereocentres in one step. These stereocentres were generated with high stereoselectivity with regards to the tertiary centres at C4 and C10, while the reaction was stereospecific for the creation of the tetrasubstituted C9 centre.

Subsequently, the researchers performed a 7-endo radical cyclisation of xanthate 8 in order to construct the 7-membered B ring. The protected daphnane diterpenoid skeleton (9) was produced as a single isomer with the desired stereochemistry at C8.

Tetracyclic skeleton 3 represents a common intermediate which, following functional group manipulations, would allow the synthesis of naturally-occurring daphnane diterpenoids, including resiniferatoxin, in addition to artificial analogues. The approach demonstrated by the Inoue group showcases the power of radical reactions in the formation of highly congested carbon frameworks and reinforces their relevance in the field of total synthesis.

Scientists studying organometallic catalysts attached to DNA have used the technology to perform cyclopropanation reactions in water, and have reported efficient yields and high enantiomeric excesses.

By attaching the catalytically active metal complex to a DNA scaffold, the chirality of the DNA helix can be directed toward the reaction, influencing its outcome.

You can read this article for free for a limited period:

DNA-based asymmetric organometallic catalysis in water
Jens Oelerich and Gerard Roelfes
Chem. Sci., 2013, DOI: 10.1039/C3SC00100H

Chad Mirkin and colleagues have investigated how the flow of block co-polymer inks from the tip of an AFM probe is affected by the ink’s viscosity. The size of the ink features was found to increase with dwell time and decrease with ink viscosity.

The technique, known as dip-pen nanolithography, was originally developed as a molecular patterning technique for printing small alkanethiol molecules onto a gold surface, but it has rapidly become popular method for synthesising all manner of nano structures.

An understanding of how different substances behave as they move between the probe tip and the surface of the substrate is crucial for designing new materials, patterns and processes.

Read the article for free today:

The role of viscosity on polymer ink transport in dip-pen nanolithography
Guoliang Liu , Yu Zhou , Resham S. Banga , Radha Boya , Keith A. Brown , Anthony J. Chipre , SonBinh T. Nguyen and Chad A. Mirkin
Chem. Sci., 2013, DOI: 10.1039/C3SC50423A

A comprehensive study of the mechanism by which electrochemical water splitting occurs on gold surfaces has been carried out by scientists writing in Chemical Science.

The researchers used in-situ surface enhanced Raman spectroscopy (SERS), on-line electrochemical mass spectrometry and density functional theory (DFT) calculations and found that more than one mechanism may be at work, depending on the voltage applied.

The results show that electrocatalytic surfaces for oxygen evolution may undergo dynamic changes as the reaction progresses. The oxygen evolved on a gold electrode at the onset of potential appears to be the product of an oxygen decomposition step.

Electrochemical water splitting by gold: evidence for an oxide decomposition mechanism
Marc T.M. Koper , Oscar Diaz-Morales , Federico Calle-Vallejo and Casper de Munck
Chem. Sci., 2013, DOI: 10.1039/C3SC50301A

Water is essential for life as we know it and we are all familiar with the sight of water in the atmosphere and on the surface of the earth.  But where is the water deep down, beyond the crust?

The mantle (the part of the earth between the crust and the core) is primarily composed of iron-bearing magnesium silicates which undergo a number of phase transitions with depth. At the transition between the upper and lower mantle, olivine transforms into wadsleyite and this mineral has received interest due to its potential for hosting hydrogen, which is termed water in the realm of the inner-Earth. If fully hydrated the amount of hydrogen potentially stored in this mineral is four times the amount present in the oceans and the atmosphere.

When a hydrogen atom is incorporated into the mineral it is balanced by the removal of a magnesium cation, this produces several possible locations for the incorporation of the hydrogen in the vacancy, as well as different possible ordering of the vacancy in the structure. To solve this complex problem Stephen Wimperis and Sharon Ashbrook have teamed up and used the developments of high-field NMR and sophisticated experimental methods to study the ‘difficult’ NMR nuclei of ²⁵Mg and ¹⁷O within hydrated wadsleyite. In addition to the experimental approach, they have also carried out extensive DFT calculations.

Anhydrous (left) and one of the hypothetical ordered structures for fully-hydrated wadsleyite (right).

Most of the hydrogen was found to be located on the O1 site with the substitution charge balanced by Mg3 cation vacancies. A smaller amount of hydrogen was also found to be present in slightly higher energy defect sites with different proton arrangements or centred at different cation vacancies.

This study demonstrates the benefits of using a combined computational and experimental approach to reveal the location of hydrogen within wadsleyite. The use of this multinuclear NMR approach can be used to investigate a wide range of other silicate phases which should lead to a better understanding of the locations and distribution of hydrogen in the Earth’s mantle.

For more, read this ‘HOT’ Chemical Science article in full:

Water in the Earth’s Mantle: A Solid-State NMR Study of Hydrous Wadsleyite

John M. Griffin, Andrew J. Berry, Daniel J. Frost, Stephen Wimperis and Sharon E. Ashbrook
Chem. Sci., 2013, Advance Article
DOI: 10.1039/C3SC21892A

The in vivo toxicity of iron nanoparticles in rats has been tested by a group of French and Tunisian scientists, who found that the compounds are not metabolised by the animals and cause no toxic effects.

The group tested three different porous iron(III) metal-organic framework (MOF) nanoparticles, injecting them intravenously and studying their distribution, metabolism and excretion. The nanoparticles are rapidly sequestered by the liver and spleen and, after biodegradation, are directly excreted from the body through urine or faeces without metabolisation, substantial toxicity or affecting organ function.

There is currently much concern and debate over the safety and toxicity of nanoparticles, especially with regard to human health. This study shows that biodegradable non-toxic iron(III) carboxylate MOF nanoparticles could have real potential for future biomedical applications.

Read the ‘HOT’ Chemical Science article in full:

In depth analysis of the in vivo toxicity of nanoparticles of porous iron(III) metal-organic frameworks
Tarek Baati , Leila Njim , Fadoua Neffati , Abdelhamid Kerkeni , Muriel Bouttemi , Ruxandra Gref , Mohamed F Najjar , Abdelfateh Zakhama , Patrick Couvreur , Christian Serre and Patricia Horcajada
Chem. Sci., 2013, DOI: 10.1039/C3SC22116D

Geoff Nelson, our new guest web-writer for Chemical Science, blogs about recent findings on van der Waals interactions in non-polar liquids.  Read his first Chem Sci blog post below:

Professor Christopher Hunter, in his latest Edge article, notes that the thermodynamic properties of the van der Waals interactions between non-polar molecules can be predicted based on their calculated molecular surface areas (0.3 kJ mol^-1 Å^-2).  His findings help simplify computational approaches to the design of molecular binding sites or self-assembled molecules.

Prof Hunter’s article includes a detailed model for the behaviour of the van der Waals interaction at liquid-vapor and liquid-liquid interfaces.  Each molecule has ‘surface contact points’ capable of van der Waals interactions with the external environment.  These contacts can be made and broken, depending on the space around the molecule.  The total number of contacts determines the total van der Waals contribution to free energy.  This model helps explain the physical basis of several thermodynamic events (e.g., melting).

The choice of non-polar liquids as the chemical system to study was ideal to isolate the van der Waals interaction, as other non-covalent interactions are minimised (e.g., electrostatic). 

Potent pharmaceuticals and stable self-assembled structures depend on effective binding between molecules.  Predicting the chemical structure necessary to promote such binding is now made easier by Professor Hunter’s research.

Read this Chemical Science Edge article in full:

van der Waals interactions in non-polar liquids

Christopher A. Hunter

Chem. Sci., 2013,4, 834-848

DOI: 10.1039/C2SC21666C

Geoff Nelson is a new guest web-writer for Chemical Science.  He currently works as a post-doctoral research associate in Dr David Payne’s research group in the Department of Materials at Imperial College, London.  Geoff’s current research concerns the synthesis and characterization of post-transition metal oxides for use in the energy sector.  His other research interests include carbon-based materials, biophysical chemistry, and surface science.