Chemical Science Blog

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

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.

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is responsible for the deaths of millions of people every year, while one third of the world’s population is infected with a dormant form of the bacteria. Researchers propose that better understanding of how the immune system copes with M. tuberculosis infection could aid the design of new vaccines to limit bacteria growth, and to eradicate any latent forms. Ac₂SGL (1) is a sulfoglycolipid found in the outer membrane of M. tuberculosis which has been implicated in the modulation of TB growth and development, and as such, has potential for use in a new vaccine.

The first asymmetric total synthesis of Ac₂SGL has been reported by Prof. Minnaard at the Stratingh Institute for Chemistry, University of Groningen. This elegant work resulted from a large international collaboration between researchers from the Stratingh Institute and the University of Toulouse, as well as a number of experimental immunologists from University Hospital Basel and the Singapore Immunology Network at the Agency for Science, Technology and Research (A*STAR).

To synthesise the hydroxyphthioceranic acid side chain, the researchers utilised an efficient and highly stereoselective iterative process for the construction of the 1,3-syn substituted fatty acid. The sequence was comprised of reduction, Horner–Wadsworth–Emmons olefination, and copper-catalysed asymmetric 1,4-addition (shown below).

The side chain was completed using copper-catalysed asymmetric allylic substitution and platinum-catalysed diboration/oxidation to install the requisite functionality with excellent diastereoselectivity. This hydroxyphthioceranic acid residue was appended to the disaccharide trehalose. Finally, a regioselective sulfonation at the 2′-position completed the total synthesis.

The biological activity of the synthesised Ac₂SGL was investigated, and found to be comparable with naturally isolated material. The researchers undertook modelling studies which shed light on Ac₂SGL’s binding interactions and 3D conformation. This information, and the efficient total synthesis, facilitates further investigation into the use of Ac₂SGL as a TB vaccine.

Read this ‘HOT’ Chemical Science article today:

Total synthesis, stereochemical elucidation and biological evaluation of Ac₂SGL; a 1,3-methyl branched sulfoglycolipid from Mycobacterium tuberculosis

Danny Geerdink, Bjorn ter Horst, Marco Lepore, Lucia Mori, Germain Puzo, Anna K. H. Hirsch, Martine Gilleron, Gennaro de Libero and Adriaan J. Minnaard.

Chem. Sci., 2013, DOI: 10.1039/C2SC21620E

Scientists in Germany have engineered peptide domains with altered substrate specificities, which leads the way for designer templates to make new bioactive products.

The group have studied hormaomycin, a structurally unusual antibiotic peptide, which is biosynthesized by a bacterial nonribosomal peptide synthetase (NRPS). Analysing amino acid residues of hormaomycin had previously revealed that the NRPS adenylation (A) domains naturally recombine during evolution. This inspired the team to create A domains with altered substrates, which in turn has uncovered new information about the NRPS pathway and suggests new strategies in peptide engineering.

Biosynthetic megaenzymes, such as NRPS, are currently of biomedical interest, as they produce a large number of bioactive metabolites via an assembly mechanism, which shows potential for artificial engineering. Nonribosomal peptides have many clinical applications, including antibiotics, antitumour agents and antifungals.

A significant advantage of this evolution-based engineering is that it requires no insight into protein structures, so while not effective for all NRPS systems, the experimental ease of this method makes it a useful addition to engineering techniques.

Read this ‘HOT’ article today:

Evolution-guided engineering of nonribosomal peptide synthetase adenylation domains
Max Crüsemann, Christoph Kohlhaas and Jörn Piel
Chem. Sci., 2012, DOI: 10.1039/C2SC21722H