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Germanium-bridged polymer for organic solar cells

25 Jan 2012

A team of scientists from the UK and US have made a novel germanium-bridged polymer that shows promise for organic solar cells.

Martin Heeney, at Imperial College London, and colleagues synthesised a heterocyclic monomer bridged with two germanium atoms. They co-polymerised it with an electron-accepting benzothiadiazole to give a polymer that, in contrast to the analogous C-bridged system, is semicrystalline.

Being able to design crystallinity into conjugated polymers in such a way is useful because crystalline polymers are better at transporting charge and hence offer more potential for solar cells. Heeney’s polymer exhibited power conversion efficiencies of over 5 % in bulk heterojunction solar cells. The team are now investigating the use of additives and co-solvents to increase this further.

Find out more – download Heeney’s ChemComm communication.

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Look into my MRI: Non-invasive detection of melanin formation

25 Jan 2012

Magnetic Resonance Imaging (MRI) is used as a medical tool to image soft tissue. It can give insights into the physical state of brain, identify sports injuries and diagnose cancer without ionising radiation. Its main drawback is the low sensitivity of the contrast enhancing probes it uses. These are the paramagnetic metal complexes, often containing gadolinium or manganese, that distinguish between healthy and damaged tissue by altering the relaxation times of the water protons in the body. The challenge is to increase the concentration of the contrast reagents in the tissues.

One solution is to use nanocarriers to deliver a high concentration of the contrast agent to the specific site of interest. Silvio Aime and his team recently used the nanocarrier, apoferritin, to transport solid MnOOH, which was subsequently reduced to paramagnetic Mn²⁺. Their recent ChemComm details how they have now taken this one step further to find a way of generating the Mn²⁺ contrast agent using a naturally occurring reductant. The result: a probe for melanin.

Melanin is produced from the successive oxidation of tyrosine – a process that is up-regulated in malignant melanoma. Aime exploited this oxidation process by introducing MnOOH-loaded apoferritin into melanoma cells – the MnOOH was reduced, generating the contrast agent and enabling the cells to be successfully imaged by MRI.

in vivo MRI images of tumour-bearing mice before and after administration of the contrast agent loaded nanocarrier

The team tested the sensor on melanogenic cells (melanin-forming cells) against non-melanogenic cells. They demonstrated that the signal generated belonged only to the melanin-producing cells which had internalised the Mn(III)OOH–apoferriton payloads. Animal studies revealed enhanced signal intensity in melanogenic tumours.

This interesting research has the potential to provide non-invasive, early diagnoses of skin cancers and evaluate the development of tumours, critical for saving people’s lives. The in vivo sensor may also be used to monitor other processes involving massive oxidative processes. The work of Silvio Aime is one to watch.

To find out more, download the ChemComm article today…

Posted on behalf of Sarah Brown, web science writer for ChemComm.

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A radical approach towards the stephacidin alkaloids

14 Dec 2011

Researchers from the University of Birmingham have developed a radical cascade process for rapid access to intermediates, which are important for the synthesis of alkaloids related to the stephacidin family.

Stephacidin A (1), along with other structurally related compounds, has been shown to possess activity against a number of tumour cell lines.

The group of Nigel Simpkins have demonstrated an elegant approach to the synthesis of the indoline core 3, present in a number of naturally occurring alkaloids.

Commencing from 4 (available in five steps from tryptophan), a one-pot prenylation and sulfenylation sequence afforded cyclisation precursor 5 in good yield. Treatment of sulfenyl diketopiperazine 5 with Bu₃SnH and ACCN facilitated a double radical cyclisation to provide 6a and 6b. Pleasingly, isomers possessing the correct C6 stereochemistry represented the major products (4.6–3:1 depending on R group), and desired product 1 could be obtained following deprotection of 6a and 6b and subsequent purification.

This work represents an efficient entry into the stephacidin alkaloids and a potentially powerful method for the synthesis of other medicinally relevant analogues.

To read more on Simpkin’s indoline synthesis, download the ChemComm article.

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Simple detection of RNA depurination by RIPs

06 Dec 2011

A class of protein toxins exists with an extremely apt acronym, RIP. This stands for Ribosome Inactivating Proteins but may as well stand for “Rest in Peace” when applied to cellular RNA. These proteins are known to attack the link between a purine base (adenine or guanosine) and its sugar causing depurination. This results in the formation of abasic sites (bases that are neither pyrimidines or purines) which in turn, causes the ribosome containing the sequence to have a lower affinity to elongation factors that are crucial for protein synthesis, ultimately leading to cell death.

Methods exist to detect RIPs such as ELISAs and antibody-based immunoassays or by monitoring the specific depurination activity through fluorescence, radiolabelling and immunoaffinity chromatography, amongst others. These techniques require sophisticated or elaborate set-ups, limiting the potential for high throughput (HTP) screening in a bid to discover potential inhibitors of these destructive toxins.

Seergazhi Srivatsan and co-workers have produced a label-free fluorescence hybridisation assay to detect the depurination activity of saporin, a RIP toxin, using a fluorescence ligand that specifically binds to the cytosine opposite an abasic site. If depurination takes place, then the ligand can bind and its fluorescence intensity will be quenched. This technique allows for a plethora of information to be obtained about the depurination activity of saporin as well as many other RIP toxins. This opens the door to using high throughput screening to find inhibitors of such toxins.

Download the article to read more…

Also of interest: Overcoming obstacles in labelling RNA

Posted on behalf of Sarah Brown, web science writer for ChemComm.

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Scattering layer for dye-sensitised solar cells

28 Nov 2011

We are all aware how important it is to find new improved ways to generate sustainable energy. One very promising candidate is the dye-sensitised solar cell which is particularly attractive due to its low production cost and mechanical robustness.

Scientists from East China Normal University have found a way to increase the efficiency of dye-sensitised solar cells by introducing an additional layer on top of the TiO₂ photoanode. The extra layer, made up of Y₃Al₅O₁₂:Ce down-converting microparticles, causes the conversion efficiency to increase from 6.97% to 7.91%.

SEM image of the Y₃Al₅O₁₂:Ce layer and the J-V curves for the solar cell with and without the additional layer

Likun Pan and his team have attributed this improved performance to the increased light absorbing and scattering properties of the microparticle layer (meaning more suitable photons are available for absorption by the dye) and reduced electron transfer resistance.

Download the ChemComm article to read more about the fabrication of Pan’s solar cell device.

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One nanoparticle, two nanoparticles, three nanoparticles, four!

22 Nov 2011

Nanoparticles might be small but they frequent the pages of many a journal due to the ongoing boom in nanotechnology research. Whilst they are useful in a myriad of fields, it is still difficult to directly characterise these extraordinarily small entities. King among the visualisation techniques is electron microscopy but this often requires the isolation of the sample on a support – hardly sufficient for analysing a dynamic sample in solution! Dynamic light scattering is another potential technique but finds limitations when it comes to much smaller nanoparticle sizes.

Ideally, you want to be able to count and size individual nanoparticles at a rate which produces reliable statistics. To address this challenge, Richard Compton and his team, including Neil Rees and Yi-ge Zhou who conducted the experiments alongside Jeseelan Pillay, Robert Tshikhudo and Sibulelo Vilakazi from Mintek, Randburg, have used anodic particle coulometry (APC) to measure gold nanoparticle collisions with a glassy carbon microelectrode and thus count and size individual nanoparticles.

With the electrode potential set above +1.0 V, they were able to record oxidative Faradaic transients from nanoparticle collisions and calculate an average nanoparticle radius which compared extremely well to the radius obtained from scanning electron microscopy measurements. They were also able to observe nanoparticle aggregation, which holds great promise for monitoring dynamic aggregation reactions.

It shouldn’t be long before this technique is routinely used to gain more information on all sizes of metallic nanoparticles which are currently being used in a variety of applications.

Read the ChemComm article by Compton and team for more.

Posted on behalf of Iain Larmour, web science writer for ChemComm.

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Super-fast crystals

22 Nov 2011

Identification of all solid forms of a pharmaceutical is important for drug delivery, due to the potential variability in the physical and chemical properties between amorphous and crystalline forms. The amorphous form of a compound is typically more soluble, but less stable than its crystalline counterpart. Not only that but different polymorphs of the crystalline compound can also have significantly different properties. To accurately characterise drug action, these polymorphs need to be identified.

Using small-scale crystallisation and in-situ Raman spectroscopic analysis of the antihypertensive drug, nifedipine, Franziska Emmerling and colleagues discovered an extraordinarily fast transition from the glassy amorphous state to the metastable β polymorph in less than a minute. The β polymorph is stable for less than ten minutes before transforming again, to the thermodynamically stable α polymorph.

The speed at which the transformations take place implies that classical diffusion is not responsible for the different polymorphs but could instead be the result of small intramolecular changes arising from a pre-ordered physical arrangement of the molecules.

In an industrial world where screening for solid drug forms is always leaning towards scale-reduction and time-reduction, three physical forms on a glass slide in less than twenty minutes is pretty impressive!

To find out more, download the ChemComm article.

Also of interest… Read Andrew Bond, U. Ramamurty, and Gautam Desiraju’s Chemical Science article on “Interaction anisotropy and shear instability of aspirin polymorphs established by nanoidentation“.

Posted on behalf of Scott McKellar, web science writer for ChemComm.

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Hybrid biofuel cell

16 Nov 2011

Scientists from Israel have designed a biofuel cell that combines the advantages of both enzymatic and microbial fuel cells.

Biofuel cells use redox enzymes to convert chemical energy into electricity. These cells can be divided into two categories: enzymatic fuel cells which require the enzymes to be purified and microbial fuel cells which make use of an entire microorganism. There are pros and cons to both strategies – enzymatic fuel cells tend to have increased power output whilst microbial fuel cells enable full oxidation of a wider range of fuels.

Yet now, Lital Alfonta and co-workers demonstrate that by designing a hybrid cell, one can have the best of both worlds. The team have modified yeast to display redox enzymes on their surface and then introduced this into both the anode and cathode compartments. This approach removes the need to purify the enzymes and enables regeneration of both fuel compartments.

To find out more about Alfonta’s biofuel cell device, read the ChemComm article today.

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Nitron, the new N-heterocyclic carbene

15 Nov 2011

N-heterocyclic carbenes (NHCs) are extremely useful reagents in synthesis and catalysis, but unfortunately they are expensive with 1 gram costing several hundred US dollars.

However, as Ulrich Siemeling and his colleagues report in their latest ChemComm, it seems that a much cheaper alternative is on the horizon…

The team of scientists based at the University of Kassel have discovered that Nitron, a low cost analytical reagent, exhibits surprising NHC reactivity, more akin to that of its tautomeric form than its conventional Lewis structure. By reacting Nitron with typical carbene trapping reagents such as elemental sulphur, CS₂, and rhodium complexes, they have proven that it is indeed Nitron’s tautomer that it responsible for its NHC-like reactivity in solution, despite being present at concentrations undetectable by NMR spectroscopy.

At a fraction of the price and already commercially available, Nitron may soon become a very popular choice in NHC reactions.

To read more about how Siemeling and co-workers established the cause for Nitron’s unusual reactivity, download the ChemComm article.

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Overcoming obstacles in labelling RNA

02 Nov 2011

If you can’t beat them, join them… or rather incorporate, and then join them together. This is the approach adopted by Srivatsan and colleagues for conjugating labels to RNA, which they achieve by incorporating azide functionality into RNA nucleotides.

DNA oligonucleotides are routinely labelled by incorporating modified nucleosides into the desired DNA sequence and introducing the label post-synthesis; however, these standard methods offer low yields of the desired conjugate. Furthermore, modification of RNA is more challenging than DNA modification due to its inherent instability.

Copper catalysed alkyne–azide cycloadditions (CuAAC) and Staudinger ligations are the conjugation reactions du jour due to their high chemoselectivity and reported high yield. So, with this in mind, were Srivatsan and his team able to incorporate the required moieties into RNA oligonucleotides?

Yes! Using some extremely nice chemistry, they synthesised an azide-modified nucleotide and incorporated it into an oligoribonucleotide using in vivo transcription reactions in the presence of a series of promoter-template oligonucleotides. After rigorous testing and analysis of their modified nucleic acid sequence, click reactions were performed to yield biotinylated and fluorescent-labelled click products. They also showed that amine functionality can be introduced through Staudinger ligation.

Although azides are not usually found in nature, they are becoming more useful and versatile in the design of diagnostic and therapeutic biological probes as has been very elegantly demonstrated here.

Read more in Srivatsan’s ChemComm article…

Also of interest: Synthetic DNA synthesises RNA by transcription: ‘click’ here for more

Posted on behalf of Sarah Brown, web science writer for ChemComm.

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