Chemical Science Blog

High nucleophilic reactivity and functional group tolerance are opposing properties of organometallic compounds. While organolithium compounds are highly nucleophilic so react with most electrophiles, organosilicon compounds are on the low reactivity end of the nucleophilicity scale. They tolerate numerous electrophilic functional groups and often require (Lewis) base activation to react.

Organoboron compounds, which are key intermediates in organic synthesis, are positioned in between. While some of them undergo non-catalysed C–C bond forming reactions with various electrophiles, others tolerate a variety of functional groups and are most valuable substrates for Pd-catalysed cross couplings (Suzuki–Miyaura reactions).

As it is difficult for the preparative chemist to decide which functional groups are tolerated and which do react with organoboron reagents in the absence of a catalyst, scientists in Germany have established a quantitative reactivity scale for representatives of the most important classes of organoboron reagents and they have applied these data for predicting the feasibility of transition metal free C–C bond forming reactions.

Reference:
Nucleophilicity parameters for designing transition metal-free C-C bond forming reactions of organoboron compounds
G Berionni, B Maji, P Knochel and H Mayr, Chem. Sci., 2012
DOI: 10.1039/c2sc00883a

Scientists in Germany have synthesised the core part of a sugar compound produced by the pathogenic bacteria responsible for meningitis – Neisseria meningitides – which could be used in a vaccine for meningococcal diseases, in particular meningitis B.

Pathogenic bacteria produce a polysaccharide layer, forming a capsule to cloak antigenic proteins on their surface that would otherwise provoke an immune response to destroy the bacteria. Currently available vaccines are based on capsular polysaccharides and provide protection against four major forms of meningitis, but not meningitis B. This is because the capsular polysaccharide for meningitis B resembles carbohydrates present in the central nervous system, so using it in a vaccine risks triggering an immune response against them as well.

Now, Peter Seeberger from the Max Planck Institute of Colloids and Interfaces, Potsdam, and colleagues have made the inner core of the lipopolysaccharide produced by N. meningitides as a safer alternative. The resulting tetrasaccharide consists of Hep (heptose) and Kdo (octulosonic acid) building blocks.

Synthesis of the core tetrasaccharide of Neisseria meningitidis lipopolysaccharide
Total synthesis of the core tetrasaccharide of Neisseria meningitidis lipopolysaccharide, a potential vaccine candidate for meningococcal diseases
You Yang, Christopher E. Martin and Peter H. Seeberger
Chem. Sci., 2012, Advance Article
DOI: 10.1039/C1SC00804H

Read the full story in Chemistry World

Link to journal article

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

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

A hybrid photocatalytic system, coupling a zeolite with graphene, has been developed by teams in the UK, India and Germany. This new system could have implications for important applications such as water and air purification, dye degradation, self-cleaning and anti-bacterial surfaces as well as various technical photosynthetic processes.

Dominik Eder at the University of Münster and colleagues at the University of Cambridge, used a titanosilicate zeolite (TS-1) with graphene incorporated, at varying concentrations, in its pores.  TS-1 was chosen becasue of its good photocatalytic properties and graphene was chosen for its exceptional electrical and optical properties, along with its ability to accept electrons via photoexcitation.

The team found that the photocatalytic activity was greater when compared to TS-1 hybrids with carbon nanotubes.

Reference:
Hybridizing photoactive zeolites with graphene: a powerful strategy towards superior photocatalytic properties
Z Ren, E Kim, S W Pattinson, K S Subrahmanyam, C N R Rao, A K Cheetham nd D Eder, Chem. Sci., 2011
DOI:10.1039/c1sc00511a

By building methotrexate – a potent leukaemia drug – into nanoscale coordination polymers as bridging ligands, scientists have achieved high drug loadings of 79 wt% and selective delivery to cancer cells, where the drug is released. The polymers have superior in vitro efficacy to the free drug and the toxic side effects of the free drug are avoided.

Reference:
Lipid-Coated Nanoscale Coordination Polymers for Targeted Delivery of Antifolates to Cancer Cells
R C Huxford, K E deKrafft, W S Boyle, D Liu and W Lin, Chem. Sci., 2011
DOI : 10.1039/c1sc00499a