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An artificial cell made from molybdenum-based building blocks whose pores can open and close has been devised by researchers in Germany, Israel, and Spain. The team investigated their capsules using nuclear magnetic resonance and infra-red spectroscopy and other techniques. The capsules' "breathing" pores allow bulky molecules that would otherwise be too large to enter the pores normally. Achmim Müller and colleagues at the University of Bielefeld have spent many years designing and synthesising sophisticated inorganic and organometallic clusters and supramolecular structures based on molybdenum oxide building blocks. He reasoned that it might be possible to extend these structures to create a synthetic capsule, reminiscent of a viral shell. Such a novel material might behave analogously to the solid, yet flexible metal-organic frameworks, solid materials that can "breathe". Such a "viral" entity would not be an artificial life form nor have any disease-causing properties but it would be able to mimic certain biological functions such as transportation of small molecules. Ira Weinstock, Ayala Ziv, Alina Grego, Sivil Kopilevich, and Leila Zeiri at the Ben Gurion University at Beer Sheva, Israel, and colleagues Pere Miro and Carles Bo of the Institute of Chemical Research of Catalonia, Tarragona, Spain, started with one of Müller's parent complexes. This compound is a member of the family of molecular metal oxide-based coordination polymers with spherical periodicity. The generic structure might have twelve pentagonal molybdenum oxide-based ligands connected via thirty chromium, iron, or dinuclear molybdenum "spacers". The structure of one of these compounds in which the team chose molybdenum spacers contains twenty Mo9O9 rings and is just 0.3 nanometres in diameter. These rings are the pore-like openings to the interior of the water-soluble capsule-like complex, the team explains. The team used carbon-13 NMR, Fourier transform infrared, and Raman spectroscopy as well as elemental analysis to test the integrity of their capsules. "The idea for the investigation is based on earlier qualitative Bielefeld experiments but my colleagues in Israel, led by Ira Weinstock, did the sophisticated experiments, which our group in Bielefeld could not have done as well," Müller told SpectroscopyNOW. The researchers then tested the transport of numerous carboxylate ions of different sizes in and out of the capsules using NMR spectroscopy. By carefully monitoring the process, they could observe how the proton-coupled uptake of the carboxylate ions allowed ions larger than the normal pore size, as determined by X-ray crystallography, to enter. Those carboxylates with smaller hydrocarbon side chains (primary and secondary alkyl group) enter most rapidly. But, even those with bulky ternary and tertiary alkyl groups, such as 1,1,1-trimethylacetic acid, and tert-butyl carboxylic acid were still able to pass through the pores when the rings expand through the breathing process. Phenyl carboyxlates, in the form of benzoic acid, however, were simply too big to enter despite the pore stretch even after several weeks of "waiting", the researchers say. "The fascinating aspect of our capsules, or artificial cells, is that with their twenty pores they behave like spherical viruses as they can get partly opened and afterwards closed again," explains Müller. "Correspondingly, molecular species can be taken up though even if they are larger than the size of the pores." The molybdenum framework is not affected irreversibly by the breathing process. "The reason for the breathing behaviour is that we have a very large number of relatively weak bonds keeping the spherical system together, which is a comparable situation to that seen in spherical viruses," Müller explains, "In the present case there are 120 metal ligand bonds." In addition, Müller revealed, is that a type of adaptation takes place as these "cells" get adapted to a new environment in which, for instance, acetate in the solution is replaced with other carboxylates. The capsules might have applications in separation chemistry, catalysis, or molecular transport of drugs and other molecules. Related links:
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