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The crystalline sponge method

18th December 2019


Structure of trans-decalin crystalline sponge

Determination of molecular structure of odour components based on crystalline sponge method. By Hiroyasu Sato and Akihito Yamano

Humans sense odours when they inhale airborne molecules through their nostrils. When substances with an identical composition formula but a different 3D structure are floating in the air, humans sense these substances as totally different odours. This is because sensing an odour starts with the coupling of an odour molecule with a G-protein coupled receptor (GPCR)(1). The state of the coupling varies depending on the shape of the odour molecule, including with which GPCR the odour molecule can couple, at which part of the GPCR, and how tightly. It is believed that this accounts for the uniqueness of odours.

An extremely low concentration of an odour component can react with the receptor in the human nose in the form of odour, which means that only a trace amount of such a component exists in the environment. To extract a specific odour component from natural raw materials for analysis, only several micrograms or less from a large quantity of raw material might be obtained. Even with the help of common analytical techniques however, directly determining the 3D molecular structure of odour components is impossible.

Single crystal X-ray structure analysis, however, is suited for the determination of the 3D structure of hitherto unknown substances. Since this analytical technique requires large quantities of high purity samples for crystallisation, it has seldom been used for the analysis of odour components in the past. The emergence of the crystalline sponge method(2) has dramatically changed this situation.

What is the crystalline method?

The crystalline sponge method is a molecular structure analysis technique based on a host crystal of porous organometallic complexes called a metal-organic framework (MOF), whose cavities are filled with cyclohexane molecules. It is a powerful technique that allows single crystal X-ray structure analysis to be applied even to target compounds that are difficult to crystallise. This method was very useful in determining the structure of volatile odour components, because it could be applied to small amounts of target compounds, on the order of nanograms.

A solution containing a target compound is poured into this host crystal, and the solvent is gradually evaporated to concentrate the solution. As a result, the target compound is trapped in the cavities of the MOF. As soon as it is removed from the solvent, the crystalline sponge containing the target compound begins to dry on the surface, so the crystalline sponge must be removed while it is fully soaked in the solvent.

With this method each volatile substance, containing multiple components, is applied directly to the crystalline sponge, without separation, and the 3D structure of the trapped molecules is determined by single crystal X-ray structure analysis. In this setting, only the structure of those molecules preferentially trapped in the crystalline sponge can be determined. If applied to each target component that is isolated and purified, and in combination with GC, LC, and MS, this method will enable molecular structure determination for extremely small amounts of target components.

Renewed interest in the technique is evidenced by the recently announced collaboration between Rigaku and Merck to develop commercially available lab solutions employing highly innovative crystalline sponge technology.(3)

References

( 1 ) M. Kotthoff: Ernahrungs Umschau, 62 (2015), No. 5, 82–91. DOI: 10.4455/eu.2015.014.

( 2 ) Y. Inokuma, S. Yoshioka, J. Ariyoshi, T. Arai, Y. Hitora, K. Takeda, S. Matsunaga, K. Rissanen and M. Fujita: Nature, 495 (2013), 461–466.

( 3 ) https://www.rigaku.com/press/rod/sponge

Hiroyasu Sato and Akihito Yamano are with Rigaku





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