But it is only in recent years that ILs and deep eutectic solvents (DES) have been revisited by chemical engineering, because such solvents can replace conventional organic solvents. The field of ILs began in 1914, when Paul Walden ( Plechkova and Seddon, 2008) reported on the physical properties of ethylammonium nitrate. The theory is based on analogy with green chemistry, where in past years various synthetic ionic liquids (ILs) have been developed for chemical and enzymatic reactions as well as for the extraction of natural products.

Here, we develop a novel theory about the role of these compounds, which may explain many questions in the biochemistry of cells and organisms. With the exception of sugars, which may serve as storage products and a source of energy, the other compounds are present in such large amounts that it does not make sense to consider them as only intermediates in metabolic pathways. These compounds include sugars, some amino acids, choline, and some organic acids such as malic acid, citric acid, lactic acid, and succinic acid. It seems that these compounds must serve some basic function in living cells and organisms. Considering all the information we have collected in recent years using our protocol for NMR-based metabolomics ( Kim et al., 2010), we asked ourselves why a few very simple molecules are always present in considerable amounts in all microbial, mammalian, and plant cells. Is there any logic in the studying the combination of compounds itself, instead of looking at the correlations between the compounds observed and any disease or applied experimental conditions? NMR-based metabolomics in particular give a clear view of the major compounds present in an organism or cell and enable the direct quantitative comparison of all major compounds. But in fact, little work has been done on understanding the meaning of the metabolomic data itself and the role of the total of the compounds observed. Most of this work involves a number of chemometric methods to identify markers in the metabolomic data for various events.

Over the past decade, metabolomics has developed into a major tool for studying the metabolism of organisms and cells, and through this approach much has been learned about metabolic networks and the reactions of organisms to various external conditions ( Lay et al., 2006).

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