R. Verpoorte1, Y.H. Choi1, and G.J. Witkamp2
1 Natural Products Laboratory, IBL, Leiden University
2 Kluyver Laboratory, Delft University of Technology
Through NMR-base metabolomics we found that in all kind of extracts of microbial, mammalian and plant cells certain organic bases, organic acids, amino acids, sugars and sugar alcohol occur in relatively large amounts. Much larger in fact than expected on the basis of these compounds being intermediates in primary metabolism. Their ubiquitous occurrence gave us the thought that they must have a function. Based on this our first hypothesis was the possibility of ionic liquids formed by the organic bases like choline and betaine with organic acid like malic acid. The first experiments confirmed this. Further studies resulted in finding that various combinations of the mentioned compounds do give deep eutectic solvents, i.e. mixing these solid crystalline compounds in certain molar ratios results in liquids at room temperature. More than 150 combinations now have been characterized. We named them Natural Deep Eutectic Solvents (NADES). They can be divided into the following groups:
- Organic bases with organic acids: ionic liquids
- Organic bases with neutral compounds like poly alcohols and sugars
- Organic acids with neutral compounds like poly alcohols and sugars
- Amino acids with neutral compounds like poly alcohols and sugars
- Mixtures of neutral compounds like poly alcohols and sugars
By 1HNMR it was shown that the NADES components are strongly bound via H-bonding, in some cases H2O is part of these liquid crystal-like structures. The NMR also shows that with dilution with water gradually the interaction between the compounds disappears. NADES are excellent solvents for medium polar compounds, such as most secondary metabolites. In our view biosynthesis of poorly water soluble compounds occurs in NADES, e.g. attached to cellular membranes, where the polar charged head groups of the membrane lipids act as anchors for NADES in which enzymes and intermediates are dissolved. Also vesicles could be formed by such metastable systems of lipids and NADES. All ingredients for NADES are found in resurrection plants, lichen etc. Also in drought or cold resistant plants the typical NADES ingredients are found to be present. An obvious example of the occurrence of NADES in Nature is the in the nectar in flowers, by direct NMR the ingredients can be identified. The nectar collected by bees is well known as honey, another clear example of a NADES. In terms of application the NADES are excellent solvents for both small molecules and macromolecules (proteins, polysaccharides, DNA, etc.) in which the compounds show better solubility and stability than in the currently used organic solvents. NADES thus are of interest as non-toxic, non-explosive, sustainable green solvents for many industrial applications. There are still many things to be learned from Nature!
YH Choi, J van Spronsen, Y Dai, M Verberne, F Hollmann, IWCE Arends, GJ. Witkamp, R Verpoorte. Are Natural Deep Eutectic Solvents the missing link in understanding cellular metabolism and physiology? Plant Physiol 156(2011)1701-1715
Y Dai, GJ. Witkamp, R Verpoorte, and YH Choi. Natural Deep Eutectic Solvents as a New Extraction Media for Phenolic Metabolites in Carthamus tinctorius L. Anal Chem., 85(2013)6272–6278
Y Dai, J van Spronsen, GJ. Witkamp, R Verpoorte, and Y H Choi. Natural deep eutectic solvents as new potential media for green technology. Anal Chim Acta 766(2013)61-68
Professor Verpoorte holds a Pharmacists degree (1972) and a PhD (1976) from Leiden. He was lecturer at Leiden University 1976-1987, and since 1987 professor and head of the department of Pharmacognosy. Since 201 he is as Emeritus professor, still connected to Leiden University. He was guest professor in London (UK), Uppsala (Sweden), Amiens (France),Reims (France), Seoul (Korea) and Hilo (USA). From 1992-1998 he was Vice-Chairman and Chairman of the committee of the Phytochemical Society of Europe (PSE). Since May 2011 he is professor Emeritus at Leiden University.
He is author/co-author of 780+ scientific papers, 4 books and 6 patent applications. September 2017 H factor 66 (Web of Science), 93 (Google Scholar) 74 (Scopus). Editor (1996-2002) and Editor-in-chief of Journal of Ethnopharmacology (IF 3.115)(2003-2016) and is editor-in-chief of Phytochemistry Reviews (IF 3.393) since 2001 and Executive Editor Biotechnology Letters (IF 1.846)since 2006. He supervised 66 PhD-theses, and 150+ MSc theses.
He received an Honorary Doctorate University of Amiens, France (2004) and of the University of Uppsala, Sweden (2012). In 2007 he received the PSE Medal. He is a honorary professor at the Hong Kong Baptist University since 2015. In 2015 he was awarded the Gusi Peace Prize in Manila, The Philippines. September 2017 he was awarded the Egon Stahl Medal in Gold by the International Society of Medicinal Plants and Natural Products Research for his lifetime scientific work. In 2017 recipient of the Qihuang International Prize of China Association of Chinese Medicine (China). He is among the 1% Highly Cited Researchers 2018 (Cross-Field)(Web of Science, Clarivate Analytics).