Operational Laboratory Methods for GDGTs Groups Separation
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Operational Laboratory Methods for GDGTs Groups Separation LIU Zhan, LI Li*, HE Juan, CHEN Lingdi, WANG Junjian, and JIA Guodong State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China (Received September 20, 2019; revised February 17, 2020; accepted March 4, 2020) © Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2020 Abstract Glycerol dialkyl glycerol tetraethers (GDGTs), specific membrane lipids synthesized mainly by bacteria and archaea, can be divided into isoprenoids and methyl branched alkyl GDGTs (iGDGTs and brGDGTs, respectively). Three GDGTs groups (iGDGTs, brGDGTs, and other membrane lipids) in a peat sample were separated and collected in this study using semi-preparative high-performance liquid chromatography (HPLC) and silica gel column chromatography. The obtained recoveries for the whole analytical procedure were 85% – 55% using semi-preparative HPLC and 70% – 20% using gel column chromatography. In addition, in each method, the recoveries of brGDGTs and iGDGTs were similar, regardless of the huge difference in their contents. High purity was found in the fractionated groups, determined based on ether cleavage and reduction. Moreover, the semi-preparative HPLC method could realize a better separation efficiency than the silica gel method, but it was time-consuming and required expensive equipment, while the silica gel chromatography method featured merits of time saving and convenient operation at the cost of a slight reduction in separation efficiency. The advantages of the silica gel method make it an operational laboratory method for batch experiments and isotopic studies. Key words
GDGTs separation; silica gel column chromatography; semi-preparative HPLC
1 Introduction Glycerol dialkyl glycerol tetraethers (GDGTs), which were initially believed to be synthesized mainly by extremophilic archaea, have their carbon skeletons connected with glycerol by four ether bonds. The monolayer structure of the ether bond is highly stable compared with traditional ester bonds (Sinninghe Damsté et al., 2007). Other studies have demonstrated that these ether membrane lipids are widespread in various environments and are not only in harsh environments (Schouten et al., 2013). Two main kinds of GDGTs groups have been distinguished according to the difference in carbon skeleton structure: iGDGTs and branched GDGTs. The former is composed of isoprenoid carbon skeleton (biphytane with 0 – 4 cyclopentane moieties), while the latter contains two n-alkyl skeletons containing up to four methyl branches and one cyclopentyl moiety substitute (Hoefs et al., 1997; Sinninghe Damsté et al., 2000; De Jonge et al., 2014). Generally, iGDGTs are believed to be synthesized by archaea, while brGDGTs are produced from soil bacteria, although there is some debate regarding possible additional sources (DeLong et al., 1998; Macalady et al., 2004; Schouten et al., 2007). In addition to their differences in carbon skeleton, the two GDGTs also feature differences in the spatial configur
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