利用NMR技术测定小麦胚芽中的脂质成分

为深入探究小麦胚芽中的脂质成分，该研究采用不同溶剂分别提取小麦胚芽中的磷脂和中性脂质，利用核磁共振技术 （nuclear magnetic resonance，NMR）对小麦胚芽中的脂质成分进行分析，结果表明：³¹P NMR在小麦胚芽中检测到6种磷脂和相对应的5种溶血性磷脂，以及非脂质含磷化合物甘油磷脂酰胆碱（GPC）；磷脂中含量最高的是磷脂酰胆碱(PC)，其摩尔浓度为0.42 μmol/g，摩尔分数为28.50%，质量浓度为0.31 mg/g，质量分数为30.2%；¹H NMR测定小麦胚芽中的多种中性脂质组成及含量，包括甘油三酯（TG）、甘油二酯（DG）、甘油单酯（MG）和游离脂肪酸（FA），其中含量最高的甘油酯是TG，占比为77.25%，含量最低的是2-甘油单酯（2-MG），占比为0.03%；小麦胚芽的甘油三酯和磷脂中检测出亚油酸（L）、油酸（O）和亚麻酸（Ln）等6种不饱和脂肪酸，其中亚油酸的含量最高，在甘油三酯中占比56.26%，在磷脂中占比45.37%。溶血性磷脂和GPC是磷脂的水解产物，DG、MG和FA是甘油三酯的水解产物，这些物质可以反映样品中脂质的水解程度。研究结果表明，利用NMR不仅能够对小麦胚芽脂质的组分进行定性定量分析，而且可以监测小麦胚芽脂质水解程度的变化。NMR技术在小麦胚芽相关产品的脂质分析研究具有重要作用。

Determining the lipid components of wheat germ by nuclear magnetic resonance

Wheat germ is rich in lipids with a very complex composition. The lipids in wheat germ are prone to be hydrolyzed and oxidized during processing and storage. In this study, phospholipids and neutral lipids were extracted from wheat germ with different solvents. Then the phospholipids, mono-, di-, and triacylglycerols, as well as fatty acyl group contents were analyzed by nuclear magnetic resonance (NMR) spectroscopy. The results showed that six kinds of phospholipids were detected by ³¹P NMR in wheat germ, including phosphatidylcholine (PC), phosphatidyl ethanolamine (PE), phosphatidyl inositol (PI), phosphatidylglycerol (PG), phosphatidic acid (PA), and phosphatidylserine (PS). Furthermore. There were also five kinds of lysophospholipids (lysophosphatidylcholine (LPC), lysophosphatidylglycerol (LPG), and lysophosphatidylethanolamine (LPE)) and two non-lipid phosphorus compounds (inorganic phosphate (pi) and phosphatidylcholine glycerol (GPC)). GPC was the hydrolytic product from the removal of fatty acyl groups by two-step hydrolysis of PC. PC had the highest content of phospholipid in the wheat germ. Specifically, the molar concentration of PC was 0.42 μmol/g wheat germ, the molar fraction of PC was 28.50 %, the mass concentration of PC was 0.31 mg/g wheat germ, and the mass fraction of PC was 30.20 %. The signals of PE, PC, LPC, GPC, and choline (Cho) in ¹H NMR spectra were also used to determine the phospholipids in wheat germ. Compared with ¹H NMR, ³¹P NMR spectra were better resolved and more suitable for qualitative and quantitative analysis of phospholipid components, as ³¹P NMR only determined the characteristic signal of phosphorus-containing compounds. The composition and content of free fatty acids, mono-, di-, and triacylglycerols were determined in the wheat germ by ¹H NMR. The proportion of free fatty acids was 12.35 % in the wheat germ. The highest content was triacylglyceride (TG), accounting for 77.25 %. The contents of 1,3-diacylglycerol (1,3-DG) and 1, 2-diacylglycerol (1,2-DG) were 5.80 % and 4.41 %, respectively, and the contents of 1-monoacylglycerol (1-MG) and 2-monoacylglycerol (2-MG) were 0.17% and 0.03%, respectively. Moreover, TG was gradually hydrolyzed to produce diglycerol (DG), monoglycerol (MG), free fatty acid (FA) and glycerol in the wheat germ during storage. FA was always produced in the wheat germ throughout the hydrolysis reaction, leading to a higher content, compared with DG and MG. Six kinds of fatty acyl groups were detected by ¹H NMR in triglycerides and phospholipids of wheat germ, including the acyl groups of docosahexaenoic acid (DHA), eicosapentaenoic acid and arachidonic acid (EPA+ARA), linolenic acid (Ln), linoleic acid (L) and oleic acid (O). Linoleic acid was the most abundant unsaturated fatty acyl group in triglycerides and phospholipids. Saturated and modified acyl groups (S+M) were also detected by ¹H NMR. There was significant difference in fatty acid composition between triglyceride and phospholipid in wheat germ. Lysophospholipids and GPC were the hydrolyzed products of phospholipids, while DG and MG were the hydrolyzed products of triglycerides. As such, their contents shared the degree of lipids hydrolysis in the wheat germ. Therefore, NMR can be used to determine the lipid compositions and hydrolysis in the wheat germ. In short, this NMR is a powerful tool for lipids analysis of wheat germ products.