HPLC-MS : Chapter Six

Chapter six

Regiospecific Analysis of Triacylglycerols Using High

Performance Liquid Chromatography/Atmospheric

Pressure Chemical Ionization Mass Spectrometry

Hazel R. Mottram

Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock’s Close,

Bristol BS8 1TS, UK

Introduction

Naturally occurring mixtures of triacylglycerols (TAG) represent a considerable challenge to the analytical chemist. A typical animal fat or vegetable oil may contain in the region of 40 different TAG molecular species, while more complex mixtures such as milk fat or fish oil may comprise 150 or more individual species. The range of fatty acyl moieties present may include a wide range of carbon numbers and double bonds. In addition to determining which fatty acyl moieties are present in a TAG, it is also important to be able to determine their positions on the glycerol backbone of the molecule, since this has considerable significance from biochemical (1,2), nutritional (3,4), and biotechnological (5) points of view. However, the ability to unambiguously identify positional isomers of individual TAG, especially when they are components of complex natural mixtures, is a long-standing problem in lipid chemistry. Generally, it is not possible to directly measure the optical rotation of naturally occurring TAG since it is too weak, although in some cases, such as milk fat, the molecules are sufficiently asymmetric to allow this (6,7). Consequently, other techniques have been developed, which typically involve either the formation and separation of diacyl-sn-glycerols or analysis of the intact molecule.

Enzymatic Approaches

The first method for the stereospecific analysis of TAG was introduced by Brockerhoff (8) and employs pancreatic lipase (E.C. 3.1.1.3) or a Grignard reagent to partially hydrolyze TAG, giving rise to a mixture of 1,2- and 2,3-diacyl-sn-glycerols, which are phosphorylated to their corresponding phosphatidylphenols (Fig. 6.1). The 2-position fatty acyl moiety of the 1,2-diacylglycerol phosphatidylphenol can then be cleaved using phospholipase A2 (E.C. 3.1.1.4) leaving a 1-monoacylglycerol lysophosphatidylphenol, …