Hsu Fong-Fu, Turk John
Mass Spectrometry Resource, Division of Endocrinology, Diabetes, Metabolism, and Lipid Research, Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Am Soc Mass Spectrom. 2008 Nov;19(11):1673-80. doi: 10.1016/j.jasms.2008.07.007. Epub 2008 Jul 16.
Linear ion-trap (LIT) MS2 mass spectrometric approach toward locating the position of double bond(s) of unsaturated long-chain fatty acids and toward discerning among isomeric unsaturated fatty acids as dilithiated adduct ([M-H+2Li]+) ions are described in this report. Upon resonance excitation in a LIT instrument, charge-remote fragmentation that involves beta-cleavage with gamma-H shift (McLafferty rearrangement) is the predominant fragmentation pathway seen for the [M-H+2Li]+ ions of monoenoic long-chain fatty acids. The fragmentation process results in a dilithiated product ion of terminally unsaturated fatty acid, which undergoes consecutive McLafferty rearrangement to eliminate a propylene residue, and gives rise to another dilithiated adduct ion of terminally unsaturated fatty acid. In addition to the above-cited fragmentation process, the [M-H+2Li]+ ions of homoconjugated dienoic long-chain fatty acids also undergo alpha-cleavage(s) with shift of the allylic hydrogen situated between the homoconjugated double bonds to the unsaturated site. These fragmentation pathways lead to two types of CC bond cleavages that are allylic (alpha-cleavage) or vinylic, respectively, to the proximal CC double bond, resulting in two distinct sets of ion series, in which each ion series is separated by a CH2CHCH (40 Da) residue. These latter fragmentations are the predominant processes seen for the polyunsaturated long-chain fatty acids. The spectrum feature dependent on the position of unsaturated double bond(s) affords unambiguous assignment of the position of double bond(s) of long-chain unsaturated fatty acids.
本报告描述了一种线性离子阱（LIT）质谱2（MS2）方法，用于确定不饱和长链脂肪酸双键的位置，并区分异构不饱和脂肪酸，该方法以二锂化加合物（[M-H+2Li]+）离子进行。在LIT仪器中进行共振激发时，涉及β-裂解和γ-H迁移（麦克拉弗蒂重排）的电荷远程裂解是单烯长链脂肪酸[M-H+2Li]+离子的主要裂解途径。裂解过程产生末端不饱和脂肪酸的二锂化产物离子，该离子经历连续的麦克拉弗蒂重排以消除丙烯残基，并产生另一个末端不饱和脂肪酸的二锂化加合物离子。除上述裂解过程外，同共轭二烯长链脂肪酸的[M-H+2Li]+离子还会发生α-裂解，位于同共轭双键之间的烯丙基氢迁移至不饱和位点。这些裂解途径分别导致两种类型的CC键裂解，即与近端CC双键的烯丙基（α-裂解）或乙烯基裂解，产生两组不同的离子系列，其中每个离子系列由一个CH2CHCH（40 Da）残基隔开。后一种裂解是多不饱和长链脂肪酸的主要过程。取决于不饱和双键位置的光谱特征可明确确定长链不饱和脂肪酸双键的位置。
