DISTINCTION AMONG ISOMERIC UNSATURATED FATTY ACIDS AS LITHIATED ADDUCTS

我们描述了双锂离子的串联质谱学特征。 电喷雾法制备不饱和脂肪酸的加合离子 低能碰撞激活电离质谱学 三级四极仪器上的解离。这些光谱 不饱和脂肪酸同分异构体的区分及许可 双键位置的指定。信息碎片离子反映 二锂离子上远离电荷中心的键的断裂 羧酸盐部分。光谱中含有自由基阳离子，反射 第一个键和第二个键之间以及第二个键之间的断裂 和脂肪酸链中的第三个碳原子。这些离子是 紧随其后的是具有14m/z间隔的成员的闭壳离子系列 反映第三和第四个之间的键断裂的单位和 然后在随后的相邻的碳原子对之间。这个离子 序列终止于成员，反映了 碳-碳单键乙烯与第一碳-碳DO 亚伯邦德。反映双键远端键断裂的离子 单不饱和脂肪酸中很少观察到键，而不是 当它们出现时，就会变得丰富起来。对于多不饱和脂肪酸， 含有由单个亚甲基、离子隔开的双键 反映碳-碳单键在双键之间的断裂 是丰富的，但离子反射的分裂远端的最终双胞胎 邦德不是。在这些分子中观察到的双键之间的断裂 光谱可以通过一个包含六个成员的方案来合理化 双烯丙基的过渡态和随后的重排 氢原子将产生一种终端不饱和的带电 中性烯烃的碎裂和消除。的位置。 蓖麻酸中的B-羟基烯部分可以通过 类似的方法。这些观察结果提供了一个机会 实验室有串联四极仪器，但没有 具有高能CAD功能的仪器可指定双键 位置 不饱和游离脂肪酸的质谱学方法 衍生化。

We describe here features of tandem mass spectra of dilithiated adduct ions of unsaturated fatty acids obtained by electrospray ionization mass spectrometry with low-energy collisionally activated dissociation on a triple stage quadrupole instrument. These spectra distinguish among isomeric unsaturated fatty acids and permit assignment of double-bond location. Informative fragment ions reflect cleavage of bonds remote from the charge site on the dilithiated carboxylate moiety. The spectra contain radical cations reflecting cleavage of bonds between the first and second and between the second and third carbon atoms in the fatty acid chain. These ions are followed by a closed-shell ion series with members separated by 14 m/z units that reflect cleavage of bonds between the third and fourth and then between subsequent adjacent pairs of carbon atoms. This ion series terminates at the member reflecting cleavage of the carbon-carbon single bond vinylic to the first carbon -carbon do ublebond. Ions reflecting cleavages of bonds distal to the double bond are rarely observed for monounsaturated fatty acids and are not abundant when they occur. For polyunsaturated fatty acids that contain double bonds separated by a single methylene group, ions reflecting cleavage of carbon-carbon single bonds between double bonds are abundant, but ions reflecting cleavages distal to the final double bond are not. Cleavages between double bonds observed in these spectra can be rationalized by a scheme involving a six-membered transition state and subsequent rearrangement of a bis-allylic hydrogen atom to yield a terminally unsaturated charge-carrying fragment and elimination of a neutral alkene. The location of the b-hydroxy-alkene moiety in ricinoleic acid can be demonstrated by similar methods. These observations offer the opportunity for laboratories that have tandem quadrupole instruments but do not have instruments with high energy CAD capabilities to assign double bond location in unsaturated free fatty acids by mass spectrometric methods without derivatization.