Department of Chemistry and Biochemistry, University of Missouri-St. Louis, St. Louis, MO 63121, USA.
Phys Chem Chem Phys. 2018 Nov 14;20(44):27897-27909. doi: 10.1039/c8cp02620c.
We investigate the gas-phase structures and fragmentation chemistry of deprotonated carbohydrate anions using combined tandem mass spectrometry, infrared spectroscopy, regioselective labelling, and theory. Our model system is deprotonated, [lactose-H]-. We computationally characterize the rate-determining barriers to glycosidic bond (C1-Z1 reactions) and cross-ring cleavages, and compare these predictions to our tandem mass spectrometric and infrared spectroscopy data. The glycosidic bond cleavage product data support complex mixtures of anion structures in both the C1 and Z1 anion populations. The specific nature of these distributions is predicted to be directly affected by the nature of the anomeric configuration of the precursor anion and the distribution of energies imparted. i.e., Z1 anions produced from the β-glucose anomeric form have a differing distribution of product ion structures than do those from the α-glucose anomeric form. The most readily formed Z1 anions ([1,4-anhydroglucose-H]- structures) are produced from the β-glucose anomers, and do not ring-open and isomerize as the hemiacetal group is no longer present. In contrast the [3,4-anhydroglucose-H]-, Z1 anion structures, which are most readily produced from α-glucose forms, can ring-open through very low barriers (<25 kJ mol-1) to form energetically and entropically favorable aldehyde isomers assigned with a carbonyl stretch at ∼1640 cm-1. Barriers to interconversion of the pyranose [β-galactose-H]-, C1 anions to ring-open forms were larger, but still modest (≥51 kJ mol-1) consistent with evidence of the presence of both forms in the infrared spectrum. For the cross-ring cleavage 0,2A2 anions, ring-opening at the glucose hemiacetal of [lactose-H]- is rate-limiting (>180 (α-), >197 kJ mol-1 (β-anomers)). This finding offers an explanation for the low abundance of these product anions in our tandem mass spectra.
我们使用串联质谱、红外光谱、区域选择性标记和理论研究了去质子化碳水化合物阴离子的气相结构和碎片化化学。我们的模型系统是去质子化的[lactose-H]-。我们对糖苷键（C1-Z1 反应）和跨环裂解的速率决定势垒进行了计算，并将这些预测与我们的串联质谱和红外光谱数据进行了比较。糖苷键裂解产物数据支持 C1 和 Z1 阴离子中存在复杂的阴离子结构混合物。这些分布的具体性质预计将直接受到前体阴离子的端基构型和赋予能量的分布的性质的影响。即，来自β-葡萄糖端基形式的 Z1 阴离子具有不同于来自α-葡萄糖端基形式的产物离子结构的分布。最容易形成的 Z1 阴离子([1,4-脱水葡萄糖-H]-结构)是由β-葡萄糖端基形成的，由于半缩醛基团不再存在，它们不会开环和异构化。相比之下，最容易由α-葡萄糖形式产生的[3,4-脱水葡萄糖-H]-,Z1 阴离子结构，可以通过非常低的势垒（<25 kJ mol-1）开环形成具有 1640 cm-1 左右羰基伸展的有利的醛异构体。吡喃糖[β-半乳糖-H]-,C1 阴离子转化为开环形式的互变势垒更大，但仍适中（≥51 kJ mol-1），与红外光谱中存在两种形式的证据一致。对于交叉环裂解 0,2A2 阴离子，[lactose-H]-的葡萄糖半缩醛的开环是限速步骤（>180（α-），>197 kJ mol-1（β-端基））。这一发现为我们的串联质谱中这些产物阴离子丰度低提供了一个解释。
