Yamagaki Tohru, Watanabe Takehiro
Suntory Foundation for Life Sciences Bioorganic Research Institute.
Mass Spectrom (Tokyo). 2012;1(1):A0005. doi: 10.5702/massspectrometry.A0005. Epub 2012 Jul 20.
We studied the ionization process of aromatic carboxylic acids, including ones with or without hydroxy groups in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), because many natural products, metabolites, and drags contain those structural units. In the actual experimental data, benzoic acid was ionized as only deprotonated molecule M-H. In contrast, both of negative molecular ion M(-) and deprotonated molecule M-H were generated from 2-naphthoic acid and 2-anthracenecarboxylic acid, and the ratio of negative molecular ion to deprotonated molecule M(-)/M-H was increased in 2-anthracenecarboxylic acid. In addition, the ratio of 2-anthracenecarboxylic acid was much higher than those of 1- and 9-anthracenecarboxylic acids among the three isomers. Therefore, 2-substitution induced the generation of the negative molecular ion M(-), which can made us prediction of the substituted positions from their overlapping peak isotope patterns. 2,5-Dihydroxybenzoic acid (2,5-DHBA) showed two deprotonated molecules, M-H and M-H*-H, which was generated from a neutral hydrogen radical (H*) removal from a phenolic hydroxy group. The deprotonated molecule M-H*-H of 2,5-DHBA was the most abundant among six DHBAs and three hydroxybenzoic acids (hBAs). This observation raises the possibility that such a property of 2,5-DHBA could be a clue to explain its highest efficiency as a MALDI matrix. The order of the hydrogen radical removal from the phenolic hydroxy groups was the 3-<4-≪5-positions in the DHBAs, and the 3-<4-positions in hBAs. We can distinguish among six DHBA isomers and three hBA isomers from their spectral pattern around the deprotonated molecules M-H*-H and M-H. The intra-molecular hydrogen bonding between 1-carboxy and 2-hydroxy groups was an important factor in hydrogen radical removal in the hydroxylbenzoic acids and dihydroxybenzoic acids.
我们研究了芳香族羧酸在基质辅助激光解吸/电离质谱（MALDI-MS）中的电离过程，这些芳香族羧酸包括含有或不含有羟基的化合物，因为许多天然产物、代谢物和药物都含有这些结构单元。在实际实验数据中，苯甲酸仅以去质子化分子M-H的形式被电离。相比之下，2-萘甲酸和2-蒽甲酸会同时产生负离子M(-)和去质子化分子M-H，并且在2-蒽甲酸中负离子与去质子化分子的比例M(-)/M-H有所增加。此外，在这三种异构体中，2-蒽甲酸的比例远高于1-蒽甲酸和9-蒽甲酸。因此，2-位取代诱导了负离子M(-)的产生，这使得我们能够从它们重叠的峰同位素模式预测取代位置。2,5-二羟基苯甲酸（2,5-DHBA）显示出两个去质子化分子M-H和M-H*-H，后者是由酚羟基上的中性氢自由基（H*）去除后产生的。2,5-DHBA的去质子化分子M-H*-H在六种二羟基苯甲酸和三种羟基苯甲酸（hBAs）中最为丰富。这一观察结果增加了一种可能性，即2,5-DHBA的这种性质可能是解释其作为MALDI基质效率最高的一个线索。在二羟基苯甲酸中，从酚羟基上去除氢自由基的顺序是3-<4-≪5-位，而在羟基苯甲酸中是3-<4-位。我们可以根据它们在去质子化分子M-H*-H和M-H周围的光谱模式区分六种二羟基苯甲酸异构体和三种羟基苯甲酸异构体。1-羧基和2-羟基之间的分子内氢键是羟基苯甲酸和二羟基苯甲酸中氢自由基去除的一个重要因素。
