In in-source decay (ISD) in matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry (MS), 1,5-diaminonaphthalene (1,5-DAN) is a most frequently used matrix probably due to the highly sensitive detection of fragment ions. 1,5-DAN is a reducing matrix generating c- and z-series ions by N–Cα bond cleavage. However, it is difficult for reducing matrices to distinguish leucine and isoleucine, and generate c_(n-1)-series ions owing to proline (Pro) at residues n. Oxidizing matrices providing a- and x-series ions accompanied by d-series ions by Cα–C bond cleavage solve the problem, but their sensitivity of the ISD fragment ions has been lower than reducing matrices such as 1,5-DAN. Recently, 3-hydroxy-4-nitrobenzoic acid (3H4NBA) had been reported as an oxidizing matrix generating a-series ions with higher intensity compared with conventional oxidizing matrices such as 5-nitrosalicylic acid, but a little lower intensity compared with 1,5-DAN (Anal Chem 88, 8058–8063, 2016). In this study, 3H4NBA isomers (2H3NBA, 2H4NBA, 2H5NBA, 2H6NBA, 3H2NBA, 3H5NBA, 4H2NBA, 4H3NBA, 5H2NBA, and 3H4NBA) were evaluated. All the isomers generated a-series ions accompanied by d-series ions, wherein 3H2NBA, 3H5NBA, 4H2NBA, 4H3NBA, and 5H2NBA were first confirmed as oxidizing matrices for ISD. Among the isomers, 3H2NBA and 4H3NBA generated a-series ions with higher peak intensity compared with 3H4NBA for several peptides. Especially, 3H2NBA generated a-series ions with almost the same or higher intensity, and clearly higher peak resolution compared with c-series ions using 1,5-DAN in several cases. 3H2NBA was expected to contribute to ISD analyses in MALDI-MS as one of the most effective oxidizing matrices.

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在基质辅助激光解吸/电离（MALDI）质谱（MS）的源内衰减（ISD）中，1,5-二氨基萘（1,5-DAN）是最常用的基质，可能是由于其灵敏度高检测碎片离子。1,5-DAN是一种还原性基质，可通过N–Cα键裂解产生c系列和z系列离子。但是，由于残基n上的脯氨酸（Pro），很难还原基质区分亮氨酸和异亮氨酸，并生成c _（n-1）系列离子。氧化矩阵提供一-和X -系列离子伴随着d通过Cα–C键裂解的系列离子可以解决该问题，但是它们对ISD碎片离子的敏感性低于还原性基质（例如1,5-DAN）。最近，据报道3-羟基-4-硝基苯甲酸（3H4NBA）是一种氧化基质，其生成的α系列离子的强度比常规氧化基质（如5-硝基水杨酸）高，但强度略低于1,5。 -DAN（Anal Chem 88，8058–8063，2016）。在这项研究中，评估了3H4NBA异构体（2H3NBA，2H4NBA，2H5NBA，2H6NBA，3H2NBA，3H5NBA，4H2NBA，4H3NBA，5H2NBA和3H4NBA）。所有异构体均产生一个伴随着d的-系列离子系列离子，其中3H2NBA，3H5NBA，4H2NBA，4H3NBA和5H2NBA首先被确认为ISD的氧化基质。在这些异构体中，与3H4NBA相比，3H2NBA和4H3NBA生成了多个肽的峰强度更高的-系列离子。特别是在某些情况下，与使用1,5-DAN的c系列离子相比，3H2NBA生成的a系列离子具有几乎相同或更高的强度，并且峰分辨率明显更高。3H2NBA被认为是MALDI-MS中ISD分析的最有效的氧化基质之一。

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