大气压电离质谱中含氮有机化合物质子化离子的产生及裂解反应研究
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摘要
大气压电离质谱是对有机化合物进行分析和结构鉴定的重要工具之一。有机分子的质子化离子是大气压电离质谱中最常见的准分子离子,是提供分子量和结构信息的重要依据,因此准确掌握质子加合物的产生机理和裂解反应规律是有机质谱学的核心内容。本论文以含氮有机化合物为研究对象,选取一些典型的模型分子,从其质子加合物在离子源中的产生机理和在串联质谱中的裂解规律两个方面开展研究,具体内容包括以下四部分：
1.以对二甲氨基查尔酮为模型分子,首次报道了大气压化学电离源中动力学控制的质子化反应。对二甲氨基查尔酮在溶液中碱性最强的位点是氮,而在气相中最稳定的质子化位点是羰基氧,根据常规的理解该化合物在大气压化学电离质谱中主要产生O-质子化离子。但是当使用乙腈/水为溶剂并且进样流速较大的时候,优先产生的是N-质子化离子,可能的原因是该条件下离子化过程中产生了更接近于液相环境中的高度溶剂化的N-质子化离子。这一发现有助于我们更好地理解大气压化学电离源的离子化机理。
2.以1,4-二苯基-3-苯甲酰基-1,4-二氢吡啶为模型分子,报道了电喷雾离子源中正离子模式下罕见的脱氢反应。当使用质子性溶剂时,该类化合物离子化产生的主要是[M+H]+离子；当使用非质子性溶剂时,该类化合物离子化产生的主要是[M-H]+离子。通过多种实验方法证明了脱氢反应是在喷雾针高电压作用下发生的电化学氧化反应,即1,4-二氢吡啶环被氧化芳构化成吡啶环,该反应可以被认为是一种特殊的离子抑制形式。
3.以N-苄基丁内酰胺为模型分子,研究了其质子加合物裂解反应中的质子迁移反应。该化合物裂解反应的发生需要质子从热力学最稳定的羰基氧迁移到内酰胺氮或者苯环本位碳上。实验和理论研究发现质子从羰基氧迁移到苯环本位碳是通过直接1,5-氢迁移完成的。但是质子从羰基氧迁移到内酰胺氮并不是通过经典的1,3-氢迁移完成的,而是先从氧上迁移到苯环的邻位碳,再迁移到内酰胺氮上,在该过程中苯环起到了类似于催化剂的作用(分子内质子转移催化)。进一步的研究发现这一裂解反应模型适用于其它N-苄基酰胺化合物和具有这种骨架结构的药物分子的质谱裂解反应。
4.以N-苄基哌啶和N-苄基哌嗪为模型分子,系统地研究了含N-苄基结构化合物的质子加合物的裂解反应规律,总结了离子/中性复合物介导的苄基阳离子参与的五种反应类型,包括氢负离子迁移、亲电取代、电子迁移、质子迁移和芳香族亲核取代,使我们对苄基阳离子这一重要的有机活性物种的反应性有了更全面的认识。这些反应规律已经在国内外其它质谱学家的研究中得到了很好的应用,并且可以被用来合理地解释相关药物分子裂解反应中一些特殊碎片离子的产生。
此外,本论文还简要总结了大气压电离源的离子化机理,并综述了大气压电离质谱中质子化离子裂解反应中的重要重排反应。
Atmospheric pressure ionization mass spectrometry (API-MS) is one of the important tools for analysis and structure elucidation of organic compounds. The protonated ion of organic compounds is the most common quasi-molecular ion in API-MS, which provides molecular weight and structural information. Therefore, understanding the formation and fragmentation mechanism of protonated ions is the core issue of organic mass spectrometry. In this dissertation, using typical N-containing compounds as models, two aspects including the generation mechanism of protonated ions in the ion source and the fragmentation rules of protonated ions in tandem mass spectrometry were studied, which specifically includes the following four parts:
1. Using p-(dimethylamino)chalcone as a model compound, the kinetically controlled protonation reaction in atmospheric pressure chemical ionization (APCI) was reported for the first time. The most basic site ofp-(dimethylamino)chalcone in solution is the N, while the most stable protonation site of it in the gas phase is the carbonyl O. Thus according to the conventional understanding, the O-protonated ion should be dominantly formed in APCI. However, when acetonitrile/water is used as the solvent and the infusion rate is relatively high, the N-protonated ion is preferentially produced. The proposed explanation is that under the present ionization conditions highly solvated N-protonated ions are formed, which is close to the situation in liquid phase. This finding will help us better understand the mechanism of APCI.
2. Using1,4-diphenyl-3-benzoyl-1,4-dihydropyridines as model compounds, a rare dehydrogenation reaction in electrospray ion source was reported. When a protic solvent is used, the [M+H]+ion is mainly formed; when an aprotic solvent is used, the generation of [M-H]+ion is dominant. With the aid of a variety of experimental methods, it is proved that the dehydrogenation reaction is an electrochemical oxidation reaction induced by the high voltage in the spray needle, in which the1,4-dihydropyridine ring is oxidized to a pyridine ring. This is a special case of ion suppression.
3. Using N-benzyl lactams as model compounds, the proton transfers in the fragmentation of their proton adducts were studied. The fragmentation of protonated N-benzyl lactams requires the ionizing proton migrating from the thermodynamically stable carbonyl oxygen to the lactam nitrogen or the ipso position of the phenyl ring. Experimental and theoretical studies indicate that a direct1,5-H shift is responsible for the proton transfer from the carbonyl oxygen to the ipso position of the phenyl ring. However, the proton transfer from the carbonyl oxygen to the lactam nitrogen is not achieved via a classical1,3-H shift. In fact, the proton first migrates from the carbonyl oxygen to the ortho position of the phenyl ring and then to the lactam nitrogen, in which the phenyl ring acts as a catalyst (intramolecular proton-transport catalysis). Further studies found that the present fragmentation reaction model is applicable to other N-benzyl amides and drugs containing such a skeleton structure.
4. Using N-benzyl piperidine and N-benzyl piperazine as model compounds, the fragmentation rules of protonated N-benzyl structure-containing compounds were systematically studied and five kinds of ion/neutral complex-mediated benzyl cation-involved reactions were concluded including hydride ion transfer, electrophilic substitution, electron transfer, proton transfer, and nucleophilic aromatic substitution. This study prompts us to obtain comprehensive understanding on the reactivity of the benzyl cations. The fragmentation rules summarized in this study have been well applied by other domestic and foreign mass spectrometrists in their researches. These fragmentation rules also can be used to rationalize the generation of special fragment ions in the fragmentation of related drugs.
In addition, the ionization mechanism of atmospheric pressure ionization source was briefly summarized and the important rearrangements of protonated ions generated by API-MS in collision-induced dissociation were reviewed.
1.以对二甲氨基查尔酮为模型分子,首次报道了大气压化学电离源中动力学控制的质子化反应。对二甲氨基查尔酮在溶液中碱性最强的位点是氮,而在气相中最稳定的质子化位点是羰基氧,根据常规的理解该化合物在大气压化学电离质谱中主要产生O-质子化离子。但是当使用乙腈/水为溶剂并且进样流速较大的时候,优先产生的是N-质子化离子,可能的原因是该条件下离子化过程中产生了更接近于液相环境中的高度溶剂化的N-质子化离子。这一发现有助于我们更好地理解大气压化学电离源的离子化机理。
2.以1,4-二苯基-3-苯甲酰基-1,4-二氢吡啶为模型分子,报道了电喷雾离子源中正离子模式下罕见的脱氢反应。当使用质子性溶剂时,该类化合物离子化产生的主要是[M+H]+离子；当使用非质子性溶剂时,该类化合物离子化产生的主要是[M-H]+离子。通过多种实验方法证明了脱氢反应是在喷雾针高电压作用下发生的电化学氧化反应,即1,4-二氢吡啶环被氧化芳构化成吡啶环,该反应可以被认为是一种特殊的离子抑制形式。
3.以N-苄基丁内酰胺为模型分子,研究了其质子加合物裂解反应中的质子迁移反应。该化合物裂解反应的发生需要质子从热力学最稳定的羰基氧迁移到内酰胺氮或者苯环本位碳上。实验和理论研究发现质子从羰基氧迁移到苯环本位碳是通过直接1,5-氢迁移完成的。但是质子从羰基氧迁移到内酰胺氮并不是通过经典的1,3-氢迁移完成的,而是先从氧上迁移到苯环的邻位碳,再迁移到内酰胺氮上,在该过程中苯环起到了类似于催化剂的作用(分子内质子转移催化)。进一步的研究发现这一裂解反应模型适用于其它N-苄基酰胺化合物和具有这种骨架结构的药物分子的质谱裂解反应。
4.以N-苄基哌啶和N-苄基哌嗪为模型分子,系统地研究了含N-苄基结构化合物的质子加合物的裂解反应规律,总结了离子/中性复合物介导的苄基阳离子参与的五种反应类型,包括氢负离子迁移、亲电取代、电子迁移、质子迁移和芳香族亲核取代,使我们对苄基阳离子这一重要的有机活性物种的反应性有了更全面的认识。这些反应规律已经在国内外其它质谱学家的研究中得到了很好的应用,并且可以被用来合理地解释相关药物分子裂解反应中一些特殊碎片离子的产生。
此外,本论文还简要总结了大气压电离源的离子化机理,并综述了大气压电离质谱中质子化离子裂解反应中的重要重排反应。
Atmospheric pressure ionization mass spectrometry (API-MS) is one of the important tools for analysis and structure elucidation of organic compounds. The protonated ion of organic compounds is the most common quasi-molecular ion in API-MS, which provides molecular weight and structural information. Therefore, understanding the formation and fragmentation mechanism of protonated ions is the core issue of organic mass spectrometry. In this dissertation, using typical N-containing compounds as models, two aspects including the generation mechanism of protonated ions in the ion source and the fragmentation rules of protonated ions in tandem mass spectrometry were studied, which specifically includes the following four parts:
1. Using p-(dimethylamino)chalcone as a model compound, the kinetically controlled protonation reaction in atmospheric pressure chemical ionization (APCI) was reported for the first time. The most basic site ofp-(dimethylamino)chalcone in solution is the N, while the most stable protonation site of it in the gas phase is the carbonyl O. Thus according to the conventional understanding, the O-protonated ion should be dominantly formed in APCI. However, when acetonitrile/water is used as the solvent and the infusion rate is relatively high, the N-protonated ion is preferentially produced. The proposed explanation is that under the present ionization conditions highly solvated N-protonated ions are formed, which is close to the situation in liquid phase. This finding will help us better understand the mechanism of APCI.
2. Using1,4-diphenyl-3-benzoyl-1,4-dihydropyridines as model compounds, a rare dehydrogenation reaction in electrospray ion source was reported. When a protic solvent is used, the [M+H]+ion is mainly formed; when an aprotic solvent is used, the generation of [M-H]+ion is dominant. With the aid of a variety of experimental methods, it is proved that the dehydrogenation reaction is an electrochemical oxidation reaction induced by the high voltage in the spray needle, in which the1,4-dihydropyridine ring is oxidized to a pyridine ring. This is a special case of ion suppression.
3. Using N-benzyl lactams as model compounds, the proton transfers in the fragmentation of their proton adducts were studied. The fragmentation of protonated N-benzyl lactams requires the ionizing proton migrating from the thermodynamically stable carbonyl oxygen to the lactam nitrogen or the ipso position of the phenyl ring. Experimental and theoretical studies indicate that a direct1,5-H shift is responsible for the proton transfer from the carbonyl oxygen to the ipso position of the phenyl ring. However, the proton transfer from the carbonyl oxygen to the lactam nitrogen is not achieved via a classical1,3-H shift. In fact, the proton first migrates from the carbonyl oxygen to the ortho position of the phenyl ring and then to the lactam nitrogen, in which the phenyl ring acts as a catalyst (intramolecular proton-transport catalysis). Further studies found that the present fragmentation reaction model is applicable to other N-benzyl amides and drugs containing such a skeleton structure.
4. Using N-benzyl piperidine and N-benzyl piperazine as model compounds, the fragmentation rules of protonated N-benzyl structure-containing compounds were systematically studied and five kinds of ion/neutral complex-mediated benzyl cation-involved reactions were concluded including hydride ion transfer, electrophilic substitution, electron transfer, proton transfer, and nucleophilic aromatic substitution. This study prompts us to obtain comprehensive understanding on the reactivity of the benzyl cations. The fragmentation rules summarized in this study have been well applied by other domestic and foreign mass spectrometrists in their researches. These fragmentation rules also can be used to rationalize the generation of special fragment ions in the fragmentation of related drugs.
In addition, the ionization mechanism of atmospheric pressure ionization source was briefly summarized and the important rearrangements of protonated ions generated by API-MS in collision-induced dissociation were reviewed.
引文
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