碱金属离子与三肽复合物的气相裂解反应

doi:10.7503/cjcu20160447

摘要/Abstract

摘要：

为了探索侧链R基团对碱金属离子与多肽复合物气相裂解反应的影响, 采用电喷雾电离质谱法研究了碱金属离子Li⁺, Na⁺和K⁺分别与甘氨酸三肽(GGG)、甘氨酰-苯丙氨酰-甘氨酸三肽(GFG)和甘氨酰-甘氨酰-苯丙氨酸三肽(GGF)形成的复合物的气相裂解反应. 质谱定性实验结果表明, Li⁺, Na⁺和K⁺与GGG, GFG或GGF在气相中可以形成稳定的复合物, 配合比为1∶1或2∶1. 竞争反应质谱图显示, GGG, GFG或GGF与碱金属离子形成的复合物的质谱峰丰度按Li⁺, Na⁺, K⁺顺序依次下降, 表明随着碱金属离子半径的增加, 它们与三肽的结合强度依次减弱. 碰撞诱导解离显示, 母体离子[GGG+Na]⁺, [GGF+Na]⁺和[GFG+Na]⁺ 的质心碰撞能量E(CM)₅₀数值分别为1.94, 1.76和1.63 eV. 通过质谱滴定法测得[GGG+Na]⁺, [GFG+ Na]⁺和[GGF+Na]⁺ 的结合常数lg $K a 1$ 分别为5.30, 5.25和5.17. 质谱法定量结果进一步确认复合物的稳定性顺序为[GGG+Na]⁺>[GGF+Na]⁺>[GFG+Na]⁺, 表明由于空间位阻的影响, 侧链R基团含有苄基的GFG或 GGF与Na⁺的键合强度要小于侧链R全部为H的GGG. 串级质谱分析结果显示, 碱金属化的GGG断裂位点较多, 可解离出丰富的金属化a₂, b和y型碎片离子, 而碱金属化的GGF和GFG解离出的金属化y型离子较多, b型离子其次, 金属化a型离子几乎没有. 此外, 双碱金属化的GGF可解离出较多金属化y型离子. 复合物[GGF+Na]⁺的裂解曲线显示, 当碰撞能量为25 eV时, [y₂+Na-H]⁺ 和[b₂+Na+OH]⁺为主要碎片离子, 当碰撞能量>40 eV时, 只有[b₂+Na+OH]⁺ 碎片离子占有优势数量. 根据质子化三肽裂解机理可以推测, 钠化GFG裂解后生成含噁唑酮的[b₂+Na]⁺离子, 该离子经过一系列过渡态生成[a₂+Na]⁺(2-苄基-4-咪唑酮), 而不是常见的亚胺离子.

关键词: 电喷雾电离质谱法, 碱金属离子, 三肽, 复合物, 气相裂解反应

Abstract:

For exploring effects of side chain R groups on dissociations of complexes of alkali metal ions with peptides in gas phase, the complexes of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and the tripeptides Glycyl-glycyl-glycine(GGG), Glycyl-phenylalanyl-glycine(GFG) and Glycyl-glycyl-phenylalanine(GGF) were chosen to probe the fragmentation reaction process by electrospray ionization mass spectrometry. The experimental results demonstrated that Li⁺, Na⁺ or K⁺and GGG, GFG or GGF can form complexes in a coordination ratio of 1∶1 or 2∶1, respectively. The mass spectra for competition reactions of GGG, GFG or GGF with Li⁺, Na⁺ or K⁺showed peak intensity of alkali metallated-tripeptide complexes decrease in the order of Li⁺, Na⁺, K⁺ similarly, indicating that the binding strength of complexes descend with the ascend of the radii of alkali metal ion. Collision-induced dissociation(CID) results showed the center of mass frame collision energy E(CM)₅₀ values were 1.94, 1.76 and 1.63 eV for the precursor of [GGG+Na]⁺, [GGF+Na]⁺ and [GFG+Na]⁺, respectively. The mass spectrometric titrations determined the lg $K a 1$ values were 5.30, 5.25 and 5.17, respectively. Quantitative analyses of mass spectra further confirmed the stability order [GGG+Na]⁺ >[GGF+Na]⁺>[GFG+Na]⁺, indicating binding strength of complexes GFG or GGF with R side chain(benzyl group) were weaker than GGG with R side chain(only hydrogen atom) due to effects of steric hindrance. The tandem mass spectrometric analysis revealed that dissociation of soliated GGG produces most abundant soliated a₂, b_n, y_n ion series, while soliated GGF or GFG ion gives abundant soliated y_n and b_n ions, but rarely soliated a_n. Moreover, doubly alkali meallated GGF tends to dissociate to more metallated y ion series. The breakdown graph for the complex [GGF+Na]⁺ showed the percentage of fragment ion [b₂+Na+OH]⁺ will go up greatly upon the increasing of collision energy. According to Siu’s fragmentation mechanisms of protonated tripeptide, it is inferred that soliated GFG will fragment to [b₂+Na]⁺ ion with an incipient oxazolone. Then [b₂+Na]⁺ will continue to fragment to [a₂+Na]⁺ with a cyclic protonted 4-imidazolidone.

Key words: Electrospray ionization mass spectrometry, Alkali metal ion, Tripeptide, Complex, Fragmentation reaction

中图分类号:

O657.6

TrendMD:

吴若菲, 储艳秋, 许崇晟, 刘智攀, 丁传凡. 碱金属离子与三肽复合物的气相裂解反应. 高等学校化学学报, 2016, 37(12): 2150.

WU Ruofei, CHU Yanqiu, XU Chongsheng, LIU Zhipan, DING Chuanfan. Fragmentation Reaction of Complexes of Alkali Metal Cations with Tripeptides in Gas Phase^†. Chem. J. Chinese Universities, 2016, 37(12): 2150.

图/表 10

Fig.1 Nomenclature used for fragments of tripeptidesGGG: R1=R2=R3=H; GFG: R1=R3=H, R2=—CH2—C6H5; GGF: R1=R2=H, R3=—CH2—C6H5.

Fig.2 Mass spectra for competition reaction of tripeptides with alkali metal ions(A) GGG with Li+, Na+, K+, Rb+ and Cs+ in a molar ratio of 1∶1∶1∶1∶1∶1; (B) GFG with Li+, Na+ and K+ in a molar ratio of 1∶1∶1∶1; (C) GGF with Li+, Na+ and K+ in a molar ratio of 1∶1∶1∶1.

Fig.3 Normalized percentages of the intact complexes plotted against collision energy of mass frame(A) GGG with a proton or an alkali ion; (B) GGG, GFG or GGF with a Na+; (C) GGG with one or two Na+.

Table 1 lg K a 1 values of GGG,GFG or GGF with Li+, Na+ or K+

Complex	lg $K a 1 a$	SD^b	RSD(%)^c	Complex	lg $K a 1 a$	SD^b	RSD(%)^c
[GGG+Li]⁺	5.32	0.20	3.4	[GFG+K]⁺	5.10	0.02	0.3
[GGG+Na]⁺	5.30	0.07	1.3	[GGF+Li]⁺	5.25	0.13	2.6
[GGG+K]⁺	5.14	0.12	2.3	[GGF+Na]⁺	5.17	0.19	3.8
[GFG+Li]⁺	5.20	0.10	2.0	[GGF+K]⁺	5.11	0.06	1.1
[GFG+Na]⁺	5.15	0.04	0.7

Table 1 lg K a 1 values of GGG,GFG or GGF with Li+, Na+ or K+

Complex	lg $K a 1 a$	SD^b	RSD(%)^c	Complex	lg $K a 1 a$	SD^b	RSD(%)^c
[GGG+Li]⁺	5.32	0.20	3.4	[GFG+K]⁺	5.10	0.02	0.3
[GGG+Na]⁺	5.30	0.07	1.3	[GGF+Li]⁺	5.25	0.13	2.6
[GGG+K]⁺	5.14	0.12	2.3	[GGF+Na]⁺	5.17	0.19	3.8
[GFG+Li]⁺	5.20	0.10	2.0	[GGF+K]⁺	5.11	0.06	1.1
[GFG+Na]⁺	5.15	0.04	0.7

Fig.4 Collision-induced dissociation(CID) spectra for complexes at collision energy of 32 eV(A) [GGF+Na]+; (B) [GGF+2Na-H]+.

Table 2 CID fragment ions formed from protonated or alkali metallated triglycine complexes

Precursor ion(#)	a_n/ $a n $ / $a n *$	b_n/ $b n $ / $b n *$	y_n/ $y n $ / $y n *$	Other
[G3+H]⁺	a₂	b₁, b₂	y₁, y₂	[#-H₂O]⁺
[G3+Li]⁺	a₂, $a 2 *$	$b 1 $ , $b 2 $ , [b₁+OH+Li]⁺, [b₂+OH+Li]⁺	$y 1 $ , $y 2 $ , $y 1 $ -H₂O, $y 2 $ -H₂O	[#-H₂O]⁺, [#-NH₃-H₂O]⁺
[G3+Na]⁺	$a 2 *$	$b 2 *$ , [b₁+OH+Na]⁺, [b₂+OH+Na]⁺	$y 1 $ , $y 2 $ , $y 2 *$ -H₂O	[#-H₂O]⁺, [#-NH₃]⁺
[G3+K]⁺	$a 2 *$			K⁺
[G3+2Li-H]⁺	a₂, $a 2 **$	$b 2 **$	$y 2 $ , $y 2 $ -H₂O, $y 1 **$	[#-H₂O]⁺ , [#-NH₃]⁺ , [#-NH₃- H₂O]⁺
[G3+2Na-H]⁺	$a 2 **$	$b 1 **$	$y 1 **$	[#-H₂O]⁺
[G3+2K-H]⁺		$b 2 **$		K⁺

Table 2 CID fragment ions formed from protonated or alkali metallated triglycine complexes

Precursor ion(#)	a_n/ $a n $ / $a n *$	b_n/ $b n $ / $b n *$	y_n/ $y n $ / $y n *$	Other
[G3+H]⁺	a₂	b₁, b₂	y₁, y₂	[#-H₂O]⁺
[G3+Li]⁺	a₂, $a 2 *$	$b 1 $ , $b 2 $ , [b₁+OH+Li]⁺, [b₂+OH+Li]⁺	$y 1 $ , $y 2 $ , $y 1 $ -H₂O, $y 2 $ -H₂O	[#-H₂O]⁺, [#-NH₃-H₂O]⁺
[G3+Na]⁺	$a 2 *$	$b 2 *$ , [b₁+OH+Na]⁺, [b₂+OH+Na]⁺	$y 1 $ , $y 2 $ , $y 2 *$ -H₂O	[#-H₂O]⁺, [#-NH₃]⁺
[G3+K]⁺	$a 2 *$			K⁺
[G3+2Li-H]⁺	a₂, $a 2 **$	$b 2 **$	$y 2 $ , $y 2 $ -H₂O, $y 1 **$	[#-H₂O]⁺ , [#-NH₃]⁺ , [#-NH₃- H₂O]⁺
[G3+2Na-H]⁺	$a 2 **$	$b 1 **$	$y 1 **$	[#-H₂O]⁺
[G3+2K-H]⁺		$b 2 **$		K⁺

Table 3 CID fragment ions formed from protonated or alkali metallated GFG complexes

Precursor ion(#)	a_n/ $a n $ / $a n *$	b_n/ $b n $ / $b n *$	y_n/ $y n $ / $y n *$	Other
[GFG+H]⁺	a₂	b₂	y₂, y₂-H₂O
[GFG+Li]⁺	a_2*	[b₁+OH+Li]⁺, [b₂+OH+Li]⁺ , $b 1 $ , $b 2 $	$y 1 $ , $y 2 $ , $y 2 $ -H₂O, $y 1 $ -H₂O, $y 1 *$ -H₂O-C₇H₇	[#-H₂O]⁺,[#-C₇H₇]⁺ , [#-H₂O-NH₃]⁺ , [#-H₂O-C₇H₇]⁺
[GFG+Na]⁺	$a 2 *$	[b₂+OH+Na]⁺, [b₁+OH+Na]⁺ ,b_2*	$y 1 $ , $y 2 $ -H₂O, $y 2 *$	[#-H₂O]⁺
[GFG+K]⁺	$a 2 *$			[#-H₂O]⁺ , K⁺
[GFG+2Li-H]⁺	$a 2 **$	$b 2 **$	$y 2 $ -H₂O, $y 1 $ , $y 2 $ , $y 2 $ -C₇H₇	[#-H₂O]⁺, [#-H₂O-C₇H₇]⁺, [#-C₇H₇]⁺
[GFG+2Na-H]⁺		$b 2 **$	$y 2 $ , $y 1 $ , $y 2 $ -H₂O, $y 2 $ -C₇H₇	[#-H₂O]⁺ , [#-NH₃-H₂O]⁺, [#-C₇H₇]⁺

Table 3 CID fragment ions formed from protonated or alkali metallated GFG complexes

Precursor ion(#)	a_n/ $a n $ / $a n *$	b_n/ $b n $ / $b n *$	y_n/ $y n $ / $y n *$	Other
[GFG+H]⁺	a₂	b₂	y₂, y₂-H₂O
[GFG+Li]⁺	a_2*	[b₁+OH+Li]⁺, [b₂+OH+Li]⁺ , $b 1 $ , $b 2 $	$y 1 $ , $y 2 $ , $y 2 $ -H₂O, $y 1 $ -H₂O, $y 1 *$ -H₂O-C₇H₇	[#-H₂O]⁺,[#-C₇H₇]⁺ , [#-H₂O-NH₃]⁺ , [#-H₂O-C₇H₇]⁺
[GFG+Na]⁺	$a 2 *$	[b₂+OH+Na]⁺, [b₁+OH+Na]⁺ ,b_2*	$y 1 $ , $y 2 $ -H₂O, $y 2 *$	[#-H₂O]⁺
[GFG+K]⁺	$a 2 *$			[#-H₂O]⁺ , K⁺
[GFG+2Li-H]⁺	$a 2 **$	$b 2 **$	$y 2 $ -H₂O, $y 1 $ , $y 2 $ , $y 2 $ -C₇H₇	[#-H₂O]⁺, [#-H₂O-C₇H₇]⁺, [#-C₇H₇]⁺
[GFG+2Na-H]⁺		$b 2 **$	$y 2 $ , $y 1 $ , $y 2 $ -H₂O, $y 2 $ -C₇H₇	[#-H₂O]⁺ , [#-NH₃-H₂O]⁺, [#-C₇H₇]⁺

Table 4 CID fragment ions formed from protonated or alkali metallated GFG complexes

Precursor ion(#)	a_n/ $a n $ / $a n *$	b_n/ $b n $ / $b n *$	y_n/ $y n $ / $y n *$	Other
[GGF+H]⁺			y₂-H₂O
[GGF+Li]⁺	$a 2 *$	[b₂+OH+Li]⁺, [b₁+OH+Li]⁺	$y 1 $ , $y 2 $ , $y 2 $ -H₂O, $y 2 $ -H₂O-C₇H₇	[#-H₂O]⁺, [#-NH₃- H₂O]⁺ , [#-H₂O-C₇H₇]⁺ , [#-C₇H₇]⁺
[GGF+Na]⁺		$b 1 *$ , [b₁+OH+Na] ⁺, [b₂+OH+Na]⁺	$y 1 $ , $y 2 $ , $y 2 *$ -H₂O	[#-H₂O]⁺ , [#-NH₃-H₂O]⁺ , [#-C₇H₇]⁺
[GGF+K]⁺				[#-H₂O]⁺ , K⁺
[GGF+2Li-H]⁺	$a 2 *$	$b 1 $ , $b 2 $	$y 1 $ -H₂O, $y 1 $ -C₇H₇, $y 2 $ , $y 2 *$ -C₇H₇	[#-H₂O]⁺, [#-NH₃]⁺ , [#-NH₃-H₂O]⁺ , [#-H₂O-C₇H₇]⁺ , [#-C₇H₇]⁺
[GGF+2Na-H]⁺		$b 1 $ , $b 2 $	$y 2 $ , $y 1 $ , $y 2 $ -H₂O, $y 1 $ -C₇H₇, $y 2 **$ -C₇H₇	[#-H₂O]⁺ , [#-NH₃-H₂O]⁺ , [#-C₇H₇]⁺

Table 4 CID fragment ions formed from protonated or alkali metallated GFG complexes

Precursor ion(#)	a_n/ $a n $ / $a n *$	b_n/ $b n $ / $b n *$	y_n/ $y n $ / $y n *$	Other
[GGF+H]⁺			y₂-H₂O
[GGF+Li]⁺	$a 2 *$	[b₂+OH+Li]⁺, [b₁+OH+Li]⁺	$y 1 $ , $y 2 $ , $y 2 $ -H₂O, $y 2 $ -H₂O-C₇H₇	[#-H₂O]⁺, [#-NH₃- H₂O]⁺ , [#-H₂O-C₇H₇]⁺ , [#-C₇H₇]⁺
[GGF+Na]⁺		$b 1 *$ , [b₁+OH+Na] ⁺, [b₂+OH+Na]⁺	$y 1 $ , $y 2 $ , $y 2 *$ -H₂O	[#-H₂O]⁺ , [#-NH₃-H₂O]⁺ , [#-C₇H₇]⁺
[GGF+K]⁺				[#-H₂O]⁺ , K⁺
[GGF+2Li-H]⁺	$a 2 *$	$b 1 $ , $b 2 $	$y 1 $ -H₂O, $y 1 $ -C₇H₇, $y 2 $ , $y 2 *$ -C₇H₇	[#-H₂O]⁺, [#-NH₃]⁺ , [#-NH₃-H₂O]⁺ , [#-H₂O-C₇H₇]⁺ , [#-C₇H₇]⁺
[GGF+2Na-H]⁺		$b 1 $ , $b 2 $	$y 2 $ , $y 1 $ , $y 2 $ -H₂O, $y 1 $ -C₇H₇, $y 2 **$ -C₇H₇	[#-H₂O]⁺ , [#-NH₃-H₂O]⁺ , [#-C₇H₇]⁺

Scheme 1 Fragmentation paths of [GFG+Na]+

Fig.5 Breakdown graph for precursor [GGF+Na]+

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