小肽的子离子比母离子还大？为什么？

wqrwxm

2016/07/07

私聊

生物质谱

有一篇文献用MRM 方法测定CYP3A4，肽段是LSLGGLLQPEKPVVLK。文献上选择母离子 m/z=564.6, 子离子m/z=746。
为什么母离子比子离子还大？
难道是母离子电荷比子离子多？

原文链接：http://www.sciencedirect.com/science/article/pii/S0006295214005838
原文方法如下：
The CYP protein quantification assays were performed on a triple-quadrupole $$lc-MS instrument (Agilent 6460A) coupled to an Agilent 1290 Infinity $$lc system (Agilent Technologies), in ESI positive ionization mode. Approximately 2 μg of the trypsin digest (5 μl) was injected onto the column (Kinetex 1.7 μ, C18 100A; 100 × 2.1 mm, Phenomenex, Torrance, CA). Mobile phases consisted of water (A) and acetonitrile (B) (both containing 0.1% formic acid) at a flow rate of 0.3 ml/min. The gradient was 3% mobile phase B for 2.0 min, followed by gradient elution program with mobile phase B concentration of 3 to 18% (2.0-4.0 min), 18 to 22% (4.0-8.0 min), 22 to 24% (8.0-10 min), 24% (10-12.2 min), 24 to 34% (12.2-12.5 min) and 34 to 38% (12.5-15.0 min). This was followed by washing with 80% mobile phase B for 0.9 min and re-equilibration for 4.9 min. Surrogate peptides and internal standards were monitored in multiple reaction monitoring (MRM) mode using instrument parameters provided in Table 1. HP$$lc-MS/MS data were processed using the MassHunter (Agilent Technologies) and Skyline software.

Table 1.

Optimized MS/MS parameters used for quantification of surrogate peptides and internal standards of CYP enzymes in trypsin digested microsomes.

Enzyme	Peptide	Precursor ion (m/z)	Product ion (m/z)	Fragmentor (V)	Collision energy (eV)	LLOQ (fmol on-column)
CYP3A4	LSLGGLLQPEKPVVLK	564.6	746	100	9	0.3
		564.6	689.4	100	10
	LSLGGLLQPEKPVVLK	567.3	750	100	9
		567.3	693.4	100	9
CYP3A5	DSIDPYIYTPFGTGPR	900.0	995.7	130	28	1
		900.0	684.9	130	20
	DSIDPYIYTPFGTGPR	904.9	1005.5	130	28
		904.9	689.9	130	28
CYP3A7	FNPLDPFVLSIK	695.4	918.4	110	16	0.1
		695.4	564.7	110	16
	FNPLDPFVLSIK	699.4	926.5	110	16
		699.4	568.8	110	16
CYP2C9	GIFPLAER	451.9	585.4	75	7	0.15
		451.9	366.8	75	7
	GIFPLAER	456.8	595.3	75	7
		456.8	371.7	75	7
CYP2C19	IYGPVFTLYFGLER	838.0	1145.4	145	28	1
		838.0	998.3	145	28
	IYGPVFTLYFGLER	843.0	1155.6	145	28
		843.0	1008.5	145	28
CYP2A6	GTGGANIDPTFFLSR	777.1	982.5	130	22	0.4
		777.1	867.2	130	26
	GTGGANIDPTFFLSR	781.9	992.5	130	26
		781.9	877.5	130	26
CYP2D6	GTTLITNLSSVLK	673.9	974.4	110	16	0.6
		673.9	861.3	110	16
	GTTLITNLSSVLK	677.9	982.4	110	16
		677.9	869.3	110	16
CYP reductase	FAVFGLGNK	476.9	734.3	100	9	0.05
		476.9	635.3	100	9
	FAVFGLGNK	480.8	742.4	100	9
		480.8	643.4	100	9