含芳香氨基酸三肽及其衍生物自由基离子的形成和解离机理研究

铜-配体(L)-三肽组成的三元复合物［Cu(L)M］²⁺，其中，L表示4′-氯-2,2′:6′,2″-三联吡啶(缩写为4Cl-tpy)；M表示酪氨酰-甘氨酰-色氨酸(YGW)及其修饰型三肽(CH₃CO-YGW-OCH₃，缩写 Ac-YGW-OMe)。使用该复合物，通过碰撞诱导解离 (collision-induced dissociation, CID)产生两种自由基离子 (［YGW］^·+和［Ac-YGW-OMe］^·+)。采用串联质谱结合密度泛函理论 (density functional theory, DFT)得到气相稳定结构，并研究其气相解离行为。研究结果表明，［YGW］^·+和［Ac-YGW-OMe］^·+的气相解离行为截然不同，［YGW］^·+主要产生［M-CO₂-116］⁺和［M-CO₂］^·+碎片离子；而［Ac-YGW-OMe］^·+在气相中主要产生［M-CH₃OH］^·+碎片离子。推测这两种离子的气相裂解机理分别为：［YGW］^·+羧基上的质子重排到多肽骨架中羰基氧上，经历 C_α-C键的断裂产生［M-CO₂］^·+、断裂色氨酸侧链 C_β-C_γ键产生［M-CO₂-116］⁺离子；［Ac-YGW-OMe］^·+则先经历质子重排到酯基氧上，然后经过C-O酯键的断裂形成［M-CH₃OH］^·+离子。参与重排的质子可能有3个来源：Ac-YGW-OMe中甘氨酸的C_α-H、色氨酸的C_α-H 或C_β-H，该机理有待进一步验证。本研究将为其他类型多肽及衍生物的结构及气相反应机理研究提供参考。

Formation and Dissociation Mechanism of the Radical Cations of Aromatic Tripeptide and Its Derivative

Structure and gas-phase fragmentation of tyrosyl-glycyl-tryptophan ［YGW］^·+ and its derivative ［Ac-YGW-OMe］^·+ have been studied using electrospray tandem mass spectrometry (ESI-MS/MS) combined with density functional theory (DFT). These peptide radical ions were generated via multistage collision-induced dissociation (CID) of transition metal-ligand-peptide tertiary complexes, ［Cu(L)M］²⁺ (L=4′-chloro-2,2′:6′,2″-terpyridine (4Cl-tpy); M=YGW or Ac-YGW-OMe). Low-energy CID experiments revealed that the fragmentation mechanisms of ［YGW］^·+ and ［Ac-YGW-OMe］^·+ are dramatically different. ［YGW］^·+ dissociate mainly through the loss of CO₂ and subsequent loss of indole radical to produce ［M-CO₂］^·+(m/z 380.05) and ［M-CO₂-116］⁺ (m/z 264.05) fragment ions. Other minor fragments include ［c₂+2H］⁺ (m/z 238.04), ［G·GW］⁺ (m/z 318.05) and ［1H-indole］·+ (m/z 117.23). In contrast, the main fragmentation of ［Ac-YGW-OMe］^·+ is the loss of CH3OH to give rise to ［M-CH₃OH］^·+ (m/z 448.14) product ion. Other minor fragments are ［z1-H］·+ (m/z 200.93), ［c₂+2H］⁺ (m/z 279.99) and ［M-CH₃COO·］⁺ (m/z 421.16). The gas-phase fragmentation mechanisms of ［YGW］^·+ and ［Ac-YGW-OMe］·+ are proposed based on the aforementioned CID results. For ［YGW］^·+, the major fragment ions ［M-CO₂］^·+ (m/z 380.05) and ［M-CO₂-116］⁺ (m/z 264.05) are generated through proton transfer from the carboxylic OH group to the amide oxygen to form carboxyl radical which undergoes cleavages of C_α-C and C_β-C_γ bonds, respectively. ［G·GW］⁺ (m/z 318.05) fragment ion is formed by C_α-C_β bond cleavage with the loss of p-quinomethide, and ［c₂+2H］⁺ (m/z 238.04) fragment ion is produced by proton transfer from the C_β-H of tryptophan to the amide oxygen to produce the β radical followed by N-C_α bond cleavage. For ［Ac-YGW-OMe］^·+, the main product ion ［M-CH₃OH］^·+ (m/z 448.14) is generated through proton transfer from three possible sources (C_α-H from glycine, Cα-H from tryptophan, C_β-H from side chain of tryptophan) to methyl ester oxygen followed by C-O bond cleavage, ［z₁-H］^·+ (m/z 200.93) fragment ion is produced through proton transfer from the C_β-H of tryptophan to the amide oxygen to form β radical which undergoes N-C_α bond cleavage. The DFT calculation results suggest that the optimized geometries of ［YGW］·+ and ［Ac-YGW-OMe］^·+ are non-zwitterionic structures with a weak hydrogen bond between the amino nitrogen and the indole nitrogen (［N₁-H⁺…N₄］ (bond length ca. 2.043Å) in ［YGW］^·+ and a strong hydrogen bond between the acetyl oxygen and the indole nitrogen (［N4-H⁺…O₁］ (bond length ca.1.666Å) in ［Ac-YGW-OMe］^·+, respectively.