Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
J Am Soc Mass Spectrom. 2012 Aug;23(8):1336-50. doi: 10.1007/s13361-012-0408-9. Epub 2012 Jun 6.
Amino acid residue-specific backbone and side-chain dissociations of peptide z ions in MS(3) spectra were elucidated for over 40 pentapeptides with arginine C-terminated sequences of the AAXAR and AAHXR type, nonapeptides of the AAHAAXX"AR and AAHAXAX"AR type, and AAHAAXX"AAR decapeptides. Peptide z(n) ions containing amino acid residues with readily transferrable benzylic or tertiary β-hydrogen atoms (Phe, Tyr, His, Trp, Val) underwent facile backbone cleavages to form dominant z(n-2) or z(n-3) ions. These backbone cleavages are thought to be triggered by a side-chain β-hydrogen atom transfer to the z ion C(α) radical site followed by homolytic dissociation of the adjacent C(α)-CO bond, forming x(n-2) cation-radicals that spontaneously dissociate by loss of HNCO. Amino acid residues that do not have readily transferrable β-hydrogen atoms (Gly, Ala) do not undergo the z(n) → z(n-2) dissociations. The backbone cleavages compete with side-chain dissociations in z ions containing Asp and Asn residues. Side-chain dissociations are thought to be triggered by α-hydrogen atom transfers that activate the C(β)-C(γ) or C(β)-heteroatom bonds for dissociations that dominate the MS(3) spectra of z ions from peptides containing Leu, Cys, Lys, Met, Ser, Arg, Glu, and Gln residues. The Lys, Arg, Gln, and Glu residues also participate in γ-hydrogen atom transfers that trigger other side-chain dissociations.
鉴定了超过 40 种具有精氨酸 C 末端序列 AAXAR 和 AAHXR 型、AAHAAXX"AR 和 AAHAXAX"AR 型非肽和 AAHAAXX"AAR 十肽的肽 z 离子中氨基酸残基特异性的主链和侧链解离。含有易转移苄基或叔β-氢原子（苯丙氨酸、色氨酸、组氨酸、色氨酸、缬氨酸）的肽 z(n)离子经历了容易的主链裂解，形成了主要的 z(n-2)或 z(n-3)离子。这些主链裂解被认为是由侧链β-氢原子转移到 z 离子 C(α)自由基位点引发的，随后相邻的 C(α)-CO 键发生均裂，形成 x(n-2)阳离子自由基，它们通过失去 HNCO 自发解离。没有易转移β-氢原子的氨基酸残基（甘氨酸、丙氨酸）不会发生 z(n)→z(n-2)裂解。含有天冬氨酸和天冬酰胺残基的 z 离子中的主链裂解与侧链裂解竞争。侧链裂解被认为是由α-氢原子转移引发的，这种转移激活了 C(β)-C(γ)或 C(β)-杂原子键，从而引发了主导包含亮氨酸、半胱氨酸、赖氨酸、甲硫氨酸、丝氨酸、精氨酸、谷氨酸和谷氨酰胺残基的肽的 z 离子的 MS(3)谱的裂解。赖氨酸、精氨酸、谷氨酰胺和谷氨酸残基还参与γ-氢原子转移，引发其他侧链解离。
