The MSⁿ spectra of the [M + H]⁺ and b ₅ peaks derived from the peptides HAAAAA, AHAAAA, AAHAAA, AAAHAA, and AAAAHA have been measured, as have the spectra of the b ₄ ions derived from the first four peptides. The MS² spectra of the [M + H]⁺ ions show a substantial series of b_n ions with enhanced cleavage at the amide bond C-terminal to His and substantial cleavage at the amide bond N-terminal to His (when there are at least two residues N-terminal to the His residue). There is compelling experimental and theoretical evidence for formation of nondirect sequence ions via cyclization/reopening chemistry in the CID spectra of the b tons when the His residue is near the C-terminus. The experimental evidence is less clear for ions when the His residue is near the N-terminus, although this may be due to the use of multiple alanine residues in the peptide making identifying scrambled peaks more difficult. The product ion mass spectra of the b ₄ and b ₅ ions from these isomeric peptides with cyclically permuted amino acid sequences are similar, but also show clear differences. This indicates less active cyclization/reopening followed by fragmentation of common structures for b _n ions containing His than for sequences of solely aliphatic residues. Despite more energetically favorable cyclization barriers for the b ₅ structures, the b ₄ ions experimental data show more clear evidence of cyclization and sequence scrambling before fragmentation. For both b ₄ and b ₅ the energetically most favored structure is a macrocyclic isomer protonated at the His side chain.

已经测量了源自肽 HAAAAA、AHAAAA、AAHAAA、AAAHAA 和 AAAAHA的 [M + H] ⁺和b ₅峰的 MS ⁿ光谱，以及源自前四种肽的b ₄离子的光谱。[M + H] ⁺离子的 MS ²光谱显示了大量 b _n离子，在 His 的酰胺键 C-末端处的裂解增强，在 His 的酰胺键 N-末端处出现大量裂解（当存在于His 残基 N 端至少有两个残基）。有令人信服的实验和理论证据表明，在 CID 光谱中通过环化/重新打开化学形成非直接序列离子。 当 His 残基靠近 C 端时b吨。当 His 残基靠近 N 端时，离子的实验证据不太清楚，尽管这可能是由于在肽中使用了多个丙氨酸残基，使得识别杂乱的峰变得更加困难。这些具有循环置换氨基酸序列的异构肽的b ₄和b ₅离子的产物离子质谱相似，但也显示出明显的差异。这表明，与仅脂肪族残基的序列相比，含有 His 的b _n离子的环化/重新打开活性较低，随后常见结构的断裂。尽管b ₅在能量上更有利于环化障碍 结构方面，b ₄离子实验数据显示了更清晰的环化和碎裂前序列扰乱的证据。对于b ₄和b _{5 而言}，能量上最有利的结构是在His侧链质子化的大环异构体。