The product ion mass spectra obtained by CID of the b₉ ions derived by loss of neutral alanine from the MH⁺ ion of the peptides Tyr(Ala)₉, (Ala)₄Tyr(Ala)₅, and (Ala)₈TyrAla are essentially identical, indicative of full cyclization reaction to a common intermediate before fragmentation. This leads to abundant nondirect sequence ions in the product ion mass spectra of the b₉ ions. The product ion mass spectra of the b₈ ions from the first two peptides also are essentially identical. The fragmentation of the MH⁺ ions also leads to low intensity nondirect sequence ions in the product ion mass spectra. N-terminal acetylation blocks the cyclization and eliminates nondirect sequence fragment ions in the product ion mass spectra.

通过从肽Tyr(Ala) ₉、(Ala) ₄ Tyr(Ala) ₅和(Ala) ₈ TyrAla的MH ⁺离子损失中性丙氨酸而衍生的b ₉离子的CID 获得的产物离子质谱是基本相同，表明在断裂前完全环化反应为共同的中间体。这导致 b ₉离子的产物离子质谱中出现大量非直接序列离子。前两个肽段的 b ₈离子的产物离子质谱图也基本相同。MH ⁺的碎片化离子还会导致产物离子质谱图中的低强度非直接序列离子。N-末端乙酰化阻止环化并消除产物离子质谱中的非直接序列碎片离子。