Department of Chemistry and Centre for Research in Mass Spectrometry , York University , 4700 Keele Street , Toronto , ON M3J 1P3 , Canada.
Department of Chemistry and Biochemistry , University of Windsor , 401 Sunset Avenue , Windsor , ON N9B 3P4 , Canada.
J Phys Chem B. 2019 Dec 5;123(48):10192-10201. doi: 10.1021/acs.jpcb.9b09090. Epub 2019 Nov 19.
[a + H] ions generated from Ln/tripeptide complexes, where Ln = La or Ce, have similar structures to the linear [a] ions but with protonation at both the terminal NH and N═CH groups. Ion stability is favored by having the basic secondary amine of the proline residue at the N-terminus and by an amino acid residue accommodating one of the protons on the side chain. Dissociation of [a + H] ions derived from peptides containing only aliphatic residues is by cleavage of the second amide bond to give [b] or [a] ions along with internal [a] ions. For [a + H] ions containing a tryptophan residue in the central location, in addition to cleavage of the amide bond, losses of neutrals NH, HN═CHR, (NH + CO), and HNCO were observed. Dissociations of some unsolvated Ln/tripeptide complexes gave [b + H] ions in low abundance; formation of these [b + H] ions was favored by the presence of a proline residue at the N-terminus and by either a histidine or tryptophan residue in the central position. Dissociation of these [b + H] ions was by the loss of (HO + CO) and not only CO, indicating that these ions did not have the same type of oxazolone structure as found for [b] ions. Density functional theory calculations suggest that the observed [b + H] ions of ProGlyGly were formed from [Ce(ProGlyGly)] complexes in which the peptide was bound to the metal ion as an enolate. Dissociation of the slightly lower-energy complex, where the peptide is bound in the keto form, would produce an oxazolone but the high barrier required to create this isomer of the [b + H] ion would be sufficient to result in further dissociation. Two isomers of the [b + H] ion of ProHisGly have been created, one from the [Ce(ProHisGly)] complex that characteristically dissociates by the combined loss of (HO + CO) and the other by the loss of glycine from [ProHisGlyGly + 2H]. The [b + H] ion derived from [ProHisGlyGly + 2H] dissociated by the loss of only CO.
从 Ln/三肽配合物中产生的[a + H]离子与线性[a]离子具有相似的结构，但在末端 NH 和 N═CH 基团处质子化。通过在 N-末端具有脯氨酸残基的碱性仲胺和容纳侧链上一个质子的氨基酸残基，有利于离子稳定性。仅含有脂族残基的肽衍生的[a + H]离子的解离是通过第二个酰胺键的断裂产生[b]或[a]离子以及内部[a]离子。对于在中心位置含有色氨酸残基的[a + H]离子，除了酰胺键的断裂之外，还观察到中性 NH、HN═CHR、(NH + CO)和 HNCO 的损失。一些未溶剂化的 Ln/三肽配合物的解离产生低丰度的[b + H]离子；在 N-末端存在脯氨酸残基和在中心位置存在组氨酸或色氨酸残基时，有利于这些[b + H]离子的形成。这些[b + H]离子的解离是通过(HO + CO)的损失而不是仅仅 CO 的损失，表明这些离子与[b]离子的相同类型的噁唑啉结构不同。密度泛函理论计算表明，观察到的 ProGlyGly 的[b + H]离子是由[Ce(ProGlyGly)]配合物形成的，其中肽作为烯醇化物结合在金属离子上。略微低能量配合物的解离，其中肽以酮形式结合，会产生噁唑啉，但形成[b + H]离子的这种异构体所需的高势垒足以导致进一步的解离。ProHisGly 的[b + H]离子的两种异构体已经被创建，一种来自[Ce(ProHisGly)]配合物，其特征性地通过(HO + CO)的联合损失和另一种通过从[ProHisGlyGly + 2H]损失甘氨酸来解离。源自[ProHisGlyGly + 2H]的[b + H]离子通过仅 CO 的损失而解离。
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