Fragmentation of protonated tripeptides: The proline effect revisited
Fragmentation of protonated tripeptides: The proline effect revisited
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DOI：
10.1021/jp031093k
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发表时间：
2004-04-15
影响因子：
3.3
通讯作者：
Hopkinson, AC
中科院分区：
文献类型：
--
作者：
Grewal, RN;El Aribi, H;Hopkinson, AC
报道了质子化三肽在亚稳离子条件和碰撞诱导条件下的断裂。大多数质子化三肽在C-末端酰胺键处裂解以形成B（2）和y（1）离子，相对量取决于源自C-末端氨基酸残基的氨基酸的质子亲和力。在C-末端位置有一个脯氨酸残基的质子化三肽，几乎完全通过与脯氨酸相邻的酰胺键断裂而片段化，并主要产生Y（1）离子;在中心位置有脯氨酸残基的情况下，质子化三肽的断裂发生在N-末端和C-末端酰胺键上，形成Y-2和B（2）离子。最后，在N-末端位置具有脯氨酸的三肽中的两种，Pro-Pro-Pro和Pro-Gly-Gly，主要通过在N-末端的裂解而断裂，前者通过酰胺键的裂解而断裂，后者通过α 1离子的直接形成而断裂。质子化的Pro-Val-Gly片段在低能量下主要产生B（2）和α 1离子，但α 1离子在较高能量下成为主要产物。在质子化肽的片段化中，缺乏通过在脯氨酸的酰胺键C-末端裂解形成的B离子（即，在恶唑酮环中具有脯氨酸残基）通常归因于双环恶唑酮中的环张力。然而，在这里，我们表明，双环恶唑酮结构含有脯氨酸衍生前质子化的Gly-Pro-Gly的自由能仅高2.7千卡/摩尔，在B3 LYP/6-31++G（d，p）理论水平，比异构体nonbycloxazolone衍生质子化的Pro-Gly-Gly;也就是说，应变似乎是一个小的或可忽略的因素。Y-2离子在质子化Gly-Pro-Gly中的形成具有比B离子高3 kcal/mol的自由能的势垒。相比之下，质子化Gly-Phe-Gly中的差异增加至10 kcal/mol。与Y-2离子一起沿着形成的中性产物是CO和甲亚胺，而不是曾经提出的氮丙啶酮。
The fragmentation of protonated tripeptides under metastable ion conditions and collision-induced conditions are reported. The majority of protonated tripeptides cleave at the C-terminal amide bond to form b(2) and y(1) ions, with the relative amounts depending on the proton affinities of the amino acids derived from the C-terminal amino acid residue. Protonated tripeptides that have a proline residue at the C-terminal position fragment almost exclusively by cleavage of the amide bond adjacent to the proline and give mainly y(1) ions; where there is a proline residue in the central position, fragmentation of the protonated tripeptide occurs at both the N-terminal and the C-terminal amide bonds to form Y-2 and b(2) ions-, finally, two of the tripeptides with proline at the N-terminal position, Pro-Pro-Pro and Pro-Gly-Gly, fragment largely by cleavage at the N-terminal, the former by cleavage of the amide bond and the latter by direct formation of the a, ion. Protonated Pro-Val-Gly fragments to give predominantly b(2) and a, ions at low energy, but the a, ion becomes the dominant product at higher energies. In the fragmentation of protonated peptides, the lack of b ions formed by cleavage at the amide bond C-terminal to proline (i.e., with the proline residue in the oxazolone ring) has conventionally been attributed to ring strain in a bicyclic oxazolone. Here we show, however, that the bicyclic oxazolone structure containing proline derived front protonated Gly-Pro-Gly is only 2.7 kcal/mol higher in free energy, at the B3LYP/6-31++G(d,p) level of theory, than the isomeric nonbicyclic oxazolone derived from protonated Pro-Gly-Gly; that is, strain appears to be a small or negligible factor. Formation of the Y-2 ion in protonated Gly-Pro-Gly has a barrier of 3 kcal/mol higher free energy than that of the b ion. In comparison, the difference increases to 10 kcal/mol in protonated Gly-Phe-Gly. The neutral products that are formed along with the Y-2 ion are CO and methanimine, not aziridinone as once proposed.