Neutral loss of water and ammonia are often significant fragmentation channels upon collisional activation of protonated peptides. Here, we deploy infrared ion spectroscopy to investigate the dehydration reactions of protonated AlaSer, AlaThr, GlySer, GlyThr, PheSer, PheThr, ProSer, ProThr, AsnSer, and AsnThr, focusing on the question of the structure of the resulting [M + H - H2O]+ fragment ion and the site from which H2O is expelled. In all cases, the second residue of the selected peptides contains a hydroxyl moiety, so that H2O loss can potentially occur from this side-chain, as an alternative to loss from the C-terminal free acid of the dipeptide. Infrared action spectra of the product ions along with quantum-chemical calculations unambiguously show that dehydration consistently produces fragment ions containing an oxazoline moiety. This contrasts with the common oxazolone structure that would result from dehydration at the C-terminus analogous to the common b/y dissociation forming regular b2-type sequence ions. The oxazoline product structure suggests a reaction mechanism involving water loss from the Ser/Thr side-chain with concomitant nucleophilic attack of the amide carbonyl oxygen at its β-carbon, forming an oxazoline ring. However, an extensive quantum-chemical investigation comparing the potential energy surfaces for three entirely different dehydration reaction pathways indicates that it is actually the backbone amide oxygen atom that leaves as the water molecule.

中文翻译：

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质子化的XxxSer和XxxThr二肽的失水会产生恶唑啉而不是恶唑酮产物离子。

水和氨的中性损失通常是质子化肽碰撞活化后的重要片段化通道。在这里，我们部署红外离子光谱技术来研究质子化的AlaSer，AlaThr，GlySer，GlyThr，PheSer，PheThr，ProSer，ProThr，AnsSer和AsnThr的脱水反应，重点是生成的[M + H- H2O] +碎片离子和H2O排出的位置。在所有情况下，所选肽的第二个残基均包含羟基部分，因此可能会从该侧链发生H2O损失，作为从二肽C端游离酸损失的替代方法。产物离子的红外作用谱以及量子化学计算清楚地表明，脱水作用始终产生含有恶唑啉部分的碎片离子。这与普通的恶唑酮结构形成对比，后者是由C端的脱水产生的，类似于形成常规b2型序列离子的常见b / y解离。恶唑啉产物结构表明反应机理涉及水从Ser / Thr侧链失水，同时酰胺羰基氧在其β-碳上发生亲核攻击，形成恶唑啉环。但是，一项广泛的量子化学研究比较了三种完全不同的脱水反应途径的势能面，结果表明实际上是作为水分子离开的主链酰胺氧原子。恶唑啉产物结构表明反应机理涉及水从Ser / Thr侧链失水，同时酰胺羰基氧在其β-碳上发生亲核攻击，形成恶唑啉环。但是，一项广泛的量子化学研究比较了三种完全不同的脱水反应途径的势能面，结果表明实际上是作为水分子离开的主链酰胺氧原子。恶唑啉产物结构表明反应机理涉及水从Ser / Thr侧链失水，同时酰胺羰基氧在其β-碳上发生亲核攻击，形成恶唑啉环。但是，一项广泛的量子化学研究比较了三种完全不同的脱水反应途径的势能面，结果表明实际上是作为水分子离开的主链酰胺氧原子。