Probing the Stability of Proline Cis/Trans Isomers in the Gas Phase with Ultraviolet Photodissociation
Probing the Stability of Proline Cis/Trans Isomers in the Gas Phase with Ultraviolet Photodissociation
复制标题
用紫外光解法探讨脯氨酸顺/反异构体在气相中的稳定性
DOI：
10.1021/jasms.0c00242
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发表时间：
2020
影响因子：
3.2
通讯作者：
Julian, Ryan R.
中科院分区：
文献类型：
--
作者：
Silzel, Jacob W.;Murphree, Taylor A.;Paranji, Rajan K.;Guttman, Miklos M.;Julian, Ryan R.
虽然蛋白质中的大多数肽键以反式构型存在，但当顺式肽键确实存在时，它们会对蛋白质结构和功能产生重大影响。因此，快速鉴定顺式肽键是一项重要的任务。含有脯氨酸的肽键更有可能采用顺式构型，因为连接脯氨酸侧链和主链的环相对于具有游离侧链的氨基酸具有能量景观。顺式脯氨酸异构体的例子已被确定在溶液中，并在气相中通过各种结构探测方法。质谱法是一个有吸引力的潜在的方法，用于识别顺式脯氨酸由于其速度和灵敏度，然而，问题仍然存在的顺式/反式脯氨酸异构体起源于溶液中的电离和操作过程中保留在质谱仪。在这里，我们调查的气相稳定性分离的溶液相顺式和反式脯氨酸异构体使用合成肽序列与Tyr-Pro-Pro基序。探索了各种解离方法来评估它们区分顺式/反式构型的潜力，包括碰撞诱导解离、自由基定向解离和光解。只有光解离结合极其温和的电喷雾和电荷溶剂化的18-冠-6醚能够区分顺式/反式异构体为我们的模型肽，这表明任何热活化过程中转移或同时在气相中导致异构体混乱。此外，18-冠-6的必要性表明，在电喷雾电离过程中发生的分子内电荷溶剂化可以覆盖顺式/反式异构体的均匀性。总体而言，结果表明，溶液相的顺式/反式脯氨酸异构体是脆弱的，很容易失去电喷雾过程中，需要仔细选择仪器参数和考虑的电荷溶剂化，以防止顺式/反式乱序。
Although most peptide bonds in proteins exist in the trans configuration, when cis peptide bonds do occur, they can have major impact on protein structure and function. The rapid identification of cis peptide bonds is therefore an important task. Peptide bonds containing proline are more likely to adopt the cis configuration because the ring connecting the side chain and backbone in proline flattens the energetic landscape relative to amino acids with free side chains. Examples of cis proline isomers have been identified in both solution and in the gas phase by a variety of structure-probing methods. Mass spectrometry is an attractive potential method for identifying cis proline due to its speed and sensitivity; however, the question remains of whether cis/trans proline isomers originating in solution are preserved during ionization and manipulation within a mass spectrometer. Herein, we investigate the gas-phase stability of isolated solution-phase cis and trans proline isomers using a synthetic peptide sequence with a Tyr-Pro-Pro motif. A variety of dissociation methods were explored to evaluate their potential to distinguish cis/trans configuration, including collision-induced dissociation, radical-directed dissociation, and photodissociation. Only photodissociation employed in conjunction with extremely gentle electrospray and charge solvation by 18-crown-6 ether was able to distinguish cis/trans isomers for our model peptide, suggesting that any thermal activation during transfer or while in the gas phase leads to isomer scrambling. Furthermore, the necessity for 18-crown-6 suggests that intramolecular charge solvation taking place during electrospray ionization can override cis/trans isomer homogeneity. Overall, the results suggest that solution-phase cis/trans proline isomers are fragile and easily lost during electrospray, requiring careful selection of instrument parameters and consideration of charge solvation to prevent cis/trans scrambling.