Single-Conformation Spectroscopy of Capped Aminoisobutyric Acid Dipeptides: The Effect of C-Terminal Cap Chromophores on Conformation
Single-Conformation Spectroscopy of Capped Aminoisobutyric Acid Dipeptides: The Effect of C-Terminal Cap Chromophores on Conformation
复制标题
加帽氨基异丁酸二肽的单构象光谱：C 端帽发色团对构象的影响
DOI：
10.1021/acs.jpca.9b01698
复制
发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Zwier, Timothy S.
中科院分区：
文献类型：
--
作者：
Fischer, Joshua L.;Elvir, Brayan R.;DeLucia, Sally-Ann;Blodgett, Karl N.;Zeller, Matthias;Kubasik, Matthew A.;Zwier, Timothy S.
已知氨基异丁酸（Aib）低聚物形成对映体左手和右手结构的外消旋混合物。引入手性帽将对映异构体结构转化为非对映异构体，其原则上提供光谱差异。在这里，我们使用双共振激光光谱在气相中基于模型Aib二肽筛选不同的C-末端帽，以记录超声膨胀中存在的各个构象的IR和UV光谱：NH-苄基（NHBn）作为参考结构，因为它在类似研究中通常用作荧光团，NH-对-氟苄基（NHBn-F）和α-甲基苄胺（AMBA）。对于NHBn和NHBn-F帽，观察到单一构象异构体，红外光谱可归属于这些分子中缺乏手性中心的II/II ′ β-转角对映体对。NHBn-F帽的较高振荡器强度使得能够实现UV-UV烧孔，这在NHBn帽的情况下不易实现。带有手性中心的AMBA封端结构产生了两种独特的构象异构体，其中一种是几乎相同的左手II型β-转角，而次要构象异构体被分配到C7-C7顺序双环，这是27-带的涌现形式。虽然没有观察到，但具有相反手性的II ′ β-转角非对映异构体的能量高出11 kJ/mol，这是一个令人惊讶的大差异。这种不稳定主要归因于肽的C-末端酰基氧与AMBA基团的手性诱导甲基之间的空间干扰。最后，计算证据表明，使用N-末端芳香族帽阻碍了Ac-Aib 2二肽中310-螺旋的形成。
Aminoisobutyric acid (Aib) oligomers are known to form racemic mixtures of enantiomeric left- and right-handed structures. The introduction of a chiral cap converts the enantiomeric structures into diastereomers that, in principle, afford spectroscopic differentiation. Here, we screen different C-terminal caps based on a model Aib dipeptide using double resonance laser spectroscopy in the gas phase to record IR and UV spectra of individual conformations present in the supersonic expansion: NH-benzyl (NHBn) as a reference structure because of its common use as a fluorophore in similar studies, NH-p-fluorobenzyl (NHBn-F), and α-methylbenzylamine (AMBA). For both the NHBn and NHBn-F caps, a single conformer is observed, with infrared spectra assignable to an enantiomeric pair of type II/II′ β-turns in these molecules lacking a chiral center. The higher oscillator strength of the NHBn-F cap enabled UV–UV hole burning, not readily accomplished with the NHBn cap. The AMBA-capped structure, with its chiral center, produced two unique conformers, one of which was a nearly identical left-handed type II β-turn, while the minor conformer is assigned to a C7–C7 sequential double ring, which is an emergent form of a 27-ribbon. Although not observed, the type II′ β-turn diastereomer, with opposite handedness, is calculated to be 11 kJ/mol higher in energy, a surprisingly large difference. This destabilization is attributed primarily to steric interference between the C-terminal acyl oxygen of the peptide and the chirality-inducing methyl of the AMBA group. Last, computational evidence indicates that the use of an N-terminal aromatic cap hinders the formation of a 310-helix in Ac-Aib2dipeptides.