Jami-Alahmadi Yasaman, Linford Bryan D, Fridgen Travis D
Department of Chemistry, Memorial University , St. John's, Newfoundland A1N 4T8, Canada.
J Phys Chem B. 2016 Dec 29;120(51):13039-13046. doi: 10.1021/acs.jpcb.6b09588. Epub 2016 Dec 15.
The unimolecular chemistries and structures of gas-phase (ProLeu)M and (LeuPro)M complexes when M = Li, Na, Rb, and Cs have been explored using a combination of SORI-CID, IRMPD spectroscopy, and computational methods. CID of both (LeuPro)M and (ProLeu)M showed identical fragmentation pathways and could not be differentiated. Two of the fragmentation routes of both peptides produced ions at the same nominal mass as (Pro)M and (Leu)M, respectively. For the litiated peptides, experiments revealed identical IRMPD spectra for each of the m/z 122 and 138 ions coming from both peptides. Comparison with computed IR spectra identified them as the (Pro)Li and (Leu)Li, and it is concluded that both zwitterionic and canonical forms of (Pro)Li exist in the ion population from CID of both (ProLeu)Li and (LeuPro)Li. The two isomeric peptide complexes could be distinguished using IRMPD spectroscopy in both the fingerprint and the CH/NH/OH regions. The computed IR spectra for the lowest energy structures of each charge solvated complexes are consistent with the IRMPD spectra in both regions for all metal cation complexes. Through comparison between the experimental spectra, it was determined that in lithiated and sodiated ProLeu, metal cation is bound to both carbonyl oxygens and the amine nitrogen. In contrast, the larger metal cations are bound to the two carbonyls, while the amine nitrogen is hydrogen bonded to the amide hydrogen. In the lithiated and sodiated LeuPro complexes, the metal cation is bound to the amide carbonyl and the amine nitrogen while the amine nitrogen is hydrogen bonded to the carboxylic acid carbonyl. However, there is no hydrogen bond in the rubidiated and cesiated complexes; the metal cation is bound to both carbonyl oxygens and the amine nitrogen. Details of the position of the carboxylic acid C═O stretch were especially informative in the spectroscopic confirmation of the lowest energy computed structures.
利用串级脉冲解离碰撞诱导解离（SORI-CID）、红外多光子解离光谱（IRMPD）和计算方法相结合的方式，研究了M = Li、Na、Rb和Cs时气相（ProLeu）M和（LeuPro）M配合物的单分子化学性质和结构。（LeuPro）M和（ProLeu）M的CID显示出相同的碎裂途径，无法区分。两种肽的两条碎裂途径分别产生了与（Pro）M和（Leu）M具有相同标称质量的离子。对于锂化肽，实验表明来自两种肽的m/z 122和138离子的IRMPD光谱相同。与计算得到的红外光谱进行比较，确定它们为（Pro）Li和（Leu）Li，并且得出结论，（ProLeu）Li和（LeuPro）Li的CID产生的离子群体中同时存在（Pro）Li的两性离子形式和标准形式。在指纹区和CH/NH/OH区，利用IRMPD光谱可以区分这两种异构肽配合物。每种电荷溶剂化配合物最低能量结构的计算红外光谱与所有金属阳离子配合物在这两个区域的IRMPD光谱一致。通过实验光谱之间的比较，确定在锂化和钠化的ProLeu中，金属阳离子与两个羰基氧和胺氮结合。相比之下，较大的金属阳离子与两个羰基结合，而胺氮与酰胺氢形成氢键。在锂化和钠化的LeuPro配合物中，金属阳离子与酰胺羰基和胺氮结合，而胺氮与羧酸羰基形成氢键。然而，在铷化和铯化的配合物中没有氢键；金属阳离子与两个羰基氧和胺氮结合。羧酸C═O伸缩振动位置的细节在最低能量计算结构的光谱确认中特别有用。
