Dunbar Robert C, Steill Jeffrey D, Polfer Nick C, Oomens Jos
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, USA.
J Phys Chem A. 2009 Feb 5;113(5):845-51. doi: 10.1021/jp8087176.
IRMPD spectroscopy using the FELIX free electron laser and a Fourier transform ICR mass spectrometer was used to characterize the structures of electrosprayed dimer complexes M(2+)Trp(2) of tryptophan with a series of eight doubly charged metal ions, including alkaline earths Ca, Sr, and Ba, and transition metals Zn, Cd, Mn, Co, and Ni. With the support of DFT thermochemical calculations, at least three different structural motifs were distinguished spectroscopically, depending critically on the nature of the metal ion. The spectral signatures of a ligand in the charge-solvated (CS) configuration, namely peaks near 1730 and 1150 cm(-1), were prominent in all the spectra, and it was clear that all the dimer complexes contain at least one CS ligand. The spectra indicated that the second ligand is zwitterionic (ZW) for all complexes except the Ni case, with the second ligand having an extended binding geometry with smaller metals but showing some admixture of a compact chelated geometry with larger alkaline earths. It was concluded that these dimer complexes have a mixed configuration of ligands, denoted CS/ZW. The Ni(2+)Trp(2) complex is exceptional, with the spectroscopy and the thermochemical calculation both indicating a CS/CS configuration of ligands. This geometry appears to correlate with the exceptionally small size and high binding strength of the Ni(2+) cation. The complex CdClTrp(1+) was also obtained and gave a clear spectrum showing a CS ligand configuration. The presence of a CS ligand in all the dimeric complexes of the 2+ metals is an interesting contrast with the monomer complex Ba(2+)Trp, in which the ligand is ZW.
利用费利克斯自由电子激光和傅里叶变换离子回旋共振质谱仪的红外多光子解离光谱（IRMPD）对色氨酸与一系列八种二价金属离子（包括碱土金属钙、锶和钡以及过渡金属锌、镉、锰、钴和镍）形成的电喷雾二聚体配合物M(2+)Trp(2)的结构进行了表征。在密度泛函理论（DFT）热化学计算的支持下，通过光谱区分出了至少三种不同的结构基序，这主要取决于金属离子的性质。电荷溶剂化（CS）构型中配体的光谱特征，即1730和1150 cm(-1)附近的峰，在所有光谱中都很突出，并且很明显所有二聚体配合物都至少包含一个CS配体。光谱表明，除了镍的情况外，所有配合物的第二个配体都是两性离子（ZW），第二个配体与较小的金属形成扩展的结合几何结构，但与较大的碱土金属显示出一些紧凑螯合几何结构的混合。得出的结论是，这些二聚体配合物具有配体的混合构型，记为CS/ZW。Ni(2+)Trp(2)配合物是个例外，光谱和热化学计算均表明其配体为CS/CS构型。这种几何结构似乎与Ni(2+)阳离子异常小的尺寸和高结合强度相关。还获得了配合物CdClTrp(1+)，并给出了清晰的光谱，显示出CS配体构型。二价金属的所有二聚体配合物中都存在CS配体，这与单体配合物Ba(2+)Trp形成了有趣的对比，后者的配体是ZW。
