Investigation on the Inclusions of PCB52 with Cyclodextrins by Performing DFT Calculations and Molecular Dynamics Simulations

The effective enrichment and identification of lowly concentrated polychlorinated biphenyls (PCBs) in the environment is attracting enormous research attention due to human health concerns. Cyclodextrins (CDs) are known to be capable of forming inclusion complexes with a variety of organic molecules. The purpose of this study is to provide theoretical evidence of whether CDs as host molecules can include the guest molecules PCBs to form stable host-guest inclusion complexes, and if so, whether the general infrared and Raman techniques are suitable for the direction of CD-modified PCBs. Focusing on a representative PCB molecule, 2,2',5,5'-tetrachlorobiphenyl (PCB52), we carried out density functional theory calculations and molecular dynamics (MD) simulations on its complexes with alpha-, beta-, and gamma-CDs with different host-guest stoichiometry ratios, including 1:1, 1:2, 2:1, and 2:2. On the basis of both the optimized geometries and calculated energy changes raised from encapsulating the guest molecule into the cavities of CDs, the CDs are believed to be suitable hosts for accommodating PCB52 guest molecules. The stability of inclusion complexes depends on both the type of CD and host-guest stoichiometry ratio. MD simulations give a clear picture of the scene on how the PCB52 molecule enters the cavity of beta-CD. The vibrational analyses on the 1:1 complexes of CDs provide information for the spectral characterization of the inclusion complexes: Raman spectroscopy can deliver the characteristic bands of PCB52, whereas IR spectroscopy cannot uniquely assign them, implying that Raman spectroscopy is a useful technique for the identification of CD-modified PCBs. The present theoretical results are expected to provide guidance for the relevant experimental research.