Probing the Structure and Charge State of Glutathione-Capped Au25(SG)18 Clusters by NMR and Mass Spectrometry
Zhikun Wu, Chakicherla Gayathri, Roberto R. Gil and Rongchao Jin*
Despite the recent crystallographic determination of the crystal structure of Au25(SCH2CH2Ph)18 clusters, the question−whether all thiolate-capped, 25-atom gold clusters adopt the same structure, regardless of the types of thiols (e.g., long-chain alkylthiols, aromatic thiols, or other functionalized ones)−still remains unanswered. To crystallize long-chain or bulky ligand (e.g., glutathione)-capped Au25(SR)18 clusters has proven to be difficult due to the major amorphousness caused by such ligands; therefore, one needs to seek other strategies to probe the structural information of such gold clusters. Herein, we report a strategy to probe the Au25 core structure and surface thiolate ligand distribution by means of NMR in combination with mass spectrometry. We use glutathione-capped Au25(SG)18 clusters as an example to demonstrate the utility of this strategy. One-dimensional (1D) and two-dimensional (2D) correlation NMR spectroscopic investigation of Au25(SG)18 reveals fine spectral features that explicitly indicate two types of surface binding modes of thiolates, which is consistent with the ligand distribution in the Au25(SCH2CH2Ph)18 cluster. Laser desorption ionization (LDI) mass spectrometry analysis shows that Au25(SG)18 exhibits an identical ionization and core fragmentation pattern with phenylethylthiolate-capped Au25 clusters. The charge state of the native Au25(SG)18 clusters was determined to be −1 by comparing their optical spectrum with those of [Au25(SCH2CH2Ph)18]q of different charge states (q = −1, 0). Taken together, our results led to the conclusion that glutathione-capped Au25(SG)18 clusters indeed adopt the same structure as that of Au25(SCH2CH2Ph)18. This conclusion is also valid for other types of thiolate-capped Au25 clusters, including hexyl- and dodecylthiolates. Interestingly, the chiral optical responses (e.g., circular dichroism (CD) signals in the visible wavelength region) from the Au25(SG)18 clusters seem to be imparted by the chiral glutathione ligands because no similar CD signals were observed in Au25(SCH2CH2Ph)18.