Spectroscopic and Theoretical Studies of Hg(II) Complexation with Some Dicysteinyl Tetrapeptides

Tetrapeptides containing a Cys-Gly-Cys motif and a propensity to adopt a reverse-turn structure were synthesized to evaluate how O-, N-, H-, and aromatic π donor groups might contribute to mercury(II) complex formation. Tetrapeptides Xaa-Cys-Gly-Cys, where Xaa is glycine, glutamate, histidine, or tryptophan, were prepared and reacted with mercury(II) chloride. Their complexation with mercury(II) was studied by spectroscopic methods and computational modeling. UV-vis studies confirmed that mercury(II) binds to the cysteinyl thiolates as indicated by characteristic ligand-to-metal-charge-transfer transitions for bisthiolated S-Hg-S complexes, which correspond to 1 : 1 mercury-peptide complex formation. ESI-MS data also showed dominant 1 : 1 mercury-peptide adducts that are consistent with double deprotonations from the cysteinyl thiols to form thiolates. These complexes exhibited a strong positive circular dichroism band at 210 nm and a negative band at 193 nm, indicating that these peptides adopted a β-turn structure after binding mercury(II). Theoretical studies confirmed that optimized 1 : 1 mercury-peptide complexes adopt β-turns stabilized by intramolecular hydrogen bonds. These optimized structures also illustrate how specific N-terminal side-chain donor groups can assume intramolecular interactions and contribute to complex stability. Fluorescence quenching results provided supporting data that the indole donor group could interact with the coordinated mercury. The results from this study indicate that N-terminal side-chain residues containing carboxylate, imidazole, or indole groups can participate in stabilizing dithiolated mercury(II) complexes. These structural insights on peripheral mercury-peptide interactions provide additional understanding of the chemistry of mercury(II) with side-chain donor groups in peptides.

Syntheses and Structural Investigations of Penta-Coordinated Co(II) Complexes with Bis-Pyrazolo-S-Triazine Pincer Ligands, and Evaluation of Their Antimicrobial and Antioxidant Activities

Two penta-coordinated [Co(MorphBPT)Cl2]; 1 and [Co(PipBPT)Cl2]; 2 complexes with the bis-pyrazolyl-s-triazine pincer ligands MorphBPT and PipBPT were synthesized and characterized. Both MorphBPT and PipBPT act as NNN-tridentate pincer chelates coordinating the Co(II) center with one short Co-N(s-triazine) and two longer Co-N(pyrazole) bonds. The coordination number of Co(II) is five in both complexes, and the geometry around Co(II) ion is a distorted square pyramidal in 1, while 2 shows more distortion. In both complexes, the packing is dominated by Cl…H, C-H…π, and Cl…C (anion-π stacking) interactions in addition to O…H interactions, which are found only in 1. The UV-Vis spectral band at 564 nm was assigned to metal–ligand charge transfer transitions based on TD-DFT calculations. Complexes 1 and 2 showed higher antimicrobial activity compared to the respective free ligand MorphBPT and PipBPT, which were not active. MIC values indicated that 2 had better activity against S. aureus, B. subtilis, and P. vulgaris than 1. DPPH free radical scavenging assay revealed that all the studied compounds showed weak to moderate antioxidant activity where the nature of the substituent at the s-triazine core has a significant impact on the antioxidant activity.

Photoluminescence of Pentavalent Uranyl Amide Complexes

We report the synthesis and characterization of a new family of pentavalent uranyl amide complexes, supported also by photoluminescence and theoretical investigations. These studies reveal for the first time that the UV-visible emission of uranyl(V) is an admixture of charge transfer transitions accompanied by vibronic coupling of the quartet excited state with uranyl oxo and amide vibrations, thereby offering new insights into the electronic structure of the reactive uranyl(V)

Photoluminescence of Pentavalent Uranyl Amide Complexes

We report the synthesis and characterization of a new family of pentavalent uranyl amide complexes, supported also by photoluminescence and theoretical investigations. These studies reveal for the first time that the UV-visible emission of uranyl(V) is an admixture of charge transfer transitions accompanied by vibronic coupling of the quartet excited state with uranyl oxo and amide vibrations, thereby offering new insights into the electronic structure of the reactive uranyl(V)