skip to main content

Title: 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 more » 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. « less
; ; ; ;
Osella, Domenico
Award ID(s):
Publication Date:
Journal Name:
Bioinorganic Chemistry and Applications
Page Range or eLocation-ID:
1 to 12
Sponsoring Org:
National Science Foundation
More Like this
  1. Non-covalent complexes of the short amyloid peptide motif Gly-Asn-Asn-Gln-Gln-Asn-Tyr (GNNQQNY) with peptide counterparts that were tagged with a diazirine ring at the N-termini (*GNNQQNY) were generated as singly charged ions in the gas phase. Specific laser photodissociation (UVPD) of the diazirine tag in the gas-phase complexes at 355 nm generated transient carbene intermediates that underwent covalent cross-linking with the target GNNQQNY peptide. The crosslinking yields ranged between 0.8 and 4.5%, depending on the combinations of peptide C-terminal amides and carboxylates. The covalent complexes were analyzed by collision-induced dissociation tandem mass spectrometry (CID-MS 3 ), providing distributions of cross-links at the target peptide amino acid residues. A general preference for cross-linking at the target peptide Gln-4-Gln-5-Asn-6-Tyr-7 segment was observed. Born–Oppenheimer molecular dynamics calculations were used to obtain 100 ps trajectories for nine lowest free-energy conformers identified by ωB97X-D/6-31+G(d,p) gradient geometry optimizations. The trajectories were analyzed for close contacts between the incipient carbene atom and the X–H bonds in the target peptide. The close-contact analysis pointed to the Gln-5 and Tyr-7 residues as the most likely sites of cross-linking, consistent with the experimental CID-MS 3 results. Non-covalent binding in the amide complexes was evaluated by DFT calculations of structures and energies. Althoughmore »antiparallel arrangements of the GNNQQNY and *GNNQQNY peptides were favored in low-energy gas-phase and solvated complexes, the conformations and peptide–peptide interface surfaces were found to differ from the secondary structure of the dry interface in GNNQQNY motifs of amyloid aggregates.« less
  2. Asparagine-linked glycosylation, also known as N-linked glycosylation is an essential and highly conserved post-translational protein modification that occurs in all three domains of life. This modification is essential for specific molecular recognition, protein folding, sorting in the endoplasmic reticulum, cell–cell communication, and stability. Defects in N-linked glycosylation results in a class of inherited diseases known as congenital disorders of glycosylation (CDG). N-linked glycosylation occurs in the endoplasmic reticulum (ER) lumen by a membrane associated enzyme complex called the oligosaccharyltransferase (OST). In the central step of this reaction, an oligosaccharide group is transferred from a lipid-linked dolichol pyrophosphate donor to the acceptor substrate, the side chain of a specific asparagine residue of a newly synthesized protein. The prokaryotic OST enzyme consists of a single polypeptide chain, also known as single subunit OST or ssOST. In contrast, the eukaryotic OST is a complex of multiple non-identical subunits. In this review, we will discuss the biochemical and structural characterization of the prokaryotic, yeast, and mammalian OST enzymes. This review explains the most recent high-resolution structures of OST determined thus far and the mechanistic implication of N-linked glycosylation throughout all domains of life. It has been shown that the ssOST enzyme, AglB protein ofmore »the archaeon Archaeoglobus fulgidus, and the PglB protein of the bacterium Campylobactor lari are structurally and functionally similar to the catalytic Stt3 subunit of the eukaryotic OST enzyme complex. Yeast OST enzyme complex contains a single Stt3 subunit, whereas the human OST complex is formed with either STT3A or STT3B, two paralogues of Stt3. Both human OST complexes, OST-A (with STT3A) and OST-B (containing STT3B), are involved in the N-linked glycosylation of proteins in the ER. The cryo-EM structures of both human OST-A and OST-B complexes were reported recently. An acceptor peptide and a donor substrate (dolichylphosphate) were observed to be bound to the OST-B complex whereas only dolichylphosphate was bound to the OST-A complex suggesting disparate affinities of two OST complexes for the acceptor substrates. However, we still lack an understanding of the independent role of each eukaryotic OST subunit in N-linked glycosylation or in the stabilization of the enzyme complex. Discerning the role of each subunit through structure and function studies will potentially reveal the mechanistic details of N-linked glycosylation in higher organisms. Thus, getting an insight into the requirement of multiple non-identical subunits in the N-linked glycosylation process in eukaryotes poses an important future goal.« less
  3. Donor–π-acceptor (D–π-A) fluorophores consisting of a donor unit, a π linker, and an acceptor moiety have attracted attention in the last decade. In this study, we report the synthesis, characterization, optical properties, TD-DFT, and cytotoxicity studies of 17 near infrared (NIR) D–π-A analogs which have not been reported so far to the best of our knowledge. These fluorophores have chloroacrylic acid as the acceptor unit and various donor units such as indole, benzothiazole, benzo[ e ]indole, and quinoline. The fluorophores showed strong absorption in the NIR (700–970 nm) region due to their enhanced intramolecular charge transfer (ICT) between chloroacrylic acid and the donor moieties connected with the Vilsmeier–Haack linker. The emission wavelength maxima of the fluorophores were in between 798 and 870 nm. Compound 20 with a 4-quinoline donor moiety showed an emission wavelength above 1000 nm in the NIR II window. The synthesized fluorophores were characterized by 1 H NMR and 13 C NMR, and their optical properties were studied. Time dependent density functional theory (TD-DFT) calculations showed that the charge transfer occurs from the donor groups (indole, benzothiazole, benzo[ e ]indole, and quinoline) to the acceptor chloroacrylic acid moiety. Fluorophores with [HOMO] to [LUMO+1] transitions were shown tomore »possess a charge separation character. The cytotoxicity of selected fluorophores, 4 , 7 , 10 and 12 was investigated against breast cancer cell lines and they showed better activity than the anti-cancer agent docetaxel.« less
  4. In biological systems, chemical and physical transformations of engineered silver nanomaterials (AgENMs) are mediated, in part, by proteins and other biomolecules. Metalloprotein interactions with AgENMs are also central in understanding toxicity, antimicrobial, and resistance mechanisms. Despite their readily available thiolate and amine ligands, zinc finger (ZF) peptides have thus far escaped study in reaction with AgENMs and their Ag( i ) oxidative dissolution product. We report spectroscopic studies that characterize AgENM and Ag( i ) interactions with two ZF peptides that differ in sequence, but not in metal binding ligands: the ZF consensus peptide CP-CCHC and the C-terminal zinc finger domain of HIV-1 nucleocapsid protein p7 (NCp7_C). Both ZF peptides catalyze AgENM (10 and 40 nm, citrate coated) dissolution and agglomeration, two important AgENM transformations that impact bioreactivity. AgENMs and their oxidative dissolution product, Ag( i )(aq), mediate changes to ZF peptide structure and metalation as well. Spectroscopic titrations of Ag( i ) into apo-ZF peptides show an Ag( i )–thiolate charge transfer band, indicative of Ag( i )–ZF binding. Fluorescence studies of the Zn( ii )–NCp_7 complex indicate that the Ag( i ) also effectively competes with the Zn( ii ) to drive Zn( ii ) displacement from themore »ZFs. Upon interaction with AgENMs, Zn( ii ) bound ZF peptides show a secondary structural change in circular dichroism spectroscopy toward an apo-like structure. The results suggest that Ag( i ) and AgENMs may alter ZF protein function within the cell.« less
  5. Arginine (Arg) complexes with Zn 2+ and Cd 2+ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light from a free electron laser. Electrospray ionization generated complexes of deprotonated Arg with Zn 2+ , [Zn(Arg–H)] + , and Arg with CdCl + , CdCl + (Arg). Possible low-energy conformers of these species were found using quantum chemical calculations, and their calculated IR spectra were compared to experimentally measured IRMPD spectra. Calculations were performed at the B3LYP/6-311+G(d,p) level for Zn 2+ complexes and B3LYP/def2-TZVP with an SDD effective core potential on cadmium for CdCl + complexes. [Zn(Arg–H)] + was found to adopt a charge-solvated, tridentate [N,CO − ,N ω ′] structure where Zn 2+ binds to the backbone amine, carbonyl oxygen, and side-chain terminal guanidine nitrogen (N ω ′). The CdCl + (Arg) species was suggested to be a mixture of a dominant (∼85%) charge-solvated, tridentate [N,CO,N ω ′] structure where the CdCl + binds to the backbone amine, carbonyl, and side-chain imine (N ω ′) and a minor (∼15%) bidentate [N,CO − ](N ω ′H 2 + ) zwitterionic structure where the metal center binds to the backbone amine and carbonyl oxygen with intramolecular proton migration frommore »the hydroxyl to the N ω ′ guanidine nitrogen (as designated in parenthesis).« less