Gold Nanoparticles Augment N-Terminal Cleavage and Splicing Reactions in Mycobacterium tuberculosis SufB

Protein splicing is a self-catalyzed event where the intervening sequence intein cleaves off, joining the flanking exteins together to generate a functional protein. Attempts have been made to regulate the splicing rate through variations in temperature, pH, and metals. Although metal-regulated protein splicing has been more captivating to researchers, metals were shown to only inhibit splicing reactions that confine their application. This is the first study to show the effect of nanoparticles (NPs) on protein splicing. We found that gold nanoparticles (AuNPs) of various sizes can increase the splicing efficiency by more than 50% and the N-terminal cleavage efficiency by more than 45% in Mycobacterium tuberculosis SufB precursor protein. This study provides an effective strategy for engineering splicing-enhanced intein platforms. UV-vis absorption spectroscopy, isothermal titration calorimetry (ITC), and transmission electron microscopy (TEM) confirmed AuNP interaction with the native protein. Quantum mechanics/molecular mechanics (QM/MM) analysis suggested a significant reduction in the energy barrier at the N-terminal cleavage site in the presence of gold atom, strengthening our experimental evidence on heightened the N-terminal cleavage reaction. The encouraging observation of enhanced N-terminal cleavage and splicing reaction can have potential implementations from developing a rapid drug delivery system to designing a contemporary protein purification system.

Homogeneous Gold Catalysis: Mechanistic Investigation of Hydroalkoxylation of Various Alkynes

<p>The reaction mechanism of the gold(III)-catalysed hydroalkoxylation of alkynes is studied to provide a deeper understanding of homogeneous gold catalysis. The study is conducted computationally using Density Functional Theory (DFT), with the PBE0 and BP86 functionals and basis sets of triple-ζ quality (aug-cc-pVTZ and aug-cc-pVTZ-PP for the gold atom). It emphasises the mechanisms undergone by various alkynes when they are activated by gold(III) catalysts towards nucleophilic attack to first form an enol ether and followed by a second nucleophilic attack to form a ketal as the final product. Hydrogen bonding networks formed by the solvent methanols are found to play a crucial role in the mechanism especially in the hydrogen migration steps that follow after the nucleophilic attacks. The first nucleophilic attacks are predicted to have rather low activation energies and hence they are expected to proceed fast while the second additions vary in activation barriers, depending on the steric effects in the substrates. The activation barrier for the last hydrogen migration is highest for all of the three reactions investigated and is expected to be the rate determining step. Investigations of internal alkyne reactions reveal that each elementary step requires a higher activation energy compared to terminal alkynes, which explains the low experimental rate of such reactions. Due to the regioselectivity problem in internal alkyne reactions, this results in a mixture of products which is difficult to isolate due to the similarities in their reaction energies. The study also highlights the calculated thermodynamics and kinetics of the reactions, which can be useful in predicting experimental outcomes. Arrhenius plots of concentration of each intermediate species against time were produced to further help the understanding of these mechanisms, whether or not the reactions go to full completion or stop at the formation of enol ether.</p>

Homogeneous Gold Catalysis: Mechanistic Investigation of Hydroalkoxylation of Various Alkynes

<p>The reaction mechanism of the gold(III)-catalysed hydroalkoxylation of alkynes is studied to provide a deeper understanding of homogeneous gold catalysis. The study is conducted computationally using Density Functional Theory (DFT), with the PBE0 and BP86 functionals and basis sets of triple-ζ quality (aug-cc-pVTZ and aug-cc-pVTZ-PP for the gold atom). It emphasises the mechanisms undergone by various alkynes when they are activated by gold(III) catalysts towards nucleophilic attack to first form an enol ether and followed by a second nucleophilic attack to form a ketal as the final product. Hydrogen bonding networks formed by the solvent methanols are found to play a crucial role in the mechanism especially in the hydrogen migration steps that follow after the nucleophilic attacks. The first nucleophilic attacks are predicted to have rather low activation energies and hence they are expected to proceed fast while the second additions vary in activation barriers, depending on the steric effects in the substrates. The activation barrier for the last hydrogen migration is highest for all of the three reactions investigated and is expected to be the rate determining step. Investigations of internal alkyne reactions reveal that each elementary step requires a higher activation energy compared to terminal alkynes, which explains the low experimental rate of such reactions. Due to the regioselectivity problem in internal alkyne reactions, this results in a mixture of products which is difficult to isolate due to the similarities in their reaction energies. The study also highlights the calculated thermodynamics and kinetics of the reactions, which can be useful in predicting experimental outcomes. Arrhenius plots of concentration of each intermediate species against time were produced to further help the understanding of these mechanisms, whether or not the reactions go to full completion or stop at the formation of enol ether.</p>

Unique Digoldgermanes: Structural Characteristics, Dynamic Behaviour and Distinctive Reactions

Digoldgermanes with a gold coordinated by a dialkylgermylene ligand, R’2Ge(AuPR3)(AuGeR’2) (3a; R = Me, 3b; R = Et), were synthesized as green solids through the reactions of stable dialkylgermylene 1 with R3PAuCl followed by the reduction with KC8 at ambient temperatures. The structural characteristics of 3a and 3b were elucidated using NMR spectroscopy, X-ray crystallography, and DFT calculations. An intense absorption maximum was observed at 590 nm in the UV-vis spectrum of 3a in hexane. A pendular motion of AuPR3 group between Ge(IV) and Ge(II) of 3a and 3b occurring in the NMR time scale was found by the dynamic 1H NMR analysis, suggesting that the Ge(II) ligand has an enhanced electrophilicity to be attacked by the nucleophilic gold atom which closes to ‒1 oxidation state. DFT calculations of 3a revealed the existence of high-lying σ(Ge-Au) type HOMO and low-lying LUMO with germylene pπ nature. We show the bond formation and activation alternatively at Au or Ge atom, a methylation of digoldgermane 3a with MeOTf affords methylgermane 5. Moreover, the digoldgermane 3a reacts with Cl− ion of Ph4PCl and CH3C(O)Cl smoothly to form the corresponding chloride-addition product 7 and chlorogoldgermane 9, respectively. Cyclic trimerization reactions of aromatic isocyanates were high-efficiently catalyzed by 3a giving the corresponding 1,3,5-triaryl isocyanurates.

Gold nanoparticle–Quercetin composite formulation: A Computational Model Structure

Nanoparticle mediated drug delivery is an emerging area of research now a days. In our present study, we emphasized on the mode of interaction of a widely used drug, Quercetin with frequently worked metallic nanoparticle, Gold (Au). At first five –OH groups have been attached separately with gold atom and energy minimization was performed using Avogadro Software for windows system. From this, we found that the –OH groups present at 7 position of ‘A’ ring, 3’ and 4’ positions of ‘B’ ring are most suitable site for gold atom to bind. In the next level of study, a gold atom has been interacted with two quercetin molecules at a time. The gold atom was attached to –OH group of 7 position of one quercetin molecule and 4’ position of the other. The calculated energy was found to be 482.319 KJ/Mol. Further, gold atoms were interacted with all –OH groups of quercetin molecule at a time to see its stability and the structure was found to have quite stable with an energy level of 218.074 KJ/Mol. Lastly we tried to make a quercetin–gold nanoparticle model structure which mimics the actual nanocomposite synthesized in vitro where one gold atom was interacted with two quercetin molecules and the other –OH groups of quercetin molecules were again attached with gold atoms. This structure possesses energy of 439.880 KJ/Mol. The bond lengths and bond angle of interacting C, O and Au atoms were measured to characterize the complex.

Relativistic Hirshfeld atom refinement of an organo-gold(I) compound

The main goal of this study is the validation of relativistic Hirshfeld atom refinement (HAR) as implemented in Tonto for high-resolution X-ray diffraction datasets of an organo-gold(I) compound. The influence of the relativistic effects on statistical parameters, geometries and electron density properties was analyzed and compared with the influence of electron correlation and anharmonic atomic motions. Recent work in this field has indicated the importance of relativistic effects in the static electron density distribution of organo-mercury compounds. This study confirms that differences in electron density due to relativistic effects are also of significant magnitude for organo-gold compounds. Relativistic effects dominate not only the core region of the gold atom, but also influence the electron density in the valence and bonding region, which has measurable consequences for the HAR refinement model parameters. To study the effects of anharmonic motion on the electron density distribution, dynamic electron density difference maps were constructed. Unlike relativistic and electron correlation effects, the effects of anharmonic nuclear motion are mostly observed in the core area of the gold atom.

The role of gold atom concentration in the formation of Cu–Au nanoparticles from the gas phase

The synthesis of bimetallic nanoparticles need to be controlled in order to obtain particles of a desired size, spatial structure, and chemical composition. In the synthesis of the Cu–Au nanoparticles studied here, nanoparticles can be obtained through either chemical or physical methods, each of which has its own drawbacks. Although it is very difficult to achieve the required target chemical composition of nanoparticles during chemical synthesis, their size can be stabilized quite well. In turn, physical synthesis methods mainly allow to maintain the required chemical composition; however, the size of the resulting particles varies significantly. To solve this issue, we studied the formation of Cu–Au nanoparticles with different chemical compositions from a gaseous medium using computer molecular dynamics (MD) simulation. The aim was to determine the effect of the concentration of gold atoms on the size and on the actual chemical composition of the formed bimetallic nanoparticles. The modeled region had a cubic shape with a face length of 1350 Bohr radii and contained a total of 91125 copper and gold atoms uniformly distributed in space. Thus, based on the results of the MD simulation, it was concluded that an increase in the percentage of gold atoms in the initial vapor phase led to a decrease in the size of the synthesized nanoparticles. In addition, it was found that clusters with a size of more than 400–500 atoms, regardless of the chemical composition of the initial vapor phase, basically corresponded to a given target composition.

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex added to CO/N double bond to form Au–C and B–O/N bonds. DFT calculations revealed a two-step mechanism consisting of the coordination of O/N atom to B atom followed by nucleophilic migration of Au atom.