Combining the Single-Atom Engineering and Ligand-Exchange Strategies: Obtaining the Single-Heteroatom-Doped Au₁₆Ag₁(S-Adm)₁₃ Nanocluster with Atomically Precise Structure

Obtaining cognate single-heteroatom doping is highly desirable but least feasible in the research of nanoclusters (NCs). In this work, we reported a new Au₁₆Ag₁(S-Adm)₁₃ NC, which is synthesized by the combination of single-atom engineering and ligand-exchange strategies. This new NC is so far the smallest crystallographically characterized Au-based NC protected by thiolate. The Au₁₆Ag₁(S-Adm)₁₃ exhibited a tristratified Au₃–Au₂Ag₁–Au₁ kernel capped by staple-like motifs including one dimer and two tetramers. In stark contrast to the size-growth from Au₁₈(S–C₆H₁₁)₁₄ to Au₂₁(S-Adm)₁₅ via just the ligand-exchange method, combining single Ag doping on Au₁₈(S–C₆H₁₁)₁₄ resulted in the size-decrease from Au₁₇Ag₁(S–C₆H₁₁)₁₄ to Au₁₆Ag₁(S-Adm)₁₃. DFT calculations were performed to both homogold Au₁₈ and single-heteroatom-doped Au₁₇Ag₁ to explain the opposite results under the same ligand-exchange reaction. Our work is expected to inspire the synthesis of new cognate single-atom-doped NCs by combining single-atom engineering and ligand-exchange strategies and also shed light on extensive understanding of the metal synergism effect in the NC range

The Inductive Effect in Nitridosilicates and Oxysilicates and Its Effects on 5d Energy Levels of Ce³⁺

The inductive effect exists widely in inorganic compounds and accounts well for many physicochemical properties. However, until now this effect has not been characterized quantitatively. In this work, we collected and analyzed the structural data of more than 100 nitridosilicates and oxysilicates, whose structures typically consist of [SiN₄] or [SiO₄] tetrahedra. We introduce a new parameter, the inductive effect factor μΔχ, related to the difference of electronegativity between constituent metal elements and silicon. Then, a linear relationship is established between average length of Si–N/Si–O bonds and the inductive factor with the help of statistical method, that is, <i>l̅</i> = 1.7313 + 0.0166 μΔχ (Å) with adjusted (adj) <i>R</i>² = 0.800 for Si–N and <i>l̅</i> = 1.6221 + 0.0035 μΔχ­(Å) with adj <i>R</i>² = 0.240 for Si–O. Furthermore, our research shows that the distinct positive correlation does exist between the inductive factor and the centroid shift of 5d levels of Ce³⁺. This work will help us understanding the inductive effect deeply and quantitatively

Citric Acid-Assisted Two-Step Enrichment with TiO₂ Enhances the Separation of Multi- and Monophosphorylated Peptides and Increases Phosphoprotein Profiling

Phosphopeptide enrichment is essential for large-scale phosphoprotein profiling. Titanium dioxide (TiO₂) is widely used in phosphopeptide enrichment, but it is limited in the isolation of multiphosphorylated peptides due to their strong binding. In this study, we found that citric acid greatly affects the binding of mono- and multiphosphopeptides with TiO₂, which can be used for stepwise phosphopeptide separation coupled with mass spectrum (MS) identification. We first loaded approximately 1 mg of peptide mixture of HeLa cell digests onto TiO₂ beads in highly concentrated citric acid (1 M). Then the flow-through fraction was diluted to ensure low concentration of citric acid (50 mM) and followed by loading onto another aliquot of TiO₂ beads. The two eluted fractions were subjected to nanoLC–MS/MS analysis. We identified 1,500 phosphorylated peptides, of which 69% were multiphosphorylated after the first enrichment. After the second enrichment, 2,167 phosphopeptides, of which 92% were monophosphorylated, were identified. In total, we successfully identified 3,136 unique phosphopeptides containing 3,973 phosphosites utilizing this strategy. Finally, more than 37% of the total phosphopeptides and 2.6-fold more of the multiphosphorylated peptides were identified as compared to the frequently used DHB/TiO₂ enrichment strategy. Combining SCX with CATSET, we identified 14,783 phosphopeptides and 15,713 phosphosites, of which 3,678 were unrecorded in PhosphoSitePlus database. This two-step separation procedure for sequentially enriching multi- and monophosphorylated peptides by using citric acid is advantageous in multiphosphorylated peptide separation, as well as for more comprehensive phosphoprotein profiling

Citric Acid-Assisted Two-Step Enrichment with TiO₂ Enhances the Separation of Multi- and Monophosphorylated Peptides and Increases Phosphoprotein Profiling

Phosphopeptide enrichment is essential for large-scale phosphoprotein profiling. Titanium dioxide (TiO₂) is widely used in phosphopeptide enrichment, but it is limited in the isolation of multiphosphorylated peptides due to their strong binding. In this study, we found that citric acid greatly affects the binding of mono- and multiphosphopeptides with TiO₂, which can be used for stepwise phosphopeptide separation coupled with mass spectrum (MS) identification. We first loaded approximately 1 mg of peptide mixture of HeLa cell digests onto TiO₂ beads in highly concentrated citric acid (1 M). Then the flow-through fraction was diluted to ensure low concentration of citric acid (50 mM) and followed by loading onto another aliquot of TiO₂ beads. The two eluted fractions were subjected to nanoLC–MS/MS analysis. We identified 1,500 phosphorylated peptides, of which 69% were multiphosphorylated after the first enrichment. After the second enrichment, 2,167 phosphopeptides, of which 92% were monophosphorylated, were identified. In total, we successfully identified 3,136 unique phosphopeptides containing 3,973 phosphosites utilizing this strategy. Finally, more than 37% of the total phosphopeptides and 2.6-fold more of the multiphosphorylated peptides were identified as compared to the frequently used DHB/TiO₂ enrichment strategy. Combining SCX with CATSET, we identified 14,783 phosphopeptides and 15,713 phosphosites, of which 3,678 were unrecorded in PhosphoSitePlus database. This two-step separation procedure for sequentially enriching multi- and monophosphorylated peptides by using citric acid is advantageous in multiphosphorylated peptide separation, as well as for more comprehensive phosphoprotein profiling

Citric Acid-Assisted Two-Step Enrichment with TiO₂ Enhances the Separation of Multi- and Monophosphorylated Peptides and Increases Phosphoprotein Profiling

Phosphopeptide enrichment is essential for large-scale phosphoprotein profiling. Titanium dioxide (TiO₂) is widely used in phosphopeptide enrichment, but it is limited in the isolation of multiphosphorylated peptides due to their strong binding. In this study, we found that citric acid greatly affects the binding of mono- and multiphosphopeptides with TiO₂, which can be used for stepwise phosphopeptide separation coupled with mass spectrum (MS) identification. We first loaded approximately 1 mg of peptide mixture of HeLa cell digests onto TiO₂ beads in highly concentrated citric acid (1 M). Then the flow-through fraction was diluted to ensure low concentration of citric acid (50 mM) and followed by loading onto another aliquot of TiO₂ beads. The two eluted fractions were subjected to nanoLC–MS/MS analysis. We identified 1,500 phosphorylated peptides, of which 69% were multiphosphorylated after the first enrichment. After the second enrichment, 2,167 phosphopeptides, of which 92% were monophosphorylated, were identified. In total, we successfully identified 3,136 unique phosphopeptides containing 3,973 phosphosites utilizing this strategy. Finally, more than 37% of the total phosphopeptides and 2.6-fold more of the multiphosphorylated peptides were identified as compared to the frequently used DHB/TiO₂ enrichment strategy. Combining SCX with CATSET, we identified 14,783 phosphopeptides and 15,713 phosphosites, of which 3,678 were unrecorded in PhosphoSitePlus database. This two-step separation procedure for sequentially enriching multi- and monophosphorylated peptides by using citric acid is advantageous in multiphosphorylated peptide separation, as well as for more comprehensive phosphoprotein profiling

Sodium Caseinate–Enzyme Conjugates as Biocatalysts for Recyclable Pickering Interfacial Biocatalysis

Stimuli-responsive Pickering interfacial biocatalysis is a prominent topic in biphasic biocatalysis for its high efficiency and flexible tunability. Herein, we designed CO2/N2-responsive sodium caseinate (NaCas)–enzyme conjugates that acted as both catalytic sites and stabilizers to construct a responsive Pickering interfacial biocatalytic system. The conjugates were prepared by a one-step strategy in which amino groups reacted with carboxyl groups between NaCas and enzymes. In the meantime, NaCas, with a disordered structure, could act as a buffer in the microenvironment to improve enzyme stability in harsh environments. The emulsion system stabilized by horseradish peroxidase (HRP)–NaCas displayed a higher catalytic efficiency and conversion rate compared with the conventional two-phase system, and HRP–NaCas could be easily recycled at least five times by bubbling CO2 and N2. Furthermore, the coupled NaCas system was implemented in Candida antarctica lipase B (CaLB), which extended the excellent interfacial characteristics and application field of NaCas

Synthesis and Structure of Self-Assembled Pd₂Au₂₃(PPh₃)₁₀Br₇ Nanocluster: Exploiting Factors That Promote Assembly of Icosahedral Nano-Building-Blocks

The essential force of self-assembly in the nanocluster range is not intrinsically understood to date. In this work, the synergistic effect between metals was exploited to render the self-assembly from the icosahedral M₁₃ (M = Pd, Au) nano-building-blocks. Single-crystal X-ray diffraction analysis revealed that the two Pd₁Au₁₂ icosahedrons were linked by five halogen linkages, and the assembled structure was determined to be Pd₂Au₂₃­(PPh₃)₁₀Br₇. The finding of Au–halogen linkages in the rod-like M₂₅ nanoclusters has not been previously reported. Furthermore, the calculations on Hirshfeld charge analysis were performed, which implied that the reduced electronic repulsion (induced by the synergistic effect of Pd and Au metals) between two icosahedral units promoted the assembly. This study sheds light on the deep understanding of the essential force of self-assembly from icosahedral nano-building-blocks

A Protein-Capsid-Based System for Cell Delivery of Selenocysteine

Selenocysteine (Sec) has received a lot of attention as a potential anticancer drug. However, its broad cytotoxicity limits its therapeutic usefulness. Thus, Sec is an attractive candidate for targeted drug delivery. Here, we demonstrate for the first time that an engineered version of the capsid formed by Aquifex aeolicus lumazine synthase (AaLS) can act as a nanocarrier for delivery of Sec to cells. Specifically, a previously reported variant of AaLS (AaLS-IC), which contains a single cysteine per subunit that projects into the capsid interior, was modified by reaction with the diselenide dimer of Sec (Sec₂) to generate a selenenylsulfide conjugate between the capsid and Sec (AaLS-IC-Sec). Importantly, it was determined that the structural context of the reactive cysteine was important for efficient capsid loading. Further, the encapsulated Sec could be quantitatively released from AaLS-IC-Sec by reducing agents such as glutathione or dithiothreitol. To assess cellular penetrance capabilities of AaLS-IC-Sec and subsequent cytotoxic response, six different cells line models were examined. Across the cell lines analyzed, cytotoxic sensitivity correlated with cellular uptake and intracellular trafficking patterns. Together these findings suggest that the engineered AaLS-IC capsid is a promising vehicle for targeted cell delivery of Sec

Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver-Doped Ag_<i>x</i>Au_{25–<i>x</i>} Clusters

The luminescent ligand protected metal clusters have attracted considerable attentions while the origin of the emission still remains elusive. As recently reported in our previous work, the rod-shaped Au₂₅ cluster possesses a low photoluminescence quantum yield (QY = 0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au₂₅ cluster can drastically enhance the photoluminescence with high quantum yield (QY = 40.1%). To explore the enhancement mechanism of fluorescence, femtosecond transient absorption spectroscopy is performed to determine the electronic structure and ultrafast relaxation dynamics of the highly luminescent silver-doped Ag_<i>x</i>Au_{25–<i>x</i>} cluster by comparing the excited state dynamics of doped and undoped Au₂₅ rod cluster, it is found that the excited state relaxation in Ag_<i>x</i>Au_{25–<i>x</i>} is proceeded with an ultrafast (∼0.58 ps) internal conversion and a subsequent nuclear relaxation (∼20.7 ps), followed by slow (7.4 μs) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in Ag_<i>x</i>Au_{25–<i>x</i>} (∼20.7 ps) compared to that in undoped Au₂₅ rod (∼52 ps). We conclude that it is the central Ag atom that stabilizes the charges on LUMO orbital and enhances the rigidity of Ag_<i>x</i>Au_{25–<i>x</i>} cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around ∼0.8 THz were observed in both clusters, indicating the symmetry preservation from Au cluster to Ag alloying Au clusters. The present results provide new insights for the structure-related excited state behaviors of luminescent ligand protected Ag alloying Au clusters

Total Structure Determination of Au₂₁(S-Adm)₁₅ and Geometrical/Electronic Structure Evolution of Thiolated Gold Nanoclusters

The larger size gold nanoparticles typically adopt a face-centered cubic (fcc) atomic packing, while in the ultrasmall nanoclusters the packing styles of Au atoms are diverse, including fcc, hexagonal close packing (hcp), and body-centered cubic (bcc), depending on the ligand protection. The possible conversion between these packing structures is largely unknown. Herein, we report the growth of a new Au₂₁(S-Adm)₁₅ nanocluster (S-Adm = adamantanethiolate) from Au₁₈(SR)₁₄ (SR = cyclohexylthiol), with the total structure determined by X-ray crystallography. It is discovered that the hcp Au₉-core in Au₁₈(SR)₁₄ is transformed to a fcc Au₁₀-core in Au₂₁(S-Adm)₁₅. Combining with density functional theory (DFT) calculations, we provide critical information about the growth mechanism (geometrical and electronic structure) and the origin of fcc-structure formation for the thiolate-protected gold nanoclusters