ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting High Plasmonic Character

The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cluster systems exploited in many avenues of research. Herein we report a comparative electrospray ionization-mass spectrometry (ESI-MS) investigation of the 60-fold thiolated Au₁₄₄ and CuAu₁₄₄ clusters, in various positive charge-states, in conjunction with a density-functional theoretical (DFT) analysis based upon the icosahedral Pd₁₄₅-structure-type applicable to these systems. Samples rich in the hexanethiolate-protected CuAu₁₄₄ clusters are obtained via a single-phase reduction process. The predicted electronic structure of the vacancy-centered Au₁₄₄(SR)₆₀ system provided a simple rationale for the limiting [4+] charge-state observed of Au₁₄₄, whereas the maximal [3+] charge detected on the CuAu₁₄₄(SR)₆₀ cluster can be explained if the 145th atom occupies the central site. Occupancy of the center-site stabilizes the superatomic 3S-orbital, and thereby shifts the shell-closing count from 82 to 84 free electrons. The DFT-calculated energetics also predicts a strong (0.65 eV) preference for placing the smaller Cu ion in this central site. Remarkably, the optical absorption spectra of dilute tetrahydrofuran (THF) solutions feature a broad band centered near 2.3 eV, in contrast to the previously reported “nonplasmonic” response of sub-2.0-nm all-gold or -copper clusters. Other methods (matrix-assisted laser desorption ionization mass spectrometry and high-resolution electron microscopy) were used to investigate whether aggregation phenomena might account for this observed plasmon emergence. This unusual result points to the need to obtain highly purified samples of copper-doped gold clusters of ca. 145 atoms total

Comprehensive Analysis of Aspergillus nidulans PKA Phosphorylome Identifies a Novel Mode of CreA Regulation

In filamentous fungi, an important kinase responsible for adaptation to changes in available nutrients is cyclic AMP (cAMP)-dependent protein kinase (protein kinase A [PKA]). This kinase has been well characterized at a molecular level, but its systemic action and direct/indirect targets are generally not well understood in filamentous fungi. In this work, we used a pkaA deletion strain (ΔpkaA) to identify Aspergillus nidulans proteins for which phosphorylation is dependent (either directly or indirectly) on PKA. A combination of phosphoproteomic and transcriptomic analyses revealed both direct and indirect targets of PKA and provided a global perspective on its function. One of these targets was the transcription factor CreA, the main repressor responsible for carbon catabolite repression (CCR). In the ΔpkaA strain, we identified a previously unreported phosphosite in CreA, S319, which (based on motif analysis) appears to be a direct target of Stk22 kinase (AN5728). Upon replacement of CreA S319 with an alanine (i.e., phosphonull mutant), the dynamics of CreA import to the nucleus are affected. Collectively, this work provides a global overview of PKA function while also providing novel insight regarding significance of a specific PKA-mediated phosphorylation event