Elemental 2D imaging of paintings with a mobile EDXRF system

Imaging techniques are now used commonly and intensively in cultural heritage object analysis. Nowadays, many different techniques in nature as well as many applications exist, where they can be applied. X-ray radiography and infrared reflectography as well as UV photography are some of the most applied techniques. The study of works of art usually requires these techniques to be non-invasive. Furthermore, they are frequently required to perform in situ analysis. A few years ago, our laboratory developed a mobile energy-dispersive X-ray fluorescence and UV–vis–NIR coupled spectrometer, especially designed for fieldwork studies, where all three techniques can be applied strictly at the same site of analysis. Recent developments on a new positioning system have now allowed us to perform 2D elemental mappings with our equipment, which is especially well adapted to painting analysis. The system control is carried out entirely through a laptop computer running a dedicated homemade software. The positioning is achieved by means of a CCD camera embedded in the system and controlled via a Wi-Fi connection through the computer. The data acquisition system, which is made through a homemade multichannel pulse height analyzer, being also managed via the software mentioned above, goes through an Ethernet connection. We will present here the new developments of the system and an example of in situ 2D elemental mapping applied on an anonymous oil painting on wood panel. The discovery of a hidden painting under this oil painting makes it a good choice for a first example of 2D large scan with a mobile instrument.     

DFT and ab initio potential energy scan and hydrogen bond analysis of N-substituted hydrazino acetamides: Characterization of the “hydrazinoturn” hydrogen bonding pattern

We studied a series of model primary amides in gas phase at the DFT (B3LYP) and HF at 6-31+G/6-31+G** levels of theory in order to shed light on their conformation, structure, and intramolecular hydrogen bonding network. A potential energy scan was performed by rotating around the appropriate bond for each molecule studied in this paper. In this manner, it was possible to show that the amidic group of these model compounds acts as H-bond donor and interacts with two different H-bond acceptors which stabilizes a C₈ pseudocycle, the so called “hydrazinoturn”. This study was addressed theoretically in order to understand the conformation adopted by hydrazino acetamides as model compounds for aza-β³-peptides. We thus investigated the conformational analysis of hydrazinoturns computationally and showed that these systems represent a very stabilizing folding driving force, provided that the neighboring molecular functional groups do not imply other competing hydrogen bonding patterns.     

Ga掺杂ZnO的电子结构与电性能的研究

采用密度泛函理论广义梯度近似第一性原理计算的方法研究了n型Ga掺杂的纤锌矿结构氧化物ZnO的晶格结构、能带结构和态密度,在此基础上分析了其电性能.计算结果表明,掺杂ZnO氧化物晶格a,b轴增大,c轴略有减小;Ga掺杂ZnO氧化物两能带之间具有0.6eV的直接带隙,需要载流子(电子)跃迁的能隙宽度较未掺杂的ZnO氧化物减小;掺杂体系费米能级附近的态密度大大提高,其能带主要由Gas态、Zns态和Os态电子构成,且他们之间存在着强相互作用,其中Gas态电子对导带贡献最大.电输运性能分析结果表明,Ga掺杂ZnO氧化物导电机构由Znp-Op电子在价带与导带的跃迁转变为Gas-Znd-Os电子在价带与导带的跃迁,这也表明Gas态电子在导电过程中的重要作用;掺杂体系费米能级附近的载流子有效质量较未掺杂体系增大,且价带中的载流子有效质量较大,导带中的载流子有效质量较小.     

The mononuclear metal center of type-I dihydroorotase from aquifex aeolicus

Background

Dihydroorotase (DHO) is a zinc metalloenzyme, although the number of active site zinc ions has been controversial. E. coli DHO was initially thought to have a mononuclear metal center, but the subsequent X-ray structure clearly showed two zinc ions, α and β, at the catalytic site. Aquifex aeolicus DHO, is a dodecamer comprised of six DHO and six aspartate transcarbamoylase (ATC) subunits. The isolated DHO monomer, which lacks catalytic activity, has an intact α-site and conserved β-site ligands, but the geometry of the second metal binding site is completely disrupted. However, the putative β-site is restored when the complex with ATC is formed and DHO activity is regained. Nevertheless, the X-ray structure of the complex revealed a single zinc ion at the active site. The structure of DHO from the pathogenic organism, S. aureus showed that it also has a single active site metal ion.

Results

Zinc analysis showed that the enzyme has one zinc/DHO subunit and the addition of excess metal ion did not stimulate catalytic activity, nor alter the kinetic parameters. The metal free apoenzyme was inactive, but the full activity was restored upon the addition of one equivalent of Zn²⁺ or Co²⁺. Moreover, deletion of the β-site by replacing the His180 and His232 with alanine had no effect on catalysis in the presence or absence of excess zinc. The 2.2 Å structure of the double mutant confirmed that the β-site was eliminated but that the active site remained otherwise intact.

Conclusions

Thus, kinetically competent A. aeolicus DHO has a mononuclear metal center. In contrast, elimination of the putative second metal binding site in amidohydrolyases with a binuclear metal center, resulted in the abolition of catalytic activity. The number of active site metal ions may be a consideration in the design of inhibitors that selectively target either the mononuclear or binuclear enzymes.