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The measurement of the composition of ε-Ga2O3 and the quantification of Sn doping in ε-Ga2O3:Sn by laser-assisted atom probe tomography (APT) may be inaccurate depending on the experimental conditions. Both the role of the laser energy and surface electric field were investigated, and the results clearly indicate that deviations from stoichiometry are observed changing the electric field conditions during APT. The measured atomic fraction of Ga can change from 0.45 at low field to 0.38 at high field, to be compared with the expected 0.4. This was interpreted in terms of preferential evaporation of Ga at high field and deficit of O at low field, which was caused by the formation of neutrals. The quantification of Sn-doping is accurate at low-field conditions, with an overestimation of the detected Sn-metallic fraction at high field. This suggests that Sn has a higher evaporation field compared to Ga. Finally, multiple detection events were in-depth studied, revealing that three dissociation reactions occur during APT: GaO2+ → Ga+ + O+; Ga2O22+ → Ga+ + GaO2+; Ga3O22+ → Ga+ + Ga2O2+. Nevertheless, only 2% of the detected events are related to such dissociation reactions, too small a fraction to fully explain the observed deviation from the stoichiometric composition in ε-Ga2O3.
The composition of GaAs measured by laser-assisted atom probe tomography may be inaccurate depending on the experimental conditions. In this work, we assess the role of the DC field and the impinging laser energy on such compositional bias. The DC field is found to have a major influence, while the laser energy has a weaker one within the range of parameters explored. The atomic fraction of Ga may vary from 0.55 at low-field conditions to 0.35 at high field. These results have been interpreted in terms of preferential evaporation of Ga at high field. The deficit of As is most likely explained by the formation of neutral As complexes either by direct ejection from the tip surface or upon the dissociation of large clusters. The study of multiple detection events supports this interpretation.
Thin films synthesized by assembling clusters present interesting chemical and physical properties and a large specific surface, and are appealing for functional applications (e.g. sensing and catalysis). Also, clusters supported on surfaces are interesting both for nanocatalysis applications and for fundamental research. By means of pulsed laser deposition (PLD) in a background atmosphere we can induce cluster aggregation in the ablation plume and control the deposition kinetic energy of the clusters. These phenomena depend on the plume expansion dynamics and their influence on the properties of the deposited films has been investigated as a function of the background gas mass and pressure. The control of these parameters permits variation of the film surface morphology, from a compact structure with a very smooth surface, to a film with a controlled roughness at the nanoscale, to an open, low density meso- and nanostructure characterized by a high fraction of voids and by a large specific area. Thin films of WOx, TiOx, Pd/PdO, and Ag were deposited and characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. Post-deposition annealing permits control of the crystalline degree of the films, which in the case of tungsten and titanium oxide is found to depend on the original nanostructure, while a different degree of oxidation can be induced by controlling the amount of oxygen in the deposition chamber. In-situ scanning tunneling microscopy (STM) was employed to study the first stages of growth of W films on different substrates. This opens the possibility to tailor the material properties through the control of the building nano-units.
Hirudin is an anticoagulant polypeptide isolated
from a medicinal leech that inhibits thrombin with extraordinary
potency (Kd = 0.2–1.0 pM) and
selectivity. Hirudin is composed of a compact N-terminal
region (residues 1–47, cross-linked by three disulfide
bridges) that binds to the active site of thrombin, and
a flexible C-terminal tail (residues 48–64) that
interacts with the exosite I of the enzyme. To minimize
the sequence of hirudin able to bind thrombin and also
to improve its therapeutic profile, several N-terminal
fragments have been prepared as potential anticoagulants.
However, the practical use of these fragments has been
impaired by their relatively poor affinity for the enzyme,
as given by the increased value of the dissociation constant
(Kd) of the corresponding thrombin
complexes (Kd = 30–400 nM). The
aim of the present study is to obtain a derivative of the
N-terminal domain 1–47 of hirudin displaying enhanced
inhibitory potency for thrombin compared to the natural
product. In this view, we have synthesized an analogue
of fragment 1–47 of hirudin HM2 in which Val1 has
been replaced by tert-butylglycine, Ser2 by Arg,
and Tyr3 by β-naphthylalanine, to give the BugArgNal
analogue. The results of chemical and conformational characterization
indicate that the synthetic peptide is able to fold efficiently
with the correct disulfide topology (Cys6–Cys14,
Cys16–Cys28, Cys22–Cys37), while retaining
the conformational properties of the natural fragment.
Thrombin inhibition data indicate that the effects of amino
acid replacements are perfectly additive if compared to
the singly substituted analogues (De Filippis V, Quarzago
D, Vindigni A, Di Cera E, Fontana A, 1998, Biochemistry
37:13507–13515), yielding a molecule that inhibits
the fast or slow form of thrombin by 2,670- and 6,818-fold
more effectively than the natural fragment, and that binds
exclusively at the active site of the enzyme with an affinity
(Kd,fast = 15.4 pM,
Kd,slow = 220 pM) comparable
to that of full-length hirudin (Kd,fast
= 0.2 pM, Kd,slow = 5.5 pM). Moreover,
BugArgNal displays absolute selectivity for thrombin over
the other physiologically important serine proteases trypsin,
plasmin, factor Xa, and tissue plasminogen activator, up
to the highest concentration of inhibitor tested (10 μM).
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