Metal centres in biomolecules are recognized as being particularly sensitive to radiation damage by X-ray photons. This results in such molecules being both susceptible to an effective X-ray-induced ...loss of function and problematic to study using X-ray diffraction methods, with reliable structures of the metal centres difficult to obtain. Despite the abundance of experimental evidence, the mechanistic details of radiation damage at metal centres are unclear. Here, using ab initio calculations, we show that the absorption of X-rays by microsolvated Mg(2+) results in a complicated chain of ultrafast electronic relaxation steps that comprise both intra- and intermolecular processes and last for a few hundred femtoseconds. At the end of this cascade the metal reverts to its original charge state, the immediate environment becomes multiply ionized and large concentrations of radicals and slow electrons build up in the metal's vicinity. We conclude that such cascades involving metal ions are essential to our understanding of radiation chemistry and radiation damage in biological environments.
We report the observation of electron-transfer-mediated decay (ETMD) involving magnesium (Mg) clusters embedded in helium (He) nanodroplets. ETMD is initiated by the ionization of He followed by ...removal of two electrons from the Mg clusters of which one is transferred to the He ion while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. For Mg clusters larger than five atoms we observe stable doubly ionized clusters. Thus, ETMD provides an efficient pathway to the formation of doubly ionized cold species in doped nanodroplets.
After core ionization of an atom or molecule by an x-ray photon, multiply charged ions are produced in the Auger decay process. These ions tend to neutralize their charge when embedded in an ...environment. We demonstrate that, depending on the atom or molecule and its neighbors, electron transfer mediated decay (ETMD) provides a particularly efficient neutralization pathway for the majority of the ions produced by Auger decay. The mechanism is rather general. As a showcase example, we conducted an ab initio study of the NeKr2 cluster after core ionization of the Ne atom. This example has been chosen because it is amenable to both ab initio calculations and coincidence experiments. We find that even for frozen nuclei, the neutralization rate can be as fast as 0.130 ps(-1). We also show that nuclear dynamics may increase the rate by about an order of magnitude. The generality of the mechanism makes this neutralization pathway important in weakly bonded environments.
Inner-shell ionization of an isolated atom typically leads to Auger decay. In an environment, for example, a liquid or a van der Waals bonded system, this process will be modified, and becomes part ...of a complex cascade of relaxation steps. Understanding these steps is important, as they determine the production of slow electrons and singly charged radicals, the most abundant products in radiation chemistry. In this communication, we present experimental evidence for a so-far unobserved, but potentially very important step in such relaxation cascades: Multiply charged ionic states after Auger decay may partially be neutralized by electron transfer, simultaneously evoking the creation of a low-energy free electron (electron transfer-mediated decay). This process is effective even after Auger decay into the dicationic ground state. In our experiment, we observe the decay of Ne
produced after Ne 1s photoionization in Ne-Kr mixed clusters.
We investigate interatomic Coulombic decay in NeKr dimers after neon inner-valence photoionization Ne
+
(2s
−1
) using a synchrotron light source. We measure with high energy resolution the two ...singly charged ions of the Coulomb-exploding dimer dication and the photoelectron in coincidence. By carefully tracing the post-collision interaction between the photoelectron and the emitted ICD electron we are able to probe the temporal evolution of the state as it decays. Although the ionizing light pulses are 80 picoseconds long, we determine the lifetime of the intermediate dimer cation state and visualize the contraction of the nuclear structure on the femtosecond time scale.
We investigate the time-resolved dynamics of interatomic Coulombic decay in NeKr dimers after neon inner-valence photoionization using a synchrotron light source.
The correlated nature of electronic states in atoms and molecules is manifested in the simultaneous emission of two electrons after absorption of a single photon close to the respective threshold. ...Numerous observations in atoms and small molecules demonstrate that the double ionization efficiency close to threshold is rather small. In this Letter we show that this efficiency can be dramatically enhanced in the environment. To be specific, we concentrate on the case where the species in question has one or several He atoms as neighbors. The enhancement is achieved by an indirect process, where a He atom of the environment absorbs a photon and the resulting He(+) cation is neutralized fast by a process known as electron transfer mediated decay, producing thereby doubly ionized species. The enhancement of the double ionization is demonstrated in detail for the example of the Mg · He cluster. We show that the double ionization cross section of Mg becomes 3 orders of magnitude larger than the respective cross section of the isolated Mg atom. The impact of more neighbors is discussed and the extension to other species and environments is addressed.
We investigated Ne+–Ar+ ion-pair formation which follows irradiation of the NeAr dimer by monochromatic soft X-rays. Using momentum-resolved electron-ion multicoincidence spectroscopy, we could ...unambiguously identify that the formation of the ion pair at photon energy of 200.5eV proceeds via interatomic Coulombic decay (ICD) of the Ne+(2s−1)Ar inner-valence ionized and NeAr+(3p−25d) ionization satellite states. Photoabsorption at higher photon energies of 268.2eV and 888.7eV leads to the emission of core electrons of Ar and Ne respectively, and to the subsequent local Auger decay process. We demonstrate that at these energies the ion pair formation originating in the doubly ionized Ar L-MM and Ne K-LL Auger final states proceeds mostly via radiative charge transfer and charge transfer driven by non-adiabatic coupling mechanisms.
Many of the chronic complications of diabetes mellitus involve defects in the connective tissue such as poor wound healing, diminished bone formation, and decreased linear growth. Because collagen is ...the major protein component of these connective tissues, we examined collagen production in diabetic rats as a probe of this generalized defect in connective tissue metabolism. Doses of streptozocin ranging from 35 to 300 mg/kg were used to induce diabetes of graded metabolic severity in rats. Parietal bone or articular cartilage was removed and incubated at 37 degrees C with 5 microCi L-5-3Hproline for 2 h, and collagen and noncollagen protein production were quantitated after separation with purified bacterial collagenase. Within 2 wk after induction of diabetes, collagen production was significantly reduced in bone and cartilage from diabetic rats to 52% (P less than .01) and 51% (P less than .01) of control (buffer-injected) levels, respectively. In contrast, noncollagen protein production in bone and cartilage from diabetic animals was no different from in tissue from control rats. The correlation between collagen relative to total protein production (relative rate) and the degree of hyperglycemia was highly significant for both bone (r = -.77, P less than .001) and cartilage (r = -.87, P less than .001). Other factors found to correlate with altered collagen production were the duration of diabetes and the amount of weight loss. Thus, diabetes is associated with a marked decrease in collagen production, which was seen early after induction of diabetes and was specific when compared with noncollagen protein production. Cumulative effects of these marked changes in collagen production may contribute to the chronic connective tissue complications in diabetes.
AIM:To present a series of covered self-expandable metal stents(CSEMS) placed for different indications and to evaluate the effectiveness,complications and extractability of these devices.METHODS:We ...therefore retrospectively reviewed the courses of patients who received CSEMS due to malignant as well as benign biliary strictures and postsphincterotomy bleeding in our endoscopic unit between January 2010 and October 2011.RESULTS:Twenty-six patients received 28 stents due to different indications(20 stents due to malignant biliary strictures,six stents due to benign biliary strictures and two stents due to post-sphincterotomy bleeding).Biliary obstruction was relieved in all cases,regardless of the underlying cause.Hemostasis could be achieved in the two patients who received the stents for this purpose.Complications occurred in five patients(18%).Two patients(7%) developed cholecystitis,stents dislocated/migrated in other two patients(7%),and in one patient(3.6%) stent occlusion was documented during the study period.Seven stents were extracted endoscopically.Removal of stents was easily possible in all cases in which it was desired using standard forceps.Twelve patients underwent surgery with pylorus preserving duodenopancreatectomy.In all patients stents could be removed during the operation without difficulties.CONCLUSION:Despite the higher costs of these devices,fully covered self-expanding metal stents may be suitable to relief biliary obstruction due to bile duct stenosis,regardless of the underlying cause.CSEMS may also represent an effective treatment strategy of severe post-sphincterotomy bleeding,not controlled by other measures.
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