Nearest-neighbor configuration in (GaIn)(NAs) probed by x-ray absorption spectroscopy

Ga(1-x)In(x)N(y)As(1-y) is a promising material system for the fabrication of inexpensive "last-mile" optoelectronic components. However, details of its atomic arrangement and the relationship to observed optical properties is not fully known. Particularly, a blueshift of emission wavelength is observed after annealing. In this work, we use x-ray absorption fine structure to study the chemical environment around N atoms in the material before and after annealing. We find that as-grown molecular beam epitaxy material consists of a nearly random distribution of atoms, while postannealed material shows segregation of In toward N. Ab initio simulations show that this short-range ordering creates a more thermodynamically stable alloy and is responsible for blueshifting the emission.     

Catalytic activity of a ceria-lanthana system for 4-methylpentan-2-ol dehydration

A ceria-lanthana catalytic system prepared by the sol-gel technique proved to be active in 4-methylpentan-2-ol conversion, mainly leading to 4-methylpent-1-ene, a monomer for manufacturing polymers of high technological properties. The product distribution strongly depends on the acid-base features of the catalyst. The catalytic behavior of the samples was examined in the light of their acid-base properties. This revised version was published online in June 2006 with corrections to the Cover Date.     

"Oscillating" metallocene catalysts: what stops the oscillation?

The 150 MHz (13)C NMR microstructural analysis of polypropylene samples produced with two representative "oscillating" metallocene catalysts of largely different steric hindrance, namely [(2-(3,5-di-tert-butyl-4-methoxyphenyl)indenyl)(2)ZrP](+) and [(2-phenylindenyl)(2)ZrP](+) (P = polymeryl), and the implications on the origin of the stereocontrol are presented and discussed in detail. The original mechanistic proposal of an "oscillation" between a rac-like (isotactic-selective) and a meso-like (nonstereoselective) conformation cannot explain the observed polymer configuration. The isotactic-stereoblock nature of the polymers obtained with the former catalyst proves unambiguously that the active cation "oscillates" between the two enantiomorphous rac-like conformations at an average frequency that, even at high propene concentration, is only slightly lower than that of monomer insertion. The less-hindered [(2-phenylindenyl)(2)ZrP](+) gives instead a largely stereoirregular polypropylene, which is the logical consequence of a faster ligand rotation; however, depending on the use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent), the polymerization products may also contain appreciable amounts of a fairly isotactic fraction. The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration spanned by very short atactic ones, rules out the possibility that the latter are due to an active species in meso-like conformation and points rather to a conformationally "locked" rac-like species with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counteranion, which would play the role of the interannular bridge in the rac-bis(indenyl) ansa-metallocenes, was tested by computer modeling on a [rac-(2-phenylindenyl)(2)ZrMe(C(3)H(6))][B(C(6)F(5))(4)] ion couple and found viable.     

Sequence and phosphorylation level determination of two donkey β‐caseins by mass spectrometry

Two coeluting components, with experimentally measured M_r values of 25529 and 24606 Da, were identified by reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and mass spectrometric analysis in the dephosphorylated casein fraction of a milk sample collected from an individual donkey belonging to the Ragusano breed of the east of Sicily. By coupling enzymatic digestions, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) and RP‐HPLC/nano‐electrospray ionization tandem mass spectrometry (nESI‐MS/MS) analysis, the two proteins were identified as donkey β‐CNs and their sequences characterized completely, using the two known β‐CNs from mare as references. The two donkey β‐CNs, showing a mass difference of 923 Da, differ by the presence of the domain E²⁷SITHINK³⁴ in the full‐length component (M_r 25529 Da). In comparison with the mare's β‐CNs used as reference, they present nine amino acid substitutions: L→S³⁷, R→H⁵², S→N⁸¹, P→V⁸⁴, L→V⁹¹, R→Q²⁰³, P→L/I²⁰⁶, L→F²¹⁰ and A→P²¹⁹. Together, these substitutions account for the increase of 18 Da in the M_r of the donkey β‐CNs with respect to the counterparts from the mare. The molecular mass determination by ESI‐MS for the phosphorylated proteins showed that the full‐length component was composed of highly multi‐phosphorylated isoforms with five to seven phosphate groups. By analogy with the homologous mare's β‐CNs, the full‐length (226 amino acids) β‐CN was termed variant A, whereas the shorter (218 amino acids) β‐CN was termed variant A^Δ5. Copyright © 2009 John Wiley & Sons, Ltd.