Photoinduced formation of N2 molecules in ammonium compounds

Via fluorescence yield (FY) and resonant inelastic scattering spectroscopy in the soft X-ray range we find that soft X-rays induce formation of N2 molecules in solid NH4Cl and in related compounds. The nitrogen molecules form weak bonds in NH4Cl, so that a substantial fraction of the molecules remains in the sample. From measurements of the FY as a function of exposure and temperature, the rates for the photochemical processes are estimated. At elevated temperatures (363 K), several nitrogen atoms are removed from the sample per incoming photon. At lower temperatures (233 K), the rate is reduced to around 0.02 nitrogen atoms for each incoming photon. Virtually all these atoms form N2 molecules which are bound in the sample. The generality and implications of these results are briefly discussed.     

Model photostable compounds

The photostabilities and syntheses of 2,2′-bis(o-hydroxyphenyl)-5,5′-bipyrimidine, 6,6′-bis(o-hydroxyphenyl)-3,3′-bipyridine and 3,8-bis(o-hydroxyphenyl)-4,7-phenanthroline are discussed. The syntheses of parent compounds lacking the o-hydroxy groups are also described.     

Water-based latex dispersions 4. Exchange dynamics and residence times of nonylphenol ethoxylate in concentrated latex dispersions

The equilibrium residence times of the nonionic surfactant nonylphenol ethoxylate (NP100) in a latex dispersion were determined using NMR diffusometry. At 16% w/w particle concentration and 0.12, 0.43 and 0.81% w/w NP100, the residence times of the surfactant were 0.16, 1.02 and 4.73 s in solution (tau(A)) and 0.3, 0.37 and 0.61 s on the surface of the particles (tau(B)), respectively. At even higher particle concentration (>45% w/w), tau(A) and tau(B) were 1.47 and 2.2 s. Calculating the number of collisions that ought to result in adsorbed species, at 16% w/w, only 2, 5 and 2 per thousand (corresponding to 0.12, 0.43 and 0.81% w/w NP100) resulted in adsorption, whereas at >45% w/w, only 12 per thousand resulted in adsorption, which suggested that the surfactant was irreversibly adsorbed on the particles. The small increase in collision frequency with increased particle concentration could be a result of a diffusion controlled adsorption, while an energy barrier for desorption controlled the overall exchange dynamics in the dispersion. The slow dynamics in the dispersion was controlled, mainly by the nonylphenol group, which gave NP100 a strong preference to surfaces. In addition, the chain length of the poly(ethylene glycol) (PEG) group changed the solution behavior from being that of a typical surfactant to that of a polymer.     

The microstructure of the base oxide on 304L steel

304L stainless steel specimens were subjected to 168 h of oxidation in dry oxygen at 600°C. The composition and microstructure of the oxide as well as the steel near the steel–oxide interface were investigated using SEM and TEM in conjunction with EDS analysis. It was found that the first oxide layers, referred to as the base oxide, consisted of 50–100 nm-sized corundum-type grains. Each grain contained oxides of chromium, iron plus a small amount of manganese in a solid solution with the chemical formula (Cr,Fe,Mn)₂O₃. In the oxide regions closest to the steel–oxide interface, the grain center compositions varied laterally from grain to grain, with the cation balance ranging between very chromium-rich (Cr:Fe=4:1) and very iron-rich (Cr:Fe=1:8). Within each oxide grain, the chromium level was generally higher in the center than near the boundary. This was due to the much quicker diffusion of iron (compared to chromium) at and through the oxide grain boundaries than in the bulk of the oxide grains, creating iron enrichment near the oxide grain boundaries. Compositional variation with respect to the distance from the steel–oxide interface was also noted. Chromium was richer in the oxide regions closer to the steel–oxide interface than farther out in the oxide and depleted from the steel regions bordering the steel–oxide interface. Manganese was also depleted in those steel regions although hardly enriched in the oxide just outside the steel–oxide interface. Nickel was less reactive than chromium, iron or manganese and hence was virtually absent from the oxide.     

Cytotoxic potency of small macrocyclic knot proteins: structure-activity and mechanistic studies of native and chemically modified cyclotides

The cyclotides are a family of circular and knotted proteins of natural origin with extreme enzymatic and thermal stability. They have a wide range of biological activities that make them promising tools for pharmaceutical and crop-protection applications. The cyclotides are divided into two subfamilies depending on the presence (M?bius) or absence (bracelet) of a cis-Pro peptide bond. In the current work we report a series of experiments to give further insight into the structure-activity relationship of cyclotides in general, and the differences between subfamilies and the role of their hydrophobic surface in particular. Selective chemical modifications of Glu, Arg, Lys and Trp residues was tested for cytotoxic activity: derivatives in which the Trp residue was modified showed low effect, demonstrating the existence of a connection between hydrophobicity and activity. However, over the full set of cyclotides examined, there was no strong correlation between the cytotoxic activity and their hydrophobicity. Instead, it seems more like that the distribution of charged and hydrophobic residues determines the ultimate degree of potency. Furthermore, we found that while the Glu residue is very important in maintaining the activity of the bracelet cyclotide cycloviolacin O2, it is much less important in the M?bius cyclotides. Despite these differences between cyclotide subfamilies, a systematic test of mixtures of cyclotides revealed that they act in an additive way.