On the mechanism of intramolecular sensitization of photocleavage of the 2-(2-nitrophenyl)propoxycarbonyl (NPPOC) protecting group

A spectroscopic study of a variety of covalently linked thioxanthone(TX)-linker-2-(2-nitrophenyl)propoxycarbonyl(NPPOC)-substrate conjugates is presented. Herein, the TX chromophore functions as an intramolecular sensitizer to the NPPOC moiety, a photolabile protecting group used in photolithographic DNA chip synthesis. The rate of electronic energy transfer between TX and NPPOC was quantified by means of stationary fluorescence as well as nanosecond and femtosecond time-resolved laser spectroscopy. A dual mechanism of triplet-triplet energy transfer has been observed comprising a slower mechanism involving the T1(pipi*) state of TX with linker-length-dependent time constants longer than 20 ns and a fast mechanism with linker-length-dependent time constants shorter than 3 ns. Evidence is provided that the latter mechanism is due to energy transfer from the T2(npi*) state which is in fast equilibrium with the fluorescent S1(pipi*) state. In the case of direct linkage between the aromatic rings of TX and NPPOC, the spectroscopic properties are indicative of one united chromophore which, however, still shows the typical NPPOC cleavage reaction triggered by intramolecular hydrogen atom transfer to the nitro group.     

Studies of transport and collection characteristics of gaseous mercury in natural gases using amalgamation and isotope dilution analysis

Transport and collection characteristics were studied for gaseous elemental mercury (Hg(0)(g)) in natural gases using newly developed methodology based on amalgamation, isotope dilution with permeation tubes and inductively coupled plasma mass spectrometry. The study involved different Au-Pt collection tube designs, tubing materials and gaseous matrices, including air, natural and sales gas, as well as methane and sales gas to which hydrogen sulfide (H(2)S) had been added. The Hg(0)(g) capacity of the Au-Pt tubes was determined to 3.5 +/- 0.1 microg. Blanks and detection limits of gaseous mercury (Hg(g)) were 58 +/- 17 pg m(-3) and 50 pg m(-3), respectively, for a 60 L sample volume. For the gases tested, added Hg(0)(g) tracers could be collected with 90% or higher efficiency at flow rates and volumes of up to 10 L min(-1) and 100 L, respectively. The collection efficiency was found to be independent of the type of gas tested, even in the presence of H(2)S. However, for the gases containing H(2)S, the apparent transport efficiency of added Hg(0)(g) tracers through stainless steel tubing varied from 50 to 150% upon changing the temperature from 25 to 100 degrees C. The interaction of stainless steel with Hg(0)(g) leading to either a sink, or source of Hg, was not observed in the absence of H(2)S, nor was it observed for PTFE tubing in the presence of H(2)S. These observations raise questions about the applicability of currently used sampling procedures for determination of Hg(g) in H(2)S rich natural gases, including the 6978-2 ISO standard method, in which stainless steel is a prescribed material for tubing and valves of the sampling apparatus.     

Hydrogen bonding in liquid water probed by resonant Auger-electron spectroscopy

We have measured resonant and off-resonant Auger-electron spectra of liquid water. Continuumlike transitions near and above the O1s vertical ionization energy are identified by the characteristic normal Auger-electron spectra. On the contrary, well-resolved spectator shifts of the main Auger-electron peak are observed at the liquid-water O1s absorption main edge and near the absorption pre-edge. The shifts of 1.4 and 1.9 eV arise from the localized nature of the excitation. Excited-state localization/delocalization is also discussed for the analogous vacuum ultraviolet (VUV) transitions, and we point out the similarities between x-ray and VUV absorption spectra of liquid water.     

Adsorption of hydrophobically modified anionic starch at oppositely charged oil/water interfaces

In this paper we study the adsorption at cationic emulsion droplets of starch which had been hydrophobically modified with octenyl succinic anhydride (OSA), a modification which also renders the starch anionic. Emulsions were formed with didodecyldimethylammonium bromide (DDAB) after which the OSA-starch was added. The emulsions were separated by centrifugation and the surface load of OSA-starch was determined through serum depletion. The results show the adsorbed amounts can become very high, in some cases reaching approximately 40 mg/m2. The surface load correlates positively with the surface charge density of the starch which depends on the degree of substitution, rms radius and molar mass. Furthermore, the surface load obtained depends on the ratio between polymer surface charge density and the interface charge density which could be varied experimentally by combining various amounts of DDAB and dioleoyl phosphatidylcholine (DOPC) in the formation of the emulsion. The very high surface loads should correspond to very thick adsorbed layers. Thus, OSA-starch should be appropriate for encapsulation applications provided a suitable adhesion substance is employed.     

Column temperature as an active variable in the isocratic, normal-phase high-performance liquid chromatography separation of lipophilic metabolites of nonylphenol ethoxylates

Normal-phase separation of technical grade nonylphenol (t-NP, about 90% 4-nonylphenol), 4-nonylphenol mono-ethoxylate (4-NP1EO) and 4-nonylphenol di-ethoxylate (4-NP2EO) was assessed, with the inclusion of column temperature as an active variable. The compound 2,4,6-trimethylphenol was evaluated for use as internal standard. Isocratic elution with 2-propanol/hexanes mixtures from an amino-silica column and spectrometric UV detection at 277 nm were employed. Technical nonylphenol presented a significant contribution from unknown substances that eluted with retention times similar to that of 4-NP1EO. GC-MS analysis of the unknowns allowed to identify them as isomers of 2-NP. The response of the system to joint variations in flow rate, eluent composition and column temperature was investigated by means of Doehlert statistical experimental design. A model for retention of the analytes as a function of the experimental variables was proposed, and separation selectivity was studied. Selection of the optimal working zone was made through desirability function (D) calculations. Potential co-elution of 2-NP isomers with 4-NP1EO was considered when optimizing the separation. The occurrence of a restricted region of the experimental space where baseline resolution of analytes, associated impurities and internal standard results feasible (D not equal to 0) is apparent.     

Competitive adsorption of a polydisperse polymer during emulsification: experiments and modeling

In this paper we study the selective adsorption of a high molar mass polymer, OSA-starch, at the cyclohexane/water interface during emulsification. This was made possible through the use of AsFlFFF-MALS-RI which enables us to characterize the size and molar mass of polydisperse ultrahigh molar mass polymers. The results show that the high molar mass components in the molar mass distribution of the polymer were selectively adsorbed. The selective adsorption is most likely due to convective mass transport in the turbulent flow fields, during formation of the emulsion, which favors transport to the interface of the high molar mass polymers in the sample. The rapid adsorption that occurs during emulsification is likely to give rise to nonequilibrium effects and jamming at the interface. Furthermore, we describe the adsorption process and illustrate its selective nature through a turbulent flow collision model.     

Thermodynamic and real-space structural evidence of a 2D critical point in phospholipid monolayers

The two-dimensional phase diagram of phospholipid monolayers at air-water interfaces has been constructed from Langmuir compression isotherms. The coexistence region between the solid and fluid phases of the monolayer ends at the critical temperature of the transition. The small-scale lateral structure of the monolayers has been imaged by atomic force microscopy in the nm to microm range at distinct points in the phase diagram. The lateral structure is immobilized by transferring the monolayer from an air-water interface to a solid mica support using Langmuir-Blodgett techniques. A transfer protocol that ensures preservation of the structure during the transfer has been established. The lateral structure reflecting the density fluctuations has been visualized and quantitatively characterized as the monolayer passes through a series of first-order phase transitions and ultimately approaches a critical point. The critical behavior inferred from the thermodynamic as well as the structural data is found to be consistent with the 2D Ising universality class. Additional results are presented demonstrating the presence of striped phases and coexisting domains in binary mixtures.     

DNA gel particles: particle preparation and release characteristics

Aqueous mixtures of oppositely charged polyelectrolytes undergo associative phase separation, resulting in coacervation, gelation, or precipitation. This phenomenon has been exploited here to form DNA gel particles by interfacial diffusion. We report the formation of DNA gel particles by mixing solutions of DNA (either single-stranded (ssDNA) or double-stranded (dsDNA)) with solutions of cationic surfactant CTAB and solutions of the protein lysozyme. Swelling, surface morphology, and DNA release determinations indicate different interaction of ssDNA and dsDNA with both the surfactant and the protein. By using CTAB and lysozyme as the base material, the formation of a DNA reservoir hydrogel, without adding any kind of cross-linker or organic solvent, was demostrated.     

An interpolation theorem for proper holomorphic embeddings

Given a Stein manifold x of dimension n > 1, a discrete sequence , and a discrete sequence where , there exists a proper holomorphic embedding satisfying f(a _j ) = b _j for every j = 1,2,... Forstnerič and Prezelj supported by grants P1-0291 and J1-6173, Republic of Slovenia. Kutzschebauch supported by Schweizerische National fonds grant 200021-107477/1. Ivarsson supported by The Wenner-Gren Foundations.