Reconciling low- and high-salt solution behavior of sulfobetaine polyzwitterions

We investigate the water-solubility upon salt additions, of homogeneous families of sulfobetaine-based polyzwitterions. These polymers bear both positive ammonium, and negative sulfonate charges on each monomer and as a result present an upper critical solution temperature (UCST) in the 0-100 degrees C temperature range. Two chemistries are investigated, with either a carboxylate-carrying function (SPE) or an amido-carrying one (SPP). In agreement with the literature published on pSPEs, we find that an addition of simple salts improves the water-solubility of pSPEs, as well as that of pSPPs, yet only once a threshold concentration of added salt has been reached in the solution. We verify using scaling arguments that the onset of solubility promotion, corresponds exactly to the complete screening of the attraction between positive and negative charges inside a polyzwitterionic coil. On the contrary, for salt concentrations smaller than the threshold concentration, we observe that an addition of salt can be adverse to the solubility of polyzwitterions, depending on the degree of polymerization, the type of salt, and the type of zwitterionic motive. Thanks to zeta-potential measurements and systematic variations of these three parameters, we demonstrate, in agreement with theoretical prediction, that this molecular weight-dependent enhanced solubility at small salt concentrations is due to charge asymmetry resulting from partial hydrolysis, combined with specific interactions between salts and zwitterion constituents, evidencing the complexity of the solution behavior of these macromolecules. We thereby reconcile the different behaviors in the domains of low- and high-salinity.     

Ab initio ONIOM-molecular dynamics (MD) study on the deamination reaction by cytidine deaminase

We applied the ONIOM-molecular dynamics (MD) method to the hydrolytic deamination of cytidine by cytidine deaminase, which is an essential step of the activation process of the anticancer drug inside the human body. The direct MD simulations were performed for the realistic model of cytidine deaminase by calculating the energy and its gradient by the ab initio ONIOM method on the fly. The ONIOM-MD calculations including the thermal motion show that the neighboring amino acid residue is an important factor of the environmental effects and significantly affects not only the geometry and energy of the substrate trapped in the pocket of the active site but also the elementary step of the catalytic reaction. We successfully simulate the second half of the catalytic cycle, which has been considered to involve the rate-determining step, and reveal that the rate-determining step is the release of the NH3 molecule.     

The role of pendant amines in the breaking and forming of molecular hydrogen catalyzed by nickel complexes

We present the results of a comprehensive theoretical investigation of the role of pendant amine ligands in the oxidation of H(2) and formation of H(2) by [Ni(P(R)(2)N(R')(2))(2)](2+) electrocatalysts (P(R)(2)N(R')(2) is the 1,5-R'-3,7-R derivative of 1,5-diaza-3,7-diphosphacyclooctane, in which R and R' are aryl or alkyl groups). We focus our analysis on the thermal steps of the catalytic cycle, as they are known to be rate-determining for both H(2) oxidation and production. We find that the presence of pendant amine functional groups greatly facilitates the heterolytic H(2) bond cleavage, resulting in a protonated amine and a Ni hydride. Only one single positioned pendant amine is required to serve this function. The pendant amine can also effectively shuttle protons to the active site, making the redistribution of protons and the H(2) evolution a very facile process. An important requirement for the overall catalytic process is the positioning of at least one amine in close proximity to the metal center. Indeed, only protonation of the pendant amines on the metal center side (endo position) leads to catalytically active intermediates, whereas protonation on the opposite side of the metal center (exo position) leads to a variety of isomers, which are detrimental to catalysis.     

A simple formula for mean multiplexing delay for independent regenerative sources

Simple expressions are given for the mean delay, mean waiting time, and mean busy period length in a multiplexer. Data streams with active periods having a general distribution are permitted, and the data rate during the active periods can be random. Data can also arrive in batches. The key restrictions of the model are that the sources are independent, idle periods are exponentially distributed, and a source generates at least enough data during an active period to keep the server busy throughout the period. The exact formulas allow evaluation of the error in approximations such as a heavy traffic diffusion approximation.Both continuous and discrete time models are considered. The discrete-time model includes that studied by Viterbi and subsequently generalized by Neuts. The Pollaczek-Khinchine formula for the mean amount of work in anM/GI/1 queue is retrieved as a limiting case.Preliminary version presented at IEEE INFOCOM, San Francisco, April 1993.     

Design of a Resonant-Cavity-Enhanced p-i-n GaN/AlxGa1-xN Photodetector

A resonant-cavity-enhanced p-i-n photodetector has been designed and analyzed to operate at a wavelength of 360 nm based on the Al_xGa_1-xN material system. The novel approach has been adopted of using epitaxial AlN/Al_xGa_1-xN quarter-wave stacks as the distributed Bragg reflector that serves as the front mirror. An Al_xGa_1-xN absorptive filter layer is incorporated to suppress all but one resonant mode to ensure single, narrow-band operation. This device structure is projected to achieve wavelength selective, high speed, and high quantum efficiency operation in the ultraviolet. MOCVD-grown 6 1/2-pair AlN/Al_xGa_1-x     

Magnetic interactions in transition metal clusters embedded in matrices prepared by LECBD technique

The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix have been realized by the co-deposition of both beams arriving at the same time on a 45°-tilted substrate. The nanoparticles were produced from an intense cluster beam of selected size (centered around 300 atoms per cluster) produced from a laser vaporization source. We used a Knudsen cell to evaporate the matrix. From the ratio of the deposition rates of both beams, we were able to adjust continuously the atomic concentration of clusters in the matrix from 3 to 70 %. The typical size distribution of embedded clusters determined from HRTEM observations on diluted samples revealed nanocrystallizedgrains with a very narrow size dispersion. Moreover, magnetoresistance (MR) and magnetization measurements versus temperature and concentration were performed. The results show clearly a magnetic threshold around 25 % Co-clusters content in matrix, corresponding to the transition from superparamagnetic to magnetically ordered state. This threshold is identical to the 3D-percolation threshold for connected particles. A maximum of MR ratio of 12 % at 4.2 K was obtained for Co_25%Ag_75% granular film at the onset of non negligible magnetic interactions between Co-particles. The magnetic properties of our systems, strongly correlated to the nucleation processes, allowed us to determine the variation of the cluster radius as a function of the concentration and of the matrix nature.     

Is ferromagnetism an intrinsic property of the CuII/GdIII couple? 1. Structures and magnetic properties of two novel dinuclear complexes with a mu-phenolato-mu-oximato (Cu,Gd) core

Two original dinuclear (Cu(II),Gd(III)) complexes (1 and 2) deriving from polydentate nonsymmetrical Schiff base ligands LiH2 have been prepared. Formally they differ by the length of the diamino chain. They crystallize in the orthorhombic Pbca (No. 61) (1) and in the monoclinic P2(1/n) (No. 14) (2) space groups. The cell parameters are a = 12.6295(7) A, b = 20.7894(9) A, c = 18.3301(13) A, and Z = 8 for 1 and a = 12.7246(16) A, b = 13.5691(17) A, c = 14.5310(19) A, beta = 94.629(16) degrees , and Z = 4 for 2. These structural studies show that in both complexes the CuII and GdIII ions are doubly bridged by a phenolato oxygen atom and an oximato (N-O) pair. The bridging network is not planar. The more important distortions are observed for the complex having the larger diamino chain. Unexpectedly the latter complex presents an antiferromagnetic interaction, but the related J value is small (J approximately equal to -0.49 cm(-1)). In the former complex the interaction is ferromagnetic (J approximately equal to 3.5 cm(-1)) as it is for complexes containing (CuO2Gd) bridging cores which yield J values varying from 1.4 to 10.1 cm(-1).     

An information theoretical evaluation of the applicability of the ASTM infrared data base for retrieval purposes

Summary The information content of the identification by retrieval with binary coded infrared spectra (ASTM infrared datafile), taking into account the correlations between the speaks and the errors in the coded spectra, amounts to about 21 bits. This amount of information can be obtained when use is made of not more than 93 wavelength intervals, selected from the total number of 140 intervals that are available for retrieval. The information content can be raised to about 29 bits when for the retrieval procedure a window of 0.3 is introduced, thereby requiring only 72 intervals. The influence of the composition of the dataset upon the information content is considerable; for hydrocarbons an amount of information of only 15 bits is obtained with a search using 79 intervals.The probability of the match between the spectrum of the unknown compound and the right reference spectrum being superior to all other possible matches will increase from virtually zero to about 88% when a window of 0.3 is used.

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Eine informationstheoretische Auswertung der Anwendung der ASTM-Sammlung — Infrarotspektren für die Bibliothekssuche

Zusammenfassung Der Informationsgehalt für die Identifizierung durch Vergleich binÄr codierter Infrarotspektren (ASTM-Sammlung Infrarotspektren) betrÄgt unter Berücksichtigung der Ungenauigkeiten und der Korrelation zwischen den Banden etwa 21 Bits. Diese Informationsmenge kann erhalten werden aus dem Vergleich von nur 93 WellenlÄngen-Intervallen, die aus einer Gesamtzahl von 140 verfügbaren selektiert wurden.Der Informationsgehalt für die Bibliothekssuche kann bis zu 29 Bits zunehmen, wenn für die Suche ein Fenster von 0,3 angewendet wird; die Zahl der benötigten Intervalle betrÄgt dann nur 72. Aus verschiedenen Spektrensammlungen erhÄlt man verschiedene Informationsmengen; zum Beispiel betrÄgt der Informationsgehalt für Kohlenwasserstoffe nur 15 Bits unter Benutzung von 79 Intervallen. Die Wahrscheinlichkeit, da\ der Vergleich des Spektrums einer unbekannten Verbindung mit dem richtigen Vergleichsspektrum eine bessere übereinstimmung liefert als der Vergleich mit allen anderen Vergleichsspektren, nimmt von ungefÄhr null bis 88% zu, wenn ein Fenster von 0,3 angewendet wird.