We analyze the repton model of Rubinstein as adapted by Duke as a model for the
A theory for the gel electrophoresis of a flexible polyelectrolyte, bearing an uncharged bulky label or an uncharged section at one end, is presented. We first consider a gel that is fully permeable to the label: we calculate the degree of stretching of the polyelectrolyte and its mobility as a function of chain size, electric field and label friction. Various regimes are identified, and their "existence domains" are calculated. For increasing friction, we predict a transition from a mobility decreasing with chain size to a mobility increasing with chain size. Secondly, we consider the possibility that the label may get trapped at some locations of the gel, a situation relevant to a method of "trapping electrophoresis" recently proposed by Ulanovsky et al. for DNA sequencing. A molecular model for detrapping by thermally activated "backward reptation" is constructed and solved using the Kramers rate-equation theory. Different closed analytical expressions and approximate scaling laws corresponding to different regimes of stretching and field strengths are predicted. The most striking result is a mobility which exponentially decreases past a critical size Np*, which decreases with increasing field. In the regime relevant to the experiments by Ulanowsky et al., we predict Np* approximately E-2/3. The predictions are in good qualitative agreement with presently available experiments, but further experimental investigations are suggested.
A theory for field-inversion gel electrophoresis of a flexible polyelectrolyte bearing an uncharged bulky label at one end is described, and the evolution of the mobility with chain length, field strength, friction of the label, and the duration of the forward and reverse pulses is predicted. A new critical size, Ndetrap, is introduced, and its value calculated. It increases roughly linearly with the duration of the reverse pulses. Chains smaller than Ndetrap are detrapped by reverse pulses, and may have a high mobility, whereas chains larger than Ndetrap are not trapped, and have a very small mobility. This leads to an increase of the mobility (as compared with constant field) in a given range of sizes, and to a strong selectivity around Ndetrap. Depending on the parameters, numerous other effects, including a secondary mobility plateau and band inversion, may appear. The corresponding regimes are predicted and discussed. All predictions are qualitatively consistent with available experimental data. We use them to suggest efficient conditions for the development of pulsed-field trapping electrophoresis, a possible tool for improved DNA sequencing. In particular, we recommend using a ramping of pulse times, with a constant ratio of forward to reverse time in the range 3 to 5.
La cristallisation des paraffines dans les pŽtroles bruts et m•me dans les gaz ˆ condensat est un probl•me majeur dans le domaine de lÕexploitation pŽtroli•re. Afin dÕŽvaluer ce risque, on utilise des mod•les thermodynamiques. Ils permettent de calculer la tempŽra-ture de cristallisation commen•ante (TCC) ou Wax Appearance Temperature (WAT) ainsi que la fraction cristallisŽe en fonction de la tempŽrature (pour des tempŽratures infŽrieures ˆ la TCC). Lors de leur dŽveloppement, ces mod•les doivent •tre ŽvaluŽs sur des donnŽes expŽrimentales. Il existe dans la littŽrature et chez les pŽtroliers diffŽrentes mŽthodes pour mesurer la TCC, malheureusement il nÕen est pas de m•me pour la fraction cristallisŽe en fonction de la tempŽ-rature. Dans cet article, nous avons dŽveloppŽ une mŽthode de RMN basse rŽsolution afin de mesurer la fraction cristallisŽe en fonction de la tempŽrature. Elle a dÕabord ŽtŽ ŽvaluŽe sur des mŽlanges de corps purs puis elle a ŽtŽ appliquŽe ˆ des pŽtroles bruts. Les rŽsultats ont ŽtŽ comparŽs ˆ ceux obtenus par analyse calorimŽ-trique diffŽrentielle ainsi quÕaux rŽsultats du mod•le. USE OF PULSED NMR SPECTROSCOPY TO MEASURE THE AMOUNT OF SOLID DEPOSITS AS A FUNCTION OF TEMPERATURE IN WAXY CRUDESParaffin crystallization in crude oils and in gas condensates is a major problem for petroleum exploitation. Thermodynamic models are used to evaluate the risk. With them, one calculates the Wax Appearance Temperature (WAT) as well as the crystallized fraction as a function of temperature, for temperatures below the WAT. During development, these models are tested against experimental data. Although different methods for measuring the WAT are available in the literature and at petroleum companies, such methods unfortunately do not exist for measuring the crystallized fraction as a function of temperature. In this work, we have developed a low resolution NMR method for measuring the crystallized fraction as a function of temperature. The method was first evaluated using pure samples and was then applied to crude oils. The results were compared to those obtained both from differential calorimetry and from the model. La cristalizaci-n de las parafinas en los petr-leos crudos, e incluso en los gases de condensados, constituye un problema agudo en el aspecto de la explotaci-n del petr-leo. Con objeto de evaluar este riesgo, se utilizan modelos termodin ‡micos que permiten calcular la temperatura de cristalizaci-n inicial (TCI) o Wax Appearance Temperature (WAT), as' como la fracci-n cristalizada en funci-n de la temperatura (para temperaturas inferiores a la TCI). Al proceder a su desarrollo, estos modelos se habr ‡n de evaluar acorde a datos experimentales. En la literatura tŽcnica profesional y tambiŽn por parte de las empresas petroleras existen diversos mŽtodos para la medici-n de la TCI, pero, desdichadamente, no ocurre lo mismo al tratarse de la fracci-n cristalizada, en funci-n de la temperatura. Al proceder a este trabajo, hemos desarrollado un mŽtodo de RMN de baja resoluci-n con objeto de medir la...
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