In the study of DNA electric birefringence, it is usual to use theories that consider that molecules in solution are small in relation to the light wavelength. In this work, we study the DNA electric birefringence using a broken-rod macroion (BRM) model composed of two cylindrical arms which does not restrict the size of the molecules. To achieve this, we include the inhomogeneity effect of the light electric field through the molecule and the interaction between its different parts. To analyze the interaction between a molecule and the incident beam of light, we apply the discrete dipole approximation (DDA), according to which each molecule is described as a finite array of electronic coupled oscillators. The electric birefringence is calculated from the oscillator polarizability. This is obtained from experimental data of electric birefringence saturation and from the increment of the solution refraction index in relation to that of the solvent. Furthermore, the oscillator polarizability is also estimated from DNA absorption spectrum using the KronigKramers relations. This allows us to analyze the contributions of the different absorption bands of DNA to the electric birefringence. We analyze the influence of the inhomogeneity of the light electric field and of the intramolecular interactions in the characterization of DNA optical properties using electric birefringence measurements.

In the present work, we study the effect of translational-rotational hydrodynamic coupling on the stationary electric linear dichroism of DNA fragments. The theoretical resolution of the problem has, so far, been dealt with analytic methods valid only in the limit of low electric fields. In this work, we apply numerical methods that allow us to study the problem and also consider electric fields of arbitrary strength. We use the bent rod molecules model to describe DNA fragments with physical properties characterized by their electric charge, electric polarizability tensor, rotational diffusion tensor, and translation-rotation coupling diffusion tensor. The necessary orientational distribution function to calculate electric dichroism is obtained by solving the Fokker-Planck equation through the finite difference method. We analyze the different contributions due to electric polarizability and translational-rotational coupling to the electric dichroism.

In the present work we make a theoretical study of the steady state electric linear dichroism of DNA fragments in aqueous solution. The here developed theoretical approach considers a flexible bent rod model with a saturating induced dipole moment. The electric polarizability tensor of bent DNA fragments is calculated considering a phenomenological model which theoretical and experimental backgroung is presented here. The model has into account the electric polarizability longitudinal and transversal to the macroion. Molecular flexibility is described using an elastic potential. We consider DNA fragments originally bent with bending fluctuations around an average bending angle. The induced dipole moment is supposed constant once the electric field strength grows up at critical value. To calculate the reduced electric linear dichroism we determine the optical factor considering the basis of the bent DNA perpendicular to the molecular axis. The orientational distribution function has into account the anisotropic electric properties and the molecule flexibility. We applied the present theoretical background to fit electric dichroism experimental data of DNA fragments reported in the bibliography in a wide range of molecular weight and electric field. From these fits, values of DNA physical properties are estimated. We compare and discuss the results here obtained with the theoretical and experimental data presented by other authors. The original contributions of this work are: the inclusion of the transversal electric polarizability saturating with the electric field, the description of the electric properties with an electric polarizability tensor dependant on the bending angle and the use of an arc model originally bent.

We study the effect of translational-rotational hydrodynamic coupling on the transient electric linear dichroism of DNA fragments in aqueous solution. As opposed to previous theoretical works, where analytic solutions valid in the limit of low electric field were reported, we present here a numerical approach which allows to obtain numerical results valid independently from the applied electric field strength. Numerical procedures here used are an extension to the transient-state of those developed in a previous work for the study of the problem in the steady-state. The molecular orientational processes induced by an electric field is characterized with statistical arguments solving the FokkerPlanck equation by means of the finite difference method to know the orientational distribution function of molecules.

In the present work we analyze the physical fundamentals of Manning’s counterion condensation using his charged line model in a simple salt solution. We extend the theory for the cases of finite saline concentration and polymeric concentration tending to zero and the case of both finite concentrations. To find the equilibrium between the phases of free and condensed counterions, besides minimizing the free energy, we deduce auxiliary equation to determine the two characteristic parameters of the theory, the fraction fo condensed counterions and the volume of condensation. We compare the obtained results in the present work for only one infinite charged line with the ones of counterion condensation theory by Schurr and Fujimoto. We find that the linear density of critical charge depends on the concentration of added salt and takes values higher than one, instead of unitary value predicted by Manning. We obtain the equations by the activity and osmotic coefficients in function of the critical charge density. We compare them with the corresponding equations by Manning for these parameters. We extend the counterion condensation theory to solutions of linear polyelectrolytes for finite saline and polymeric concentrations using a cell model. We modify the electrostatic contribution to the Gibbs energy adding, to the traditional one calculated by Manning, the energy excess due to the macroion being in a cylindrical cell. We apply the theory to obtain the osmotic coefficient and we compare our results with experimental data of DNA osmotic coefficient and with theoretical adjustment using the Poisson-Boltzmann equation.

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Las soluciones macromoleculares se convierten en birrefringentes y dicroicas al aplicar sobre ellas un campo eléctrico externo. Tanto la birrefringencia como el dicroísmo eléctricos se deben a que las moléculas en solución tienen, en general, propiedades ópticas anisotrópicas, y a que el campo eléctrico induce para éstas una orientación preferencial en el espacio. En el presente trabajo se estudia la birrefringencia y el dicroísmo eléctricos de fragmentos de la molécula ácido desoxirribonucleico (ADN) en solución salina. Se proponen tres objetivos específicos que, aunque se definen en términos de aplicaciones al ADN, contribuyen al desarrollo de las teorías electro-ópticas y a su aplicación a soluciones macromoleculares en general. El primer objetivo es analizar el efecto de la electroforesis junto con el acoplamiento hidrodinámico traslación-rotación de moléculas rígidas sobre la birrefringencia y el dicroísmo eléctricos del ADN. A tal fin se resuelve la ecuación de Fokker-Planck del sistema y se obtiene la función de distribución orientacional de las moléculas al aplicar el campo eléctrico, una vez alcanzado el estado estacionario. Los resultados obtenidos muestran que el acoplamiento hidrodinámico influye en las señales electro-ópticas en un amplio rango de intensidades de campo eléctrico, aunque resulta particularmente importante para intensidades relativamente bajas. Los desarrollos teóricos realizados se utilizan para ajustar datos experimentales de dicroísmo eléctrico de soluciones de ADN sonicado. El segundo objetivo consiste en estudiar el efecto del tamaño finito de las moléculas con respecto a la longitud de onda de la luz sobre las propiedades ópticas del ADN. El estudio se realiza utilizando la aproximación de dipolos discretos para describir las propiedades ópticas de las moléculas. Los resultados demuestran la importancia de incluir la inhomogeneidad del campo eléctrico de la luz incidente a través de las partículas y las interacciones intramoleculares, para estudiar cómo las diferentes partes de una molécula contribuyen a las propiedades ópticas de la misma. El tercer y último objetivo es estudiar la orientación de fragmentos de ADN en un campo eléctrico con métodos de simulación computacional. En virtud de la relativamente baja capacidad de cálculo que requiere, en este trabajo se optó por utilizar simulación dinámica de Langevin. Las moléculas se describen mediante un modelo simple constituido por una cadena de subunidades esféricas unidas mediante potenciales elásticos. Se introduce una función de orientación que describe el orden orientacional de las moléculas al aplicar un campo eléctrico. Los resultados muestran como el grado orientacional incrementa con la intensidad del campo eléctrico y como este orden tiende hacia la saturación.

Recibido: 16/08/16; aceptado: 27/03/17 En este trabajo se estudia la dependencia del dicroísmo lineal eléctrico reducido (DLER) de muestras de ADN en función del campo eléctrico aplicado. Para esto se somete una solución de ADN a un proceso de sonicación, que da como resultado una muestra con un rango de tamaños de las moléculas de 100 a 1000 pares de base, aproximadamente. A los fines de obtener fracciones más monodispersas se fracciona la muestra mediante el método de cromatografía en gel, obteniéndose varias fracciones de ADN útiles para el estudio mediante técnicas electro-ópticas. Estas fracciones se caracterizan mediante electroforesis en gel para obtener la distribución de longitudes de cadena y los promedios de cada fracción. Los resultados experimentales de DLER se ajustan mediante un modelo teórico de varilla arqueada flexible con polarizabilidad eléctrica que satura con el campo eléctrico aplicado donde, además de la polarizabilidad eléctrica longitudinal, se tiene en cuenta la polarizabilidad eléctrica transversal. Los resultados de este estudio son similares a los obtenidos por otros autores mediante fragmentación de ADN por enzimas de restricción. La ventaja del método de fraccionamiento utilizado radica en que se obtienen muestras de mayor volumen para la aplicación de las técnicas electroópticas. Palabras clave: ADN, dicroísmo lineal eléctricoThis work studies the dependence of reduced electric linear dichroism (RELD) of DNA samples as a function to the applied electric field. To do so, a DNA solution is subjected to sonication, which results in a sample whose molecular size range varies, approximately, between 100 and 1000 base pairs. In order to obtain more monodisperse fractions, the sample is fractionated using the gel chromatography method, obtaining various DNA fractions which are useful for the analysis by means of electrooptical techniques. These fractions are characterized using gel electrophoresis, so as to obtain the distribution of chain lengths and their average for each fraction. The RELD experimental results are fitted by means of a theoretical model of flexible curved rod with electric polarizability which saturates with the applied electric field, where it is taken into account not only the longitudinal electrical polarizability but also the transversal one. The results of this study are similar to the ones obtained by other authors using DNA fragmentation by restriction enzymes. The advantage of the fractionation technique used resides in the obtention of samples of larger volume to apply electrooptical techniques.