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DNA in Aqueous Solutions with Repulsive Interactions : Structure Determined on the Basis of Dielectric Spectroscopy Measurements

We overview the study of dynamics and structure of the most famous biopolymer deoxyribonucleic acid (DNA) aqueous solutions, which has been conducted primarily in the laboratories at Institut za fiziku in Zagreb during the last six years. For completeness we briefly comment on our study performed on another biopolymer hyaluronic acid (HA) which has been gaining popularity quite rapidly within laymen and large public community recently. The aim of this Academy Report is to bring together the motivation for this research, the description of material preparation, experimental methods and data analysis used, as well as the presentation of the main results. As far as the physics interactions are concerned, our studies were focused until now on the case of repulsive regime of intersegment interactions mediated by univalent counterions. At the end open issues are highlighted and prospects for further research with polyvalent counterions are designated in order to study the crossover from repulsive to attractive regime of intersegment interactions. In other words, this Report is a kind of compilation of the material from our peer reviewed articles published in the international physics journals. We consider that the primary achievement of our efforts is the technique of dielectric spectroscopy (DS) developed in the laboratories of Institut za fziku in Zagreb, which enables to study small quantity soft matter and biological samples in solution. We demonstrate that this method, which is basically a method to measure the dielectric relaxation under an applied small ac electric field is a powerful technique to determine the structural properties thanks to a tight connection between these two physical parameters. Moreover, the DS method can serve to successfully discern between the structural properties of DNA solutions composed of many chains, and the single chain structural properties. The latter places the DS method at the level of well developed and widely used single-molecule micro/nano-manipulation techniques like force-measuring laser or magnetic tweezers. Our data consistently show two dielectric relaxations in 100 Hz − 100 MHz frequency range that can be attributed to diffusive motion of polyion counterions. The overall study as a function of polyion length, concentration and added salt concentration demonstrates that the motion of polyion counterions detected at MHz frequencies probes the structural organization of the polyion network in solution, where as the motion at kHz range is correlated with single polyion conformational properties. Fundamental length scales found to characterize the polyelectrolyte structure differ for the dilute and semidilute regime and depend on the strength of repulsive electrostatic interactions and the flexibility ; they also compare well with the fundamental length scales predicted by theory.

biopolyelectrolyte; dielectric response; dynamics and conformation; univalent counterion

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