Electro-optics of solutions of biopolymers and their complexes

The review summarizes the results of electric-birefringence studies of the molecular properties of biopolymers and their complexes. The application of alternating electric fields makes it possible to study the kinetics of orientation of DNA, RNA, and polypeptide molecules in diluted solutions. An analysis of molecular-mass dependences of relaxation times of biopolymer molecules in terms of the rotational friction theory is used to determine the equilibrium rigidity of macromolecules characterized by Kuhn segment length A. For DNA molecules in buffer solutions with a low ionic strength, the value of A derived from electro-optical measurements is 114 nm. The high sensitivity of electric birefringence to changes in the secondary and tertiary structures of biopolymers ensures its use for the analysis of sequence curvature in short fragments of DNA and for investigation of transitions between different tertiary structures of RNA. The study of electro-optical birefringence in solutions of complexes of DNA and polypeptides with oppositely charged ions of surfactants makes it possible to gain insight into their conformational properties in organic solvents. The stoichiometric complexes of DNA in chloroform occur in the compact globular state, whereas the conformation of complexes formed by various polypeptides depends on their composition and may vary from rodlike to coiled. Electric birefringence in solutions of biopolymer complexes is associated with the orientation of molecules that is due to the external-field-induced dipole moment that appears as a result of a small displacement of ions along the contour of polyion chains. The time of reaching the induced dipole moment is equal to or greater than the time of orientational relaxation of the complex as a whole.