LIQUID-CRYSTAL FORMATION IN DNA FRAGMENT SOLUTIONS

The critical volume fractions pertaining to the formation of DNA liqui d crystals were obtained from polarization microscopy, P-31-nmr, and p hase separation experiments. The DNA length (approximately one to two times the persistence length 50 nm), ionic strength, and counterion va riety dependencies are reported. The cholesteric-isotropic transition is interpreted in terms of the coexistence equations, which are derive d from the solution free energy including orientational entropy and ex cluded volume effects. With the wormlike chain as reference system, th e electrostatic contribution to the free energy is evaluated as a ther modynamic perturbation in the second virial approximation with a Debye -Huckel potential of mean force. The hard core contribution has been e valuated with scaled particle theory and/or a simple generalization of the Carnahan-Starling equation of state for hard spheres. For suffici ently high ionic strengths, the agreement is almost quantitative. At l ower amounts of added salt deviations are observed, which are tentativ ely attributed to counterion screening effects. The contour length dep endence agrees with a DNA persistence length 50 nm. (C) 1998 John Wile y & Sons, Inc.