Critical Aspects on Optical-Kerr Effects of Macromolecules. Lack of

J. Phys. Chem. 1987, 91, 1957-1960

1957

Critical Aspects on Optical-Kerr Effects of Macromolecules. Lack of Measurable Orientation of DNA Michael Wirth, Tore Eriksson, and Bengt Nord&n* Department of Physical Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden (Received: July 17, 1986; In Final Form: November 12, 1986)

The optical electric fields of intense laser pulses can give rise to orientation effects in liquids of small anisotropic molecules. In contrast to low-frequency electric fields, optical fields give no electrophoretic polarization (and in transparent media no thermal dissipation) and have therefore been proposed for studying reorientation phenomena of biopolymers in physiological salt media. In this study the possibility of detecting such an orientation of DNA is considered. Experiments with this purpose, using on one hand a pulsed Nd:YAG laser and on the other a ruby laser, were performed on sonicated fragments of duplex DNA. An earlier reported observation of a considerable response with DNA can be explained as an effect of heat dissipation owing to absorption of Nd:YAG radiation in solvent water. Despite a higher field strength (400 kV/cm), or longer pulses, and a better measuring sensitivity, no Kerr orientation of DNA was detectable (