Oscillatory flow birefringence of deoxyribonucleic acid solutions 1

Abstract

Through measurements of oscillatory flow birefringence of macromolecular solutions information on optical properties and relaxation effects are obtained for the molecules in a nearly unperturbed state. Small, oscillatory velocity gradients are used which are then directly proportional to the birefringence. Measurements are performed using an apparatus in which monochromatic light having circular polarization passes through a thin fluid layer which is confined between a fixed surface and an oscillating surface. The motion of the surface is monitored in amplitude and phase and from this the velocity gradient is determined. The light leaving the fluid now has an oscillating ellipticity which is then monitored by passing it through a fixed plane analyzer and on to a photodetector. In the present apparatus the fluid is under precise temperature control and with its associated electronic instruments birefringence is measured in the frequency range from 0.02 to 1000 Hz. This apparatus is applied to the study of solutions of salmon sperm deoxyribonucleic acid, DNA. The relaxation times and magnitude of birefringence in solution are determined from measurements over a wide range of frequencies. This is done for several levels of shear degradation, the DNA being degraded by high speed chopping and by pressure cell induced flow. The ionic environment is considered as it influences the relaxation times. The method of oscillatory flow birefringence is also used to observe thermal denaturation of DNA. The birefringence undergoes a large change in magnitude and a reversal in sign as a consequence of the helix-coil transition. This method shows promise as a means for studying the kinetics of the process.