Continuous-channel flow linear dichroism

Linear dichroism (LD) is the difference in absorbance of light polarized parallel to a sample orientation axis and polarized perpendicular to it. Flow LD provides information about samples with high enough aspect ratios to be oriented in a flow stream. It is particularly useful for studying DNA-ligand systems, protein fibres and membrane assemblies and is the ideal technique for monitoring growth or destruction of particles. Standard Couette flow cells are limited to a dead time in kinetic processes of ∼40 s (the assembly time). Recently an injection Couette flow cell has reduced the dead time to ∼600 ms. In this paper we report an alternative system, based on syringe drives, 3 alternative flow-through cells, and the optical system of a Biologic MOS-450 spectrometer that has been adapted for LD. This system can reduce the dead time to 25 ms. The sample requirement is ∼100 μL per time point. Less sample is required for longer dead times. The system has been applied to measure the kinetics of DNase digestion of DNA and GTP-induced polymerization of the bacterial cell division protein FtsZ.