The Formation of Complex Lenses from Evaporated Droplets of DNA

Sean English

Samples containing 50 mg/mL sonicated, calf thymus DNA, of fragment lengths 500 - 1500 base pairs, were permitted to dry naturally and by the application of heat. A conscopic cross was observed for both manners of evaporation, with and without the presence of a Bertrand lens. Based on this birefringence and the reported lens behavior of nematic liquid crystals, it was suggested that the process of evaporation resulted in a lens structure. In this study, preliminary steps were taken to verify or contradict this claim, while establishing the basic molecular and macroscopic organization of the DNA in these sample droplets. The use of scanning probe microscopy allowed for the identification of a periodic structure in the evaporated samples. If this periodicity is treated as P/2, the cholesteric pitch for the scanned regions is 1.94±0.22 mm. This is in agreement with the accepted pitch range 2.2 - 3 for DNA in the cholesteric phase. A periodic ring structure was also observed for the sample with ring thicknesses determined by observation with the microscope to be (67.0±9.3) mm and (29.0±4.6) mm for the air-dried and heated samples respectively. It is not clear how this structure is related to the periodic structure observed by SPM. However, the presence of a steep depression, or well, at the center of the sample appears to be necessary for the lens behavior. Analysis of the sample diffraction suggests that this well and the surrounding convex regions of the sample result in the convergence of light at two maxima. This appears to verify the lens behavior of the sample observed visually. The structure of the sample was theoretically modeled as two sets of multi slits separated by an opaque obstacle, and the experimental data was compared with theoretical data obtained using the structural parameters of the sample determined from observation with the microscope. The model appears to be a reasonable first step in understanding the sample structure, but the experimental variables need to be fully identified and eliminated if necessary.

DNA

3D plot for a 20X20 mm scan of a 10 mL air-dried sample of DNA, located midway between the center of the sample and its edge.