Crystal Growth Habits of DNA Fractions. 2. Low-Voltage SEM and TEM Examination of Lamellar Structure and Growth Fronts

In a previous publication we reported the first documentation of the crystal growth habits in a polydisperse, heterogeneous DNA fraction of reasonable molecular weights under quiescent and isothermal conditions. In the relative humidity (RH) range where the A-DNA helix conformation is adopted, these morphologies could best be described as sheaves and spherulites. Here, we extend those light microscopic observations to the nanoscale, with the following conclusions: (1) the growth habit of A-DNA is lamellar with chain (helix) folding, exhibiting the formation of sheaves consisting of individual lamellae similar to those formed by synthetic semicrystalline polymers; (2) the lamellar crystals are thinner than the mean molecular length of the DNA fraction; (3) the lamellae exhibit topological differences in their lateral and basal aspects; (4) the lamellar tips are round and thinner than in the lamellar region behind the tips, which appear more faceted and of a different organizational structure; (5) these two zones are associated with a thickening transition zone; and (6) the interfacial zone at the lamellar growth front contains, what appear to be, bundled molecules based on dimensional and functional considerations.