Liquid Crystals of Self-Assembled DNA Bottlebrushes

Early theories for bottlebrush polymers have suggested that the so-called main-chain stiffening effect caused by the presence of a dense corona of side chains along a central main chain should lead to an increased ratio of effective persistence length (l(p,eff)) over the effective thickness (D-eff) and, hence, ultimately to lyotropic liquid crystalline behavior. More recent theories and simulations suggest that l(p,eff) similar to D-eff, such that no liquid crystalline behavior is induced by bottlebrushes. In this paper we investigate experimentally how lyotropic liquid crystalline behavior of a semiflexible polymer is affected by a dense coating of side chains. We use semiflexible DNA as the main chain. A genetically engineered diblock protein polymer C4K12 is used to physically adsorb long side chains on the DNA. The C4K12 protein polymer consists of a positively charged binding block (12 lysines, K12) and a hydrophilic random coil block of 400 amino acids (C-4). From light scattering we find that, at low ionic strength (10 mM Tris-HCl), the thickness of the self-assembled DNA bottlebrushes is on the order of 30 nm and the effective grafting density is 1 side chain per 2.7 nm of DNA main chain. We find these self-assembled DNA bottlebrushes form birefringent lyotropic liquid crystalline phases at DNA concentrations as low as 8 mg/mL, roughly 1 order of magnitude lower than for bare DNA. Using small-angle X-ray scattering (SAXS) we show that, at DNA concentrations of 12 mg/mL, there is a transition to a hexagonal phase. We also show that, while the effective persistence length increases due to the bottlebrush coating, the effective thickness of the bottlebrush increases even more, such that in our case the bottlebrush coating reduces the effective aspect ratio of the DNA. This is in agreement with theoretical estimates that show that, in most cases of practical interest, a bottlebrush coating will lead to a decrease of the effective aspect ratio, whereas, only for bottlebrushes with extremely long side chains at very high grafting densities, a bottlebrush coating may be expected to lead to an increase of the effective aspect ratio.