Optical anisotropy of synthetic polynucleotides. I. Flow birefringence and π‐electron polarizabilily of bases

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Abstract
Birefringence of poly‐A, poly‐U, and double‐stranded poly‐AU are investigated experimentally as well as theoretically. Single‐stranded poly‐A, at low ionic strength, exhibits a strong negative birefringence indicating that, adenine molecules are stacked regularly with their planes along the transverse direction. On the addition of Mg or Mn ions, the birefringence decreases markedly. Poly‐U has no noticeable birefringence indicating that poly‐U has a contracted configuration in solution. On mixing poly‐A and poly‐U with 10–3M MgCl2, the birefringence increases markedly, but the change in the chain configuration seems less pronounced. The polarizability of adenine and uracyl are ejaculated using the simple Ifïückel scheme. The calculation shows that the π‐electron polarizability normal to the base plane is zero. Therefore, the negative birefringence is probably due to the lack of π‐electron polarizability in the direction normal to the base plane and accordingly no π‐electron polarizability along the major helix axis. Conversely, the negative birefringence is an indiaction of base stacking.