The C conformation is a low salt form of sodium DNA

Abstract

Three well-defined conformations have been observed for naturally occurring DNAs in oriented fibres. These have been designated A, B and C and their detailed geometries in a variety of crystalline and semi-crystalline forms determined by X-ray fibre diffraction analysis^1–6. The C conformation is commonly observed as a low humidity form of the lithium salt³. However, when the counter-ion is Na⁺ rather than Li⁺, previous X-ray fibre diffraction studies^6,7 have suggested that the C form is at best a poorly favoured conformation. Arnott and co-workers^6,8 reported that the C form is stable at relative humidities and salt contents which are both intermediate between those which favour the A and B conformations. Zimmerman and Pheiffer⁹ observed C-like patterns from fibres of Na-DNA immersed in t-butanol/water mixtures. They emphasized that the C form should be visualized as a family of related structures and suggested that continuous smooth transitions occur between the various members of the C family and between the B and C forms. IR linear dichroism studies have shown that a C-like conformation can occur in oriented films of Na-DNA¹⁰ at very low salt contents. Transitions between the A and B forms of Na-DNA depend on the salt content of the fibre and its relative humidity¹¹. Here we describe the conditions for routinely observing the C form of Na-DNA. These are applicable to a wide variety of natural DNAs and to the synthetic polynucleo-tide poly(dA-dC) · poly(dG-dT).