Counterions which favour the C form of DNA.

Recently the relationship between the B and C forms of DNA has been questioned. We have found some amino acids and related amines which form complexes with DNA and favour the C form. We have studied such complexes by X-ray diffraction. With some of these counterions the transition between the B and C forms occurs smoothly, whereas in other cases there is a discontinuous transition. We conclude that the C form is a well-defined variant of the B form favoured by some counterions under low humidity conditions.