X-ray diffraction studies on the complex of E. coli DNA with synthetic thermospermine

X-ray studies on the complex of E. coli DNA with synthetic thermospermine in fibers were carried out. This complex showed the ordinary transition, except that it yielded a semicrystalline B-form at 66% r.h., suggesting good side-to-side interactions of the material due to some number of cross-bridges of tetramine. An idea of thymine methyl-neighbor base short contacts in A-form DNA is proposed to understand these structural changes of double helical DNA. On the assumption that the unstable A-conformation needs lateral interactions, the present X-ray results are well explained. This unstable factor is potentially of considerable biological interest in relation with DNA compaction.