Left-Handed, Complementary Double Helices—A Heresy? The Z-DNA Family

Guided by X-ray fiber diagrams of DNA and RNA double helices, molecular models with right-handed screw sense were developed. Because this experimental evidence cannot, per se, distinguish between image/mirror image or right/left-handed screw molecules, in the past serious doubts as to the DNA double-helical structure have been raised (107–114). Moreover, theoretical considerations suggested that the nucleotide unit in its standard conformation can be fitted into a left-handed double-helical screw with Watson-Crick-type base-pairing (380, 383). Only in recent years has the right-handed screw structure been fully supported by means of single-crystal X-ray studies on oligonucleotides (Sections 10.4, 11.2) and tRNA (Chapter 15).