Molecular Structure of Deoxypentose Nucleic Acids

This paper presents early experimental evidence derived from X-ray diffraction analysis regarding the structure of deoxypentose nucleic acid (DNA). Contrary to more complex models previously proposed, the authors find that the fundamental molecular configuration is surprisingly simple: a repeating helical chain.

The researchers utilized X-ray diffraction photography of oriented fibers. By analyzing the resulting fiber diagrams—specifically the spacing of layer lines and the intensity distribution—they modeled the DNA molecule as a system of intertwined, co-axial helices. The study identifies a distinct helical pitch of approximately 34 A and a diameter of about 20 A. A crucial observation is the absence of reflections on the meridian of the diffraction pattern, which serves as a diagnostic feature of a helical structure with an axis parallel to the fiber length.

The analysis strongly suggests that the phosphate groups, which form the backbone of the polynucleotide chains, are located on the outside of the helical unit. This arrangement explains the molecule's hygroscopic nature and its interaction with water. Furthermore, the intensity distribution of the X-ray data suggests that the structure consists of two co-axial molecules that are not equally spaced along the axis, though the study stops short of providing a fully definitive, solved atomic model. The authors also confirm that this helical structure is consistent across various biological samples, including bacteriophage and diverse animal tissues.

By characterizing DNA as a regular, repeating helical structure with an exterior phosphate backbone, this work laid the fundamental groundwork for understanding the physical basis of genetic material. It shifted the scientific focus toward a structure capable of orderly replication, providing the structural foundation upon which modern molecular biology was built.