Etude par diffusion des rayons X aux petits angles des gels d’acide désoxyribonucléique et de nucléoprotéines : (note préliminaire)

Abstract

Deoxyribonucleic acid (DNA), nucleoprotamine (DNP) and nucleohistone (DNH) have previously been investigated by means of X-ray diffraction techniques, the samples being oriented fibres in equilibrium with an atmosphere of known humidity. We have now studied the low-angle X-ray scattering from unoriented aqueous gels of these three substances, as a function of the concentration of dry material and of electrolyte. We have investigated the phase diagrams of these systems, and have made observations on unfixed nuclei containing DNP and DNH. This technique permits the investigation of long-range organization, and the use of concentrations and ionic strengths in the physiological range. DNA and DNH were extracted from fowl erythrocytes, DNP from trout sperm. The DNA-water system contains one phase in the range between 5% and 50% DNA (weight of DNA per weight of gel); the structure of this phase seems to consist of rigid rods, packed in a hexagonal array and swollen by water. The mass per unit length of the rods is in good agreement with Crick & Watson's model of DNA. The behaviour of the DNP water system is quite different. At all concentrations from 5% to 50% the system contains two phases, namely the saturated solution and a paracrystalline phase. At higher concentrations only the paracrystalline phase exists; the diffraction lines of this phase are very sharp. The X-ray photographs of unfixed trout sperm heads and of the paracrystalline phase of DNP are identical. In the DNH-water system there exist several phases. At very low concentrations there is only a solution of DNH in water which gives a diffuse central spot in the X-ray diagram. At higher concentrations (about 30%) there is a phase which we denote a>: its characteristic diffraction diagram is formed of three bands, at spacings 110 A, 55 A, and 37 A. At a concentration of 50% is found another phase (denoted Ψ ): its X-ray diagram contains one band at a spacing ~ 30 A. At even higher concentrations (~ 80%) a fourth phase appears (phase χ ); it gives two fairly sharp bands at spacings of 80 A and 40 A. Each phase exists over a small concentration range: the X-ray diagrams vary slightly with the concentration. At all concentrations intermediate between two phases the X-ray diagrams indicate the coexistence of the diagrams of both phases. The X-ray photographs of unfixed fowl erythrocyte nuclei show the three typical bands of phase w of DNH; the structure of DNH in the nuclei therefore seems to be identical with phase ω. The behaviour of DNP-water and DNH-water systems suggests that several DNA molecules are linked together by the basic proteins and form a three-dimensional complex which can be swollen by water only to a limited extent. In contrast, DNA can be swollen by water to any extent in the absence of basic proteins. The structure of the DNP paracrystalline phase is very compact and highly organized, and contains only 35% water. On the other hand, the ω phase of DNH is poorly organized; it contains over 60% water and it shows long-range order. This difference between DNP and DNH can be interpreted in terms of the biological role of these substances. The X-ray diagram of the ω phase of DNH is compatible with a lamellar structure having a period of 110 A. Some electron microscope observations seem to confirm the lamellar structure, but this model needs further confirmation. The action of deoxyribonuclease on DNH gels shows that the observed features must be attributed to a DNA-containing phase, and not to a lipid impurity as has been previously suggested.