DNA MESOPHASES INDUCED BY SPERMIDINE - STRUCTURAL-PROPERTIES AND BIOLOGICAL IMPLICATIONS

Conditions of formation of DNA aggregates by the addition of spermidin e were determined with 146 base pair DNA fragments as a function of sp ermidine and NaCl concentration. Two different phases of spermidine-DN A complexes are obtained: a cholesteric liquid crystalline phase with a large helical pitch, with interhelix distances ranging from 31.6 to 32.6 Angstrom, and a columnar hexagonal phase with a restricted fluidi ty in which DNA molecules are more closely packed (29.85 +/- 0.05 Angs trom). In both phases, the DNA molecule retains its B form. These phas es are always observed in equilibrium with the dilute isotropic soluti on, and their phase diagram is defined for a DNA concentration of 1 mg /ml. DNA liquid crystalline phases induced by spermidine are compared with the DNA mesophases already described in concentrated solutions in the absence of spermidine. We propose that the liquid crystalline cha racter of the spermidine DNA complexes is involved in the stimulation of the functional properties of the DNA reported in numerous experimen tal articles, and we discuss how the nature of the phase could regulat e the degree of activity of the molecule.