[Formation of the compact form of DNA in solution after reaction with spermidine]

The formation of compact particles from double-stranded DNA molecules in water-salt solutions containing spermidine was studied. It has been shown that in solutions of low ionic strength (0.01 M NaCl) DNA-spermidine complexes have the form of large particles which scatter UV-light. Electron micrographs show that such complexes formed at certain molar ratios spermidine/DNA may exist both as intermolecular aggregates and as toroidal particles 1500 A in diameter. The CD spectra of solutions containing DNA-spermidine complexes are characterized by the positive band (delta epsilon max = 10) at 265--270 nm. The appearance of the positive CD band may be caused by two factors: interaction between DNA and spermidine may lead to the alteration of the DNA secondary structure "in direction to A-form" or intermolecular aggregation, which may change the initial shape of the CD spectrum. The exclusion of spermidine molecules from DNA-spermidine complexes by Na+ ions in presence of poly(ethylene glycol) which occurs as the ionic strength increases from 0.01 to 0.3 does not lead to decompactization of DNA molecules but is accompained by the appearance of the intense negative CD band at 270 nm.