[Study of conformational differences between DNA composing chromatin and compact particles by the slow 1H--3H...

The rate of 1H leads to 3H exchange between water and C(8)H groups of purinic residues in chromatin as well as in optically active and optically inactive compact DNA particles, formed in PEG-containing water-salt solutions was measured. In chromatin the rate of exchange is shown to be about 15% lower as compared to that in the open form of DNA. Assuming the ylid mechanism of exchange the retardation observed may be interpreted as a consequence of screecing of C(8)H groups by proteins of chromatin which hampers the contacts of these groups with OH- ions of the solvent. In optically active compact particles the rate of exchange is shown to be as high as 20% but in optically inactive particles up to 60% as compared to the open form of DNA. This fact indicates the difference of the secondary structure of DNA in these two compact forms. The ratio between the rate constants in guanylic and adenylic residues (kG : kA) in the open form of DNA as well as in chromatin and in optically active compact particles is equal 3.0, whereas the ratios in mononucleotides and in optically inactive particles are equal 1.6 and 2.2, accordingly. Comparison of the 1H leads to 3H exchange kinetic data with literature data on the small-angle X-ray scattering reflexes for optically active and optically inactive compact particles of DNA, suggests that the secondary structure of DNA in chromatin and in optically active compact particles is a highly-ordered one, whereas in optically inactive compact particles of DNA the bases are to a great extent disordered.