P-31 NMR ANALYSIS OF THE HELIX TO COIL TRANSITION OF NATURAL DNA SAMPLES - EVIDENCE FOR THE EXISTENCE OF DIFFERENT CONFORMATIONAL STATES

The helix to coil transitions of calf thymus and salmon sperm DNA were probed by 31P nuclear magnetic resonance spectroscopy. Both the helical and coil forms were observed in the melting region indicating slow exchange between the two forms with an estimated rate of interconversion ≪ 36 sec-1 at 70°C. At least three different signals were also observed at temperatures significantly above the Tm, suggesting three classes of conformational states. One of these classes has a significantly lower T1 than the other two indicating considerable residual structure in this form. The four only partially resolved signals indicate that the phosphate residues have very similar chemical shift environments. From the experimentally observed small chemical shift differences between the helical and coil forms, it is concluded that the gauche, gauche, conformation predominates in the coil form as has been found for the helix. © 1979.