Nuclear magnetic resonance studies of polyamine binding to a defined DNA sequence

The binding of spermine to the self-complementary DNA sequence d(C-G-C-G-A-A-T-T-C-G-C-G) has been studied by nuclear magnetic resonance spectroscopy. Free spermine gives narrow resonance lines and positive nuclear overhauser effects are observed between the spermine protons, as expected for a small molecule rotating freely in solution. In the spermine-DNA complex, there was no broadening of the spermine spectrum and very weak positive nuclear overhauser effects were observed, indicating that the spermine still has a remarkably short rotational correlation time. Spermine induced no changes in the DNA spectrum beyond those found upon addition of other salts. Although spermine interacts with DNA with a binding constant of approximately 10(6) at the low ionic strength under which these experiments were performed, it appears that the nature of the complex and the lifetime of the ligand on the DNA are such that the mobility of the spermine molecule is effectively independent of that of the DNA molecule.