Kinetics for exchange of imino protons in deoxyribonucleic acid, ribonucleic acid, and hybrid oligonucleotide helixes

Abstract

The lifetimes for opening of individual base pairs in a DNA (dCA5G + dCT5G), an RNA (rCA5G + rCU5G), and a hybrid DNA-RNA (rCA5G + dCT5G) helix were measured by 1H NMR. The lifetimes were obtained by saturation recovery experiments performed on the H-bonding imino protons of the Watson-Crick base pairs. In these oligonucleotide helices the observed relaxation rates were dominated by exchange with water, with the magnetic spin-lattice relaxation time of the imino protons possibly being important only at the lowest temperatures in the DNA helix. Three interior base pairs in the DNA heptamer dCA5G + dCT5G were in the open-limited region, which means that these imino protons exchange every time the base pair opens. The lifetimes of the terminal G.cntdot.C base pairs in the DNA helix are much shorter than the interior A.cntdot.T base pairs. The pH dependence of the terminal base pairs indicated that the ends of the helix open and close many times before exchange of the imino protons with water takes place. The temperature dependence of the lifetimes of the interior A.cntdot.T imino protons in the DNA helix showed that these protons exchange only when the double helix has dissociated into single strands. Thus, these lifetimes measure the rate for dissociation of the double helix. The activation energy for this process was 47 kcal/mol. Comparison of the lifetimes of the interior protons in the DNA, RNA and hybrid helices showed that the rates of dissociation of the RNA and hybrid helices are very similar at 5.degree. C, whereas the rate for the DNA helix was .apprx. 1 order of magnitude smaller than that for the other 2 helices. The reasons for the differences in the kinetics of the 3 helices are discussed, as are the general dynamics of oligonucleotide helices in solution.