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2',3'-O-Isopropylidene-5-methyl(15N_2)[O^2,O^4-17O_2]uridine (= 2',3'-O-isopropylidene (15 N_2)[O^2,O^4-17O_2]-ribosylthymine; 1) was analyzed by 15N- and 17O-NMR spectroscopy. The 15N and 17O chemical shifts revealed, in the absence and presence of unlabelled 2',3'-O-isopropylideneadenosine (2), the formation of thymine-thymine and thymine-adenine base pairs in CHCl3. As expected, cyclic complexes stabilized by two H-bonds occurred at low temperatures, but at elevated temperatures, the data suggest that open complexes involving only one H-bond prevailed. The 17O-NMR data showed the cyclic thymine-adenine pair in a reverse base pair geometry. The open base pair involved contacts to the urea-derived carbonyl O-atom of thymine. The thermodynamics of complex formation of the cyclic and open forms in both home, and hetero pairs were calculated from the temperature and concentration dependence of the 15N-NMR data using a new method. It involves a fitting procedure onto the experimental isotherms using a theoretically derived function with the standard Gibbs free energy as a parameter to be optimized. Delta Hº and Delta Sº were derived from a linear regression of Delta Gº(T) vs. T. The fitting procedure circumvents the baseline problem and could be automated and used to calculate correct thermodynamics from UV-monitored melting curves of oligonucleotides. Since titrations are not involved, this dilution method should also be a useful alternative for stability studies of supramolecular complexes in H2O and in organic solvents.
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