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ABSTRACr The self-association of guanosine, cytidine, and adenosine and base pairing between guanosine, cytidine, adenosine, and uridine in dimethyl sulfoxide have been investigated by the variation of their IN NMR chemical shifts with concentration and temperature. Guanosine, cytidine, and adenosine all showed evidence of self-association by hydrogen bonding. In guanosine/cytidine mixtures, a hydrogen-bonded dimer is formed; however, no base pairing could be detected with adenosine/cytidine or adenosine/uridine mixtures. The structures of nucleic acids are featured by hydrogen bonds formed between purine and pyrimidine bases linking together polynucleotide strands. Several 1H NMR investigations have been made of base-pair interactions of the component nucleosides of the nucleic acid chains (1-4). These indicate that guanosine and cytidine form base pairs stabilized by three hydrogen bonds, as proposed in the structure of nucleic acids by Watson and Crick (5). Base pairing between adenosine and uridine or adenosine and thymidine is expected to be stabilized by only two hydrogen bonds (5). This weaker interaction has been detected by Shoup et al. (1) in dimethyl sulfoxide, but is more readily observable in dimethyl sulfoxide/benzene mixtures (2) or in chloroform (2). Furthermore, 1H NMR measurements have shown that guanosine and cytidine self-associate to form dimers in dimethyl sulfoxide (3). Self-association has also been found with adenosine, but not uridine (4).Recently, 15N NMR spectroscopy has been employed to study the general characteristics of nucleoside resonances (6, 7) and, more specifically, to investigate interactions between base pairs (7, 8). 15N NMR is expected to have an advantage for the latter purpose through being especially sensitive to hydrogen bonding at azine-type nitrogens (6). The early 15N NMR study of Hawkes et al. (7) involved only the uridine-adenosine interaction in dimethyl sulfoxide, and no effect was detected on the 15N chemical shift. However, a subsequent investigation (8) of a mixture of a substituted uridine and a substituted adenosine in deuteriochloroform showed a significant downfield shift (4.73 ppm), which was ascribed to a base-pair interaction.In the present investigation of hydrogen bonding in selfassociation and base-pair formation, the 15N chemical shifts of nucleosides alone and in mixtures were determined in dimethyl sulfoxide as a function of concentration and temperature. EXPERIMENTAL SECTIONThe nucleosides were commercial materials and were used without further purification. The 15N NMR spectra were recorded with a Bruker WH-180 spectrometer, operating at 18.25 MHz, with 1 M 2H15N03 in 2H20 or 1 M 1H'5N03 in 'H20 used as an external reference. Hexadeuteriodimethyl sulfoxide added to the solutions provided an internal deuterium fieldfrequency lock. Pulse widths of 60 ,usec and repetition rates of 20 sec were commonly used. The spectra were taken with proton decoupling, but with suppression of the nuclear Overhauser enhancement (6).RESULTS AND DISCUSSION Becaus...

It is well established that the C-C bonds of cyclopropane rings have, to a higher or lower degree, the physical and chemical characteristics of C-C double bonds, including more p-orbital character than expected for saturated C-C bonds.',' Although there is much spectroscopic evidence for resonance stabilization of excited states as the result of electron delocalization when a cyclopropane ring is directly attached to a 7~ system of bonds, less evidence is available for ground-state conjugative effects, such as can be obtained by NMR. The 13C data already do indicate that conjugation is important, and to round out the picture we have studied substituent-induced chemical shift changes (SCS) in the 13C NMR spectra of the cyclopropane ring carbons of 4-substituted cyclopropylbenzenes. It was expected that conjugative effects associated with both electron-attracting and electron-donating substituents could be evaluated by the SCS values.There are, however, some difficulties in the interpretation of SCS values. Thus, although data for C SCS of side-chain carbons have been extensively studied, the origins of the shift changes do not seem well understood. As one example, while Dhami and Stothers' found a linear correlation between SCS values for the p carbons of 4-substituted ethenylbenzenes with Hammett substituent ( a ) constants, the corresponding SCS figures for the a carbons did not correlate. Furthermore, although the simple qualitative 13