TABLE V 



Solubility of Thymine in the Presence of Interacting 

 Compounds at 25,5° " 



^ Data from Ts'o, Melvin and Olson, /. Am. Chem. Soc 85, 

 1289 (1963); reproduced with permission of the American 

 Chemical Society. 

 ^ Standard deviation. 



for such interactions between different com- 

 pounds have been calculated (Tables IV and V) 

 and the general conclusions can be summarized 

 as follows: 



Interactions between purine and purine are 

 stronger than the interactions of purine and 

 pyrimidine, which are in turn stronger than the 

 interactions of pyrimidine with pyrimidine. 



The cross interaction of the pyrimidine 

 nucleosides such as cytidine, thymidine and 

 uridine with purine or other purine nucleosides 

 can also be studied by nuclear magnetic reso- 

 nance. In contrast to the large concentration- 

 dependent chemical shifts previously reported 

 for the proton resonance of the purine or purine 

 nucleosides, the concentration dependent-chem- 

 ical shifts of the pyrimidine nucleosides them- 

 selves are negligible (5). It is because the 

 pyrimidine nucleosides are non-aromatic in 

 nature and therefore do not support ring cur- 

 rents as do the aromatic purine bases. There- 

 fore, the self-association of pyrimidine 

 nucleosides cannot be monitored by proton 

 magnetic resonance via the effect of the ring 

 current magnetic anisotropy. However, the 

 proton resonance of the pyrimidine nucleosides 

 was found to be greatly affected by the purine 

 due to cross interaction. Table VI summarizes 

 the gross purine effect upon the protons of the 



pyrimidine nucleosides. A more detailed pres- 

 entation of the data for the thymidine protons 

 is given in Fig, 2 (5). Marked upfield shifts are 

 noticed particularly for the base protons and 

 anomeric protons H-l'. This effect falls off 

 progressively as the proton distance from the 

 ring increases. The direction of the purine- 

 induced shifts plus their variation with distance 

 for the respective protons from the apparent 

 site of interaction suggests that the interaction 

 is that of vertical ring stacking of the pyrimidine 

 and purine bases. 



The analysis of NMR data on solution 

 properties of the monomers points the way for 

 further studies of nucleic acid by this technique. 

 For instance, an extensive NMR study of the 



MOLAL CONCENTRATION OF THYMIDINE' 

 0.1 02 03 0^<» 



■460r 



0.1 0.2 03 04 0.5 0.6 0.7 08 0.9 

 MOLAL CONCENTRATION OF PURINE •- 



Fig. 2. 



Chemical shift dependence of thymidine protons upon 



thymidine ( — a ) and upon purine concentration 



( — • — ) at 35° in D2O. Shifts measured from external 

 SDSS. Magnetic field increases from top to bottom along 

 ordinate. Spectra obtained at 60 Mc. (From Schweizer, 

 Chan and Ts'o, /. Am. Chem. Soc. 87, 5241, 1965; repro- 

 duced with permissionof the American Chemical Society.) 



188 



