TABLE I 

 Molal Osmotic Coefficients^ of Various Solutes Determined In Water at 25°'' 



'Data from Ts'o, Melvln and Olson, /. Am. aem. Soc. 85, 1289 (1963) and Ts'o and Chan, 

 ibid. 86, 4176 (1964); reproduced with permission of the American Chemical Society. 

 These are fitted osmotic coefficients computed from the experimental values. 



numerical integration on the fitted polynomials 

 and related molal concentration to ^ (Table II). 

 The data clearly indicated that the properties of 

 these bases and nucleosides in solution are far 

 from ideal. Values of both osmotic coefficients 

 and activity coefficients are much below unity. 

 These results establish the concept that purine 

 and pyrimidine nucleosides do interact exten- 

 sively in aqueous solution. Recently we have ex- 

 tended this type of measurement to other less 

 soluble purine nucleosides. The osmotic coef- 

 ficients at 25° at a concentration of 0. 1 molal in 

 water of 2'-methyladenosine is 0.723, of 2'- 

 deoxyadenosine, 0.668, and of N^-methyladeno- 

 sine, 0.548. Evidently, adenine nucleosides do 

 associate in water even more extensively than 

 purine and to about the same degree as the 

 6-methylpurine. 



After further analysis for their congruence 

 to different models for multiple equilibria, the 

 thermodynamic data were found to be incom- 

 patible with the model which assumes that only 

 dimers are formed. Thus, the degree of associa- 

 tion of these compounds may go beyond the 

 dimers stage to a higher degree of polymeriza- 

 tion. Most of the results are consistent with the 

 model which assumes that the association 

 process continues through many successive 

 steps (at least above five steps) with the same 

 equilibrium constant. Comparison of the equilib- 

 rium constant and thus the standard free energy 

 changes is given in Table III (1, 2). This table 

 shows that the tendency of purine to associate 

 is much greater than that of pyrimidine nucleo- 

 sides, which in turn is greater than that of urea. 



Now, what isthemodeof association of these 



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