Comparison of the acid-base properties of 5-and 6-uracilmethylphosphonate (5Umpa(2-) and 6Umpa(2-)) and some related compounds. Evidence for intramolecular hydrogen-bond formation in aqueous solution between (N1)H and the phosphonate group of 6Umpa(2-)

The acidity constants of 5-uracilmethylphosphonic acid, H-2(5Umpa), and 6-u racilmethylphosphonic acid, H-2(6Umpa), double dagger were determined by po tentiometric pH titrations in aqueous solution (25 degreesC; I = 0.1 M, NaN O3). Comparison of these constants with those of related uracil derivatives (partly taken from the literature) allows the conclusion that an intramole cular hydrogen bond is formed between (N1)H and the phosphonate group of 6U mpa(2-); the formation degree of this hydrogen-bonded isomer is estimated t o be 86 +/- 7%. The X-ray crystal structure analysis of H-2(6Umpa) is repor ted but this solid state structure is dominated by intermolecular hydrogen bonds. In the context of the properties of 5Umpa(2-) and 6Umpa(2-) those of uracil are also discussed and from various comparisons of acidity constant s it is concluded that deprotonation of uracil may occur at (N3)H as well a s at (N1)H but that the (N3)-deprotonated species dominates with about 80% in aqueous solution at 25 degreesC and Y = 0.1 Ni (Na+). The search for oth er examples of uracil derivatives which allow hydrogen-bond formation in aq ueous solution has led to orotic acid (= 6-uracilcarboxylic acid; [H(6Urca) ]) and 5-uracilcarboxylic acid [H(5Urca)]; based on acidity constant compar isons it is concluded that in aqueous solution (25 degreesC; I = 0.1 M, KCl ) H(5Urca) exists to a bout 92 +/- 10% as a species with a hydrogen bond be tween (C5)COOH and (C4)O, and 6Urca(-) to about 95 +/- 5% as a species with a hydrogen bond between (C6)COO- and (N1)H. The importance of intramolecul ar hydrogen-bond formation to the acid-base properties of compounds in solu tion is briefly emphasized.