Comparison of the acid–base properties of 5- and 6-uracilmethylphosphonate (5Umpa2− and 6Umpa2−) and some related compounds.        Evidence for intramolecular hydrogen-bond formation in aqueous solution between (N1)H and the phosphonate group of 6Umpa2−

Cristóbal F. Moreno-Luque; Eva Freisinger; Burkhard Costisella; Rolf Griesser; Justyn Ochocki; Bernhard Lippert; Helmut Sigel

doi:10.1039/B101078F

Comparison of the acid–base properties of 5- and 6-uracilmethylphosphonate (5Umpa²⁻ and 6Umpa²⁻) and some related compounds.Evidence for intramolecular hydrogen-bond formation in aqueous solution between (N1)H and the phosphonate group of 6Umpa²⁻

Cristóbal F. Moreno-Luque,^a Eva Freisinger,†^b Burkhard Costisella,^b Rolf Griesser,^a Justyn Ochocki,^c Bernhard Lippert*^b and Helmut Sigel*^a

Author affiliations

* Corresponding authors

^a Institute of Inorganic Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland
E-mail: Helmut.Sigel@unibas.ch

^b Department of Chemistry, University of Dortmund, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany
E-mail: Lippert@pop.uni-dortmund.de

^c Institute of Chemistry, Medical University, Muszyńskiego 1, PL-90-151 Lódź, Poland
E-mail: Ochocki@ich.pharm.am.lodz.pl

Abstract

The acidity constants of 5-uracilmethylphosphonic acid, H₂(5Umpa), and 6-uracilmethylphosphonic acid, H₂(6Umpa), were determined by potentiometric pH titrations in aqueous solution (25 °C; I = 0.1 M, NaNO₃). Comparison of these constants with those of related uracil derivatives (partly taken from the literature) allows the conclusion that an intramolecular hydrogen bond is formed between (N1)H and the phosphonate group of 6Umpa²⁻; the formation degree of this hydrogen-bonded isomer is estimated to be 86 ± 7%. The X-ray crystal structure analysis of H₂(6Umpa) is reported but this solid state structure is dominated by intermolecular hydrogen bonds. In the context of the properties of 5Umpa²⁻ and 6Umpa²⁻ those of uracil are also discussed and from various comparisons of acidity constants it is concluded that deprotonation of uracil may occur at (N3)H as well as at (N1)H but that the (N3)-deprotonated species dominates with about 80% in aqueous solution at 25 °C and I = 0.1 M (Na⁺). The search for other examples of uracil derivatives which allow hydrogen-bond formation in aqueous solution has led to orotic acid (= 6-uracilcarboxylic acid; [H(6Urca)]) and 5-uracilcarboxylic acid [H(5Urca)]; based on acidity constant comparisons it is concluded that in aqueous solution (25 °C; I = 0.1 M, KCl) H(5Urca) exists to about 92 ± 10% as a species with a hydrogen bond between (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 intramolecular hydrogen-bond formation to the acid–base properties of compounds in solution is briefly emphasized.

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DOI: https://doi.org/10.1039/B101078F
Article type: Paper
Submitted: 31 Jan 2001
Accepted: 19 Jul 2001
First published: 10 Sep 2001

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J. Chem. Soc., Perkin Trans. 2, 2001, 2005-2011

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Comparison of the acid–base properties of 5- and 6-uracilmethylphosphonate (5Umpa²⁻ and 6Umpa²⁻) and some related compounds.

Evidence for intramolecular hydrogen-bond formation in aqueous solution between (N1)H and the phosphonate group of 6Umpa²⁻

C. F. Moreno-Luque, E. Freisinger, B. Costisella, R. Griesser, J. Ochocki, B. Lippert and H. Sigel, J. Chem. Soc., Perkin Trans. 2, 2001, 2005 DOI: 10.1039/B101078F

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Journal of the Chemical Society, Perkin Transactions 2