A multicomponent self-diffusion NMR study of aggregation of nucleotides, nucleosides, nucleic acid bases and some derivatives in aqueous solution with divalent metal ions added.
The self-aggregation of the mononucleotides AMP, CMP, and UMP with Mg2+ added (nucleotide concentration = Mg2+ concentration) up to 0.4 molal or to their solubility limit in 2H2O has been monitored through self-diffusion measurements, using the Fourier transform NMR pulsed-gradient spin-echo multicomponent-self-diffusion technique. Also, purine, cytidine, uridine, purine with Mg2+ added and both cytidine and uridine with Mg2+, Zn2+ or Cd2+ added, were studied in the same way. The experimental data were fitted to two different aggregation models. For the mononucleotides with Mg2+ added a cooperative indefinite aggregation model, where the first (dimerization) aggregation constant is a magnitude lower than those for the higher aggregation step gives the best agreement between simulations and experiment. Typical values are 0.3 and 12 kg mol(-1), respectively. The latter value is about twice that found for the uncomplexed nucleotides. Also, purine and the nucleosides, cytidine and uridine, with divalent metal ions added fit best with this model. The degree of aggregation is increased upon metal ion addition, as previously shown for the mononucleotides. For purine, cytidine and uridine without metal ions added an 'isodesmic', indefinite aggregation model, with the aggregation constant for each step equal, fits the data as well. Here the application of the 'semi-isodesmic' model results in a higher first (dimerization) aggregation constant than is found for the nucleotides. The typical value is 2 kg mol(-1). In this case, the evaluated aggregation constants for the higher step become only about twice as large as those of the first step. The same measurements on isopropylcytidine, isopropyluridine and theophylline-7-acetic acid in water show that these three compounds aggregate to the same extent as the nucleosides, cytidine and uridine. Pyrimidine diffusion data reveal no aggregation at all; the application of either model results in essentially zero aggregation constants.
Stokkeland
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Institute of Physical Chemistry, Uppsala University, Box 532, S-751 21 Uppsala, Sweden.
Stilbs
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