The interaction of caffeine and theophylline with purine and pyrimidine was studied in deuterium oxide at 35°C by measuring the concentration‐dependent selective changes in the chemical shifts of the N‐methyl protons of the xanthine derivatives. Using a competitive dimer model, the equilibrium constants give a decreasing tendency for hetero‐association within the series caffeine‐purine (2.97 ± 0.15 1 mol−1) > theophylline‐purine (2.44 ± 0.10 1 mol−1) > caffeine‐pyrimidine (1.18 ± 0.20 1 mol−1) > theophylline‐pyrimidine (0.70 ± 0.08 1 mol−1), and the upfield dimer shifts suggest a plane‐to‐plane arrangement.
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Aradi et al. (1989) studied this question.
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