Aggregation of 1,3,7-trimethylxanthine with methylene blue in aqueous solution

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We have studied self-association of aromatic molecules of the thiazine dye methylene blue in aqueous solution, using a dimer model. We have determined the dimerization equilibrium constant for the dye molecules KD = 3900 ± 800 M−1 at T = 293 K. We have decomposed the experimental spectrum into dimer and monomer components. Using the ratio of the molar absorption coefficients for two absorption bands of the dimer spectrum, we obtained the “average” value of the angle between the electronic transition moments of the molecules in the dimers, α = 48°. We have studied heteroassociation of methylene blue (MB) and 1,3,7-trimethylxanthine (caffeine) molecules in aqueous solution. We have calculated the heteroassociation constant as 200 ± 34 M−1. We conclude that heteroassociation of methylene blue and caffeine molecules leads to a lower effective dye concentration in solution, which hypothetically may affect its biological activity. We have determined the values of the Gibbs free energy, the enthalpy, and the entropy for dimerization of methylene blue molecules: ΔG293 = −(20 ± 3) kJ/mol, ΔH = −(25 ± 9) kJ/mol, Δ S293 = −(17 ± 6) J/mol·K; and for methylene blue-caffeine heteroassociation: ΔG293 = −(13 ± 3) kJ/mol, ΔH = −(14 ± 10) kJ/mol, ΔS293 = −(2.4 ± 0.2) J/mol·K, respectively. We have shown that the methylene blue aggregates and the heteroassociates with caffeine are predominantly stabilized by dispersion interactions between the chromophore molecules in the associates.