Influence of temperature and ionization on self-association of theophylline in aqueous solution. Studies by proton nuclear magnetic resonance spectroscopy.

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Abstract
Thermodynamic parameters for self-association of theophylline in aqueous solution adjusted to ionic strength 0.20 were determined by proton nuclear magnetic resonance spectroscopy. The results obtained at pD 5.8 were as follows: .DELTA.G.degree. = -0.95 kcal/mol at 30.degree. C (K = 4.8 M-1), .DELTA.H.degree. = -6.3 kcal/mol, and .DELTA.S.degree. = -18 e.u. It was found that the driving force for self association involves electrostatic, polarization, and dispersion interactions, and the contribution of hydrophobic bonding is small. A possible mode of self-association of theophylline is postulated. It was found that self-association of theophylline occurs more easily in the neutral state than in the ionized state.

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