Interaction of Caffeine with L-Tryptophan: Study by 1H Nuclear Magnetic Resonance Spectroscopy

The interaction of caffeine with L-tryptophan was investigated in aqueous solution adjusted to an ionic strength of 0.20, at pH 6.2, by using 1H NMR spectroscopy. Caffeine interacted with L-tryptophan at a molar ratio of 1:1 by parallel stacking. The thermodynamic parameters obtained for the complexation were as follows: ΔGo = −1.70 kcal/mol at 25 oC(K = 17.6M−1), ΔHo = −3.7 kcal/mol, ΔSo = −6.7 eu. It was found that the driving force for parallel stacking is a combination of electrostatic, polarization, and dispersion interactions, and the contribution of hydrophobic bonding is not large. A possible stacking mode is postulated based on the complexation shifts and π-electron charge distributions of caffeine and L-tryptophan.