Interaction of L-tryptophan with α-cyclodextrin:: Studies with calorimetry and proton nuclear magnetic resonance spectroscopy

The interaction of L-tryptophan with alpha -cyclodextrin was investigated in a 0.1 M phosphate buffer at pH 7.4 with a LKB 2277 microcalorimeter, using flow mixed mode at 25 degreesC. The thermodynamic parameters for inclusion complex formation obtained are as follows; DeltaG(o) = -7.03 kJ/mol (K = 17.0), DeltaH(o) = -9.50 kJ/mol, DeltaS(o) = -8.3 J/mol K. The driving force for inclusion complex formation was considered to be mainly van der Waals-London dispersion force, and the contribution of hydrogen bonding was secondary in importance. Also, from the measurements of the proton nuclear magnetic resonance spectra and the model building with Corey-Pauling-Koltum atomic models, the probable structures of the complex, together with conformational change of L-tryptophan by complexation, were determined. (C) 2001 Wiley-Liss, Inc.