Nuclear magnetic resonance spectroscopic analysis of the selective complexation of the cis and trans isomers of phenylalanylproline by β-cyclodextrin

Nuclear magnetic resonance (NMR) spectroscopy was used to examine the effect of β-cyclodextrin inclusion complex formation on the cis-trans equilibrium constant of phenylalanylproline, Phe-Pro. The pKa values of the ionizable groups of Phe-Pro have been determined in solutions containing equimolar ratios of the peptide and β-cyclodextrin. Comparison of these values with acidity constants of the free peptide suggest a hydrogen bond between β-cyclodextrin and the cis isomer car☐ylate group that is not observed for the trans isomer. Diffusion coefficients for the cis and trans isomers of Phe-Pro in aqueous and in β-cyclodextrin solution have been measured using pulsed-field gradient NMR spectroscopy. Diffusion ordered NMR spectroscopy (DOSY) was used to selectively measure the diffusion coefficients of β-cyclodextrin and both isomers of Phe-Pro. The preferential formation of the β-cyclodextrin inclusion complex with cis Phe-Pro was observed in the DOSY spectra and formation constants for this complex calculated.