Taste for Chiral Guests: Investigating the Stereoselective Binding of Peptides to β-Cyclodextrins

Obtaining compounds of diastereomeric purity is extremely important in the field of biological and pharmaceutical industry, where amino acids and peptides are widely employed. In this work, we theoretically investigate the possibility of chiral separation of peptides by beta-cyclodextrins (beta-CDs), providing a description of the associated interaction mechanisms by means of molecular dynamics (MD) simulations. The formation of host/guest complexes by including a model peptide in the macrocycle cavity is analyzed and discussed. We consider the terminally blocked phenylalanine dipeptide (Ace-Phe-Nme), in the L- and n-configurations, to be involved in the host/guest recognition process. The CD-peptide free energies of binding for the two enantiomers are evaluated through a combined approach that assumes: (1) extracting a set of independent molecular structures from the MD simulation, (2) evaluating the interaction energies for the host/guest complexes by hybrid quantum mechanics/molecular mechanics (QM/MM) calculations carried out on each structure, for which we also compute, (3) the solvation energies through the Poisson-Boltzmann surface area method. We find that chiral discrimination by the CD macrocycle is of the order of 1 kcal/mol, which is comparable to experimental data for similar systems. According to our results, the Ace-(D)Phe-Nme isomer leads to a more stable complex with a beta-CD compared to the Ace-(L)Phe-Nme isomer. Nevertheless, we show that the chiral selectivity of beta-CDs may strongly depend on the secondary structure of larger peptides. Although the free energy differences are relatively small, the predicted selectivities can be rationalized in terms of host/guest hydrogen bonds and hydration effects. Indeed, the two enantiomers display different interaction modes with the cyclodextrin macrocavity and different mobility within the cavity. This finding suggests a new interpretation for the interactions that play a key role in chiral recognition, which may be exploited to design more efficient and selective chiral separations of peptides.