The conformational analysis of protonated cyclo-[(S)-phenylalanyl-(S)-histidyl], denoted as cyclo-[(S)-Phe-(S)-His-H+], within a cavity of a new IRMOFs-phen (isoreticular metal-organic frameworks with a 2,7-dicarboxylate phenanthrene substituted bridges) porous material was performed with the AM1 quantum chemical method. Two forms of cyclo-[(S)-Phe-(S)-His-H+] were considered: the bound-form, where the peptide is chemically linked to the phenanthrene moiety and the unbound-form where the peptide is free within the cavity. The most probable conformers of cyclo-[(S)-Phe-(S)-His-H+] within the cavity have been compared to the conformers in gas phase and in water, simulated by the implicit solvent SM5.4 model. The preferred conformations within the IRMOF-phen cavity are the (g-, g+) and (t, g+) in contrast to the gas and aqueous phases. The environment within the cavity is also relevant, since the substitution of a hydrogen atom by CH3 (IRMOF-phen-CH3) or Br (IRMOF-phen-Br) lead to new IRMOFs-phen that changes drastically the conformer populations, as well as the adsorption site of the dipeptide. These results have important implications in controlling the stereoselectivity of reactions in the presence of chiral inductors. Indeed, this is the first time that a folded conformation of cyclo-[(S)-Phe-(S)-His-H+] was found, as well as an stable cyclo-[(S)-Phe-(S)-His-H+]-benzaldehyde complex was obtained using the ONIOM(PBE1:AM1) method. This complex is compatible with the proposed structure for the transition state of the hydrocyanation of benzaldehyde that explains the observed enantioselectivity.
enantioselectivity conformation; quantum chemistry; confined species
