In this work, the inclusion complexes of leucine as the guest and a series of cyclic peptides with four to nine alanine as building block (host molecules) were investigated by M062X/6-31 ? G (d) level of theory. The calculations predicted the stability of the enantiomers in the cavity of cyclic peptides and the possibility of distinction between the enantiomers for the used level of theory. The results indicated that the most stable aggregate is formed between CyclAla9 and R and S-leucine enantiomers and CyclALA4 have the least interaction with R and S-leucine enantiomers. Geometrical changes of leucine due to complexation and hydrogen bonding were considered. The NBO calculations for analyzing the nature of the intermolecular hydrogen bonds were performed. In another part of this work, the UV absorption spectra of complexes of R, and S-leucine with CyclAla9 calculated using ONIOM method in the gas phase and it was showed that the UV absorption spectroscopy could be used as a chirality discriminator tool in recognition of enantiomers in the gas phase.