Enzyme catalysis plays a crucial role in all biochemical processes. Natural and artificial enzymes normally exhibit a high enantioselectivity toward chiral molecules as a consequence of shape-specific noncovalent attractive and repulsive intermolecular interactions. [1][2][3][4][5] An important step toward the elucidation of enzyme mechanisms requires a comprehensive kinetic study using simplified models under conditions, such as the gas phase, where the molecule/receptor interactions are not perturbed by medium effects. [6] Calixarenes lend themselves to this task since they are synthetic macrocycles endowed with a cavity-shaped architecture, which can be made asymmetric by the presence of chiral pendant groups. A peculiar feature of calixarene macrocycles is their nonplanar cyclophane structure, which may give rise to relatively stable stereoisomeric and conformational forms. A pertinent case is that of 2,8,14,20tetrakis(l-valinamido)[4]resorcinarene (1), which can display at least four stable stereoisomeric structures, that is, flattened cone (1 a L ), 1,2-alternate (1 b L ), chair 1 (1 c L ) (Figure 1), and chair 2. [7] Its stereoisomerism allows the tailoring of variously shaped cavities, which can be probed to evaluate their effect on ion or molecular recognition. [8,9] Herein, we report the first comparative gas-phase study along these lines. The intrinsic enantioselectivity of the 1 a L , 1 b L , and 1 c L diastereomeric structures was checked by introducing their proton-bonded complexes with the pure d and l enantiomers of some aromatic amino acids (A), such as phenylalanine (Phe), tyrosine (Tyr), and 3,4-dihydroxyphenylalanine (dopa), into the cell of a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) and measuring the exchange rate of the amino acid A with