Gas-phase structure and relative stability of proton-bound homo- and heterochiral clusters of tetra-amide macrocycles with amines

The structure, stability, and CID pattern of proton-bound homochiral and heterochiral complexes, formed in the gas phase by the combination of two molecules of a chiral macrocyclic tetra-amide and an amine B, i.e. CH3NH2, (CH3)2NH, or (S)-(–)-1-phenylethylamine, have been examined by ESI-ITMS-CID mass spectrometry. With B = CH3NH2, the CID pattern is characterized by the predominant loss of B, accompanied by a much less extensive release of one tetra-amide molecule. With (S)-(–)-1-phenylethylamine, loss of a tetra-amide molecule efficiently competes with loss of B. Finally, with (CH3)2NH, loss of a tetra-amide molecule predominates over loss of B. No appreciable isotope and chiral guest configuration effects have been detected in the fragmentation of the homochiral complexes. A distinct configurational effect has been appreciated in the CID of the homo- and the heterochiral complexes with all amines used. The results of this study have been discussed in the light of semi-empirical computational evidence. The differences in the CID patterns of the homo- and the heterochiral complexes have been rationalized in terms of structural factors and of the basicity of amine B.