Discrimination of Aminobiphenyl Isomers in the Gas Phase and Investigation of Their Complex Conformations.

The analysis of positional isomers is of great significance because their different chemical properties but similar structures make separation difficult. In this work, a simple method for simultaneously discriminating three positional isomers of 2-aminobiphenyl (2-ABP), 3-ABP, and 4-ABP was studied by ion mobility spectrometry (IMS) and quantum mechanical calculations at the molecular level. In the experiments, three ABP isomers were mixed with α-, ß-, and γ-cyclodextrins (CD), and the IMS results show that the three ABP isomers were clearly recognized by the formed complex of [α-CD + ABP + H]+ via measuring their IMS, in which the different ion mobilities of 1.515, 1.544, 1.585 V·s·com-2 with the collision cross sections (CCS) of 307.3, 312.5, 320.8 Å2 were obtained for [α-CD + 2-ABP + H]+, [α-CD + 3-ABP + H]+, and [α-CD + 4-ABP + H]+, respectively. Collision induced dissociation analysis of the three [α-CD + ABP + H]+ isomer complexes were further studied, indicating that the same fragmentation process required different collisional energies, and the greater the CCS for the [α-CD + ABP + H]+ with looser structure and the smaller energy required. Besides, the favorable conformation and the CCS value of the different [CD + ABP + H]+ isomer complexes were measured via quantum mechanical calculations to detail their intermolecular interactions. It revealed that the intermolecular binding between 2-ABP and α-CD is different from that of 3- and 4-ABP, resulting in different molecular conformations and CCS, and the interaction modes of ABP with ß-CD are similar to that with γ-CD, which are very consistent with the experimental observations. Finally, relative quantification of the method was performed, and satisfactory linearity with correlation coefficients (R2) greater than 0.99 was obtained. This method for isomer discrimination and conformation analysis possesses the advantages of simplicity, sensitivity, cost-effectiveness, and as such it may be widely applied in chemistry and pharmaceutical sciences.