Deamination of protonated amines to yield protonated imines

AbstractPrimary and secondary amines, when examined in atmospheric pressure chemical ionization, electrospray ionization, or chemical ionization, display protonated imines in their mass spectra. These products arise formally by nucleophilic substitution at the α-carbon with loss of both ammonia and molecular hydrogen. Collision-induced dissociation (CID) is used to characterize the product ions by comparison with authentic protonated imines. Gas-phase ion/molecule reactions of protonated amines with neutral amines also yield products that correspond to protonated imines (deamination and dehydrogenation), as well as providing simple deamination products. The reaction mechanism was investigated further by reacting the deamination product, the alkyl cation, with a neutral amine. The observed dehydrogenation of the nascent protonated secondary amine indicates that the reaction sequence is loss of ammonia followed by dehydrogenation even though the isolated protonated secondary amines did not undergo dehydrogenation upon CID. Formation of the deamination products in the protonated amine/amine reaction is competitive with proton-bound dimer formation. The proton-bound dimers do not yield deamination products under CID conditions in the ion trap or in experiments performed using a pentaquadrupole instrument. This demonstrates that the geometry of the proton-bound dimer, in which the α-carbons of the alkylamines are well separated [CαNHNCα], is an unsuitable entry point on the potential energy hypersurface for formation of the imine [CαNCα]. Isolation of the proton-bound dimers in the quadrupole ion trap is achieved with low efficiency and this characteristic can be used to distinguish them from their covalently bound isomers

Reaction, clustering and chiral recognition by electrospray ionization mass spectrometry

A highly efficient SN2 ion/molecule reaction was revealed in the course of atmospheric pressure ionization of amines. Intermolecular deamination resulted in the formation of protonated imines, which were confirmed by ion/molecule reactions, and by comparison with the dissociation behavior of synthesized authentic imines. Clustering of some simple inorganic salts and naturally occurring amino acids was investigated by electrospray ionization-quadruple ion trap mass spectrometry. Magic number clusters were identified and their structures were inferred. For sodium chloride, the magic numbers of the positive, doubly charged cluster series are attributed to three types of structures, and a correlation with the well known magic number clusters of the singly charged series was also observed. In addition, negatively charged clusters were also studied and the preferred structural type is related to the size of the cation. Ammonium chloride clusters show similar results to that of sodium chloride. Clustering of naturally occurring amino acids was surveyed and the capability of clustering was shown to be affected greatly by the side-chain functionalities. In particular, protonated clusters of the basic amino acid arginine showed some similarity to the sodium chloride in its clustering and a correlation between their magic numbers was observed. A striking observation was the formation of the protonated serine octamer which exhibits a strong chirality dependence. Experimental results and molecular mechanics calculations suggest that the homo-chiral cluster has a cylindrical shape in which four dimeric units are assembled. Possible implications for the evolution of homochirality are explored. Chiral recognition was achieved using the unimolecular dissociation kinetics of mass-selected transition metal ion bound amino acid complexes. The competitive dissociation of the trimeric complexes allows the application of the kinetic method and hence allows enantiomeric determination of amino acids