Chiral recognition of d- and l-amino acids is achieved and mixtures of enantiomers quantified in
the gas phase, using the kinetics of competitive unimolecular fragmentations of trimeric Cu(II)-bound complexes.
Singly charged copper(II)−amino acid cluster ions [CuII(A)(ref*)2−H]+ (A = amino acid; ref* = chiral reference
ligand, selected from among the natural α-amino acids) undergo competitive collision-induced dissociation
(CID) in a quadrupole ion trap to form the dimeric complexes [CuII(A)(ref*)−H]+ and [CuII(ref*)2−H]+. The
abundance ratio of these fragment ions depends strongly on the stereochemistry of the ligands in the precursor
[CuII(A)(ref*)2−H]+ complex ion and specifically on the chirality of the analyte amino acid. The chiral
selectivity, the ratio of the two fragment ion abundances for the complex containing one enantiomer of analyte
expressed relative to that for the fragments of the corresponding complex containing the other enantiomer,
ranges from 0.47 to 11. An energy quantity, Δ(ΔCuIIBDE), is predicted and shown to serve as a thermochemical
indicator of chiral discrimination; its value is calculated from the fragment ion abundance ratios using the
kinetic method of estimating thermochemical quantities from the kinetics of cluster ion dissociation. Large
chiral distinctions are observed with all of the natural chiral α-amino acids, except cysteine and arginine, by
appropriate choice of the reference ligand. The Δ(ΔCuIIBDE) values range from −2.2 to 6.9 kJ/mol. Amino
acids with aromatic substituents display the largest chiral distinction, which is consistent with ligand exchange
chromatographic results for analogous systems. The structures of the fragment Cu(II) complexes are discussed
in the light of the CID behavior of related compounds. The interactions within these ions that might contribute
to chiral recognition are rationalized to account for the observed chiral effects. The sensitive nature of the
methodology and the linear relationship between the logarithm of the fragment ion abundance ratio and the
optical purity, which is intrinsic to the kinetic method, allows mixtures to be analyzed for small enantiomeric
excess (ee) by simply recording ratios of fragment ion abundances in a mass spectrum.
A new Fourier transform ion cyclotron resonance (FTICR) cell based on completely new principles of formation of the effective electric potential distribution in Penning type traps, Boldin and Nikolaev (Proceedings of the 58th ASMS Conference, 2010), Boldin and Nikolaev (Rapid Commun Mass Spectrom 25:122-126, 2011) is constructed and tested experimentally. Its operation is based on the concept of electric potential space-averaging via charged particle cyclotron motion. Such an averaging process permits an effective electric force distribution in the entire volume of a cylindrical Penning trap to be equal to its distribution in the field created by hyperbolic electrodes in an ideal Penning trap. The excitation and detection electrodes of this new cell are shaped for generating a quadratic dependence on axial coordinates of an averaged (along cyclotron motion orbit) electric potential at any radius of the cyclotron motion. These electrodes together with the trapping segments form a cylindrical surface like in a conventional cylindrical cell. In excitation mode this cell being elongated behaves almost like an open cylindrical cell of the same length. It is more effective in ion motion harmonization at larger cyclotron radii than a Gabrielse et al.-type (Int J Mass Spectrom Ion Processes 88:319-332, 1989) cylindrical cell with four compensation sections. A mass resolving power of more than twenty millions of reserpine (m/z 609) and more than one million of highly charged BSA molecular ions (m/z 1357) has been obtained in a 7T magnetic field.
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