Chiral recognition of zinc(II) ion complexes composed of bicyclo[3.3.0] octane-2,6-diol and s-naproxen probed by collisional-induced dissociation

Abstract

Chiral recognition of racemic bicyclo[3.3.0] octane-2,6-diol(B) was achieved in the gas phase using s-Naproxen(A) as reference, using the kinetics of competitive unimolecule dissociation of tetrameric zinc(II)-bound complexes by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer(ESI-FTMS). As undergoing a mild competitive collision-induced dissociation(CID) experiment with a constant pressure argon gas introduced by leak valve, the tetrameric cluster ion [A 2B 2Z n(II)-H] + forms only two trimeric ions and R chiral is subsequently obtained in the kinetic method. Further studies obtained the difference of Gibbs free energy of [ABZ n(II)-H] +(ΔΔ G ABZn(II)-H] + ) by dissociating [A 2BZ n(II)-H] +, resulting two fragment ions [ABZ n(II)-H] + and [A 2Z n(II)-H] +, which can be established to a linear relationship between ΔΔ G [ABZn(II)-H] + and R chiral ′ basing on the kinetic method. The value of R chiral ′ suggested that ΔΔ G [ABZn(II)-H] + could be regarded as zero. Meanwhile, dissociation of [AB 2Z n(II)-H] + generated only one daughter ion [ABZ n(II)-H] + in a stable pressure. Thus, a linear relationship was established between the difference of Gibbs free energy of [AB 2Z n(II)-H] +(ΔΔ G [AB 2Zn(II)-H] + ) and R chiral ″ if the ΔΔ G [ABZn(II)-H] + can be negligible. Because there is also a linear relationship of R chiral in the tetrameric ion [A 2B 2Z n(II)-H] + and the Gibbs energy difference of trimeric cluster ion [A 2BZ n +(II)-H](ΔΔ G [A 2BZn(II)-H] + ) plus that of [AB 2Z n(II)-H] +, ΔΔ G [A 2BZ n(II)-H]+] is easy to be calculated in the dissociation process of tetrameric ion. Stable of R chiral , R chiral ′ and R chiral ″ under different pressures show T eff does not affect the chiral recognition of cluster ions in the condition selected. If an only-one-daughter-ion fragment process of [A 2BZ n(II)-H] + was existed, R chiral ‴ relating to this dissociation would be calculated just like R chiral ″ of [AB 2Z n(II)-H] + does. Conclusion was obtained that [A 2BZ n(II)-H] + makes more contribution to chiral recognition of tetrameric ion measured by kinetic method than [AB 2Z n(II)-H] + does as R chiral ‴ and R chiral ″ were applied as index to evaluate the Gibbs free energy difference of these two trimeric cluster ions. Further discussion shows that steric interactions and π-π stacking interactions are the major factors responsible for the observed efficient chiral recognition in this system.