The stereoselectivity of the reaction between (R)-(-)-2-butylamine and the diastereomeric proton-bound complexes of (+)-catharanthine (C) or (-)-vindoline (V) with some chiral amido[4]resorcinarenes has been investigated in the gas phase by ESI-FT-ICR-MS. The reaction stereoselectivity (0.56 < k(homo)/k(hetero) < 16.9) is found to depend critically on the functional groups present in the chiral pendants of the hosts. Rationalisation of the kinetic results is based on careful computational and spectroscopic studies of the most stable conformations of (+)-catharanthine and its protonated form in the isolated state and in water, as well as in a representative host structure. The emerging picture points to the relevant diastereomeric proton-bound complexes as quasi-degenerate, thus suggesting that their stereoselectivity in the guest exchange reaction is mostly due to kinetic factors. The results of this study may represent a starting point for a deeper comprehension of the intrinsic factors that endow these molecules, and their dimeric forms, with their biochemical properties.
The displacement processes of several guests, incorporated in a calixarene host system, were investigated in the gas phase by electrospray ionization-Fourier transform-ion cyclotron resonance (ESI-FT-ICR) mass spectrometry. The complexes resulting from a resorcin[4]arene host with ammonia and sec-butylamine guests were isolated in an ICR-cell, separately using both states of the photoswitch as well as two reference systems for the open and closed forms of the photoswitchable host. The isolated complexes were forced to exchange the guest by using methylamine, ethylamine and sec-butylamine, resulting in different reaction rates for all the measured systems. Especially, the reaction rates of both states of the photoswitch are dependent on the provided guest. Potential side effects like proton exchanges were examined by an H/D-exchange experiment. The results were investigated and supported by quantum chemical calculations (DFT).
The gas-phase enantioselectivity of cone N-linked peptidoresorc[4]arenes (generally symbolized as M) toward the homologue dipeptides (generally symbolized as A) has been evaluated by measuring the kinetics of the A release from the diastereomeric [M x H x A](+) complexes induced by (R)-(-)-2-butylamine (B). In most cases investigated, the heterochiral [M x H x A](+) complexes, namely those wherein the configuration of the A guest is opposite to that of the host M pendants, react faster (up to 5 times) than the homochiral analogues, wherein guest A guest has the same configuration of the host M pendants. The kinetic results, discussed in the light of previous MS and NMR evidence, indicate that both the efficiency and the enantioselectivity of the guest exchange reaction depend essentially on the structure and the relative stability of the diastereomeric [M x H x A](+) complexes. These, in turn, depend on the functional groups and the configuration of both the guest and the host pendants. The absence of any significant effects of the B configuration indicates that, in all systems investigated, the dipeptide guest A is predominantly located outside the host chiral cavity.
Electrospray ionization (ESI) of dilute solutions of 1,1=-bi-2-naphthol (BINOL) and iron(II) or iron(III) sulfate in methanol/water allows the generation of monocationic complexes of iron and deprotonated BINOL ligands with additional methanol molecules in the coordination sphere, and the types of complexes formed can be controlled by the valence of the iron precursors used in ESI. INOL (1,1=-bi-2-naphthol) is among the most important chiral ligands used in metal-mediated asymmetric synthesis [1]. Despite a huge base of knowledge, however, gas-phase models that would permit a detailed understanding of enantioselective reactions at a molecular level are still relatively scarce [2][3][4]. In the course of ongoing studies of possible systems that allow probing of chiral reactions with mass spectrometric means [3,5,6], we here report on the generation of BINOL complexes of iron(II) and iron(III) by electrospray ionization (ESI) and on the fragmentation behavior of the ions formed to provide a foundation for future studies about possible enantioselective effects in the ion/molecule reactions of these gaseous complexes. ExperimentalThe experiments were performed using a VG BIO-Q mass spectrometer, which consists of an ESI source combined with a tandem mass spectrometer of QHQ configuration (Q stands for quadrupole and H for hexapole) as described in detail elsewhere [7]. In the present experiments, millimolar solutions of BINOL and iron(II) or iron(III) sulfate, respectively, in methanol/water (1:1) were introduced through a fused-silica capillary to the ESI source by a syringe pump (ϳ5 L/min). Nitrogen was used as nebulizing and drying gas at a source temperature of about 100°C. Maximal yields of the desired ions (see following text) were achieved by adjusting the cone voltage to between 30 and 90 V, respectively. For collision-induced dissociation (CID) at low collision energies, the ions of interest were mass-selected using Q1, interacted with xenon as a collision gas in the hexapole H at variable collision energies (E lab ϭ 0 -20 eV), while scanning Q2 to monitor the ionic products. In most cases, this pressure of xenon (typically 2 ϫ 10 Ϫ4 mbar) corresponds to single-collision conditions, but for some of the more weakly bound methanol complexes, the CID cross sections are very large, such that a considerable number of multiple collisions take place [8]. Given that the primary aim of the CID experiments reported here is the illustration of trends in relative binding energies, this aspect is not pursued any further. The same holds true for the internal energy content of the ions, which somewhat depends on size of the complexes as well as the cone voltage used, but poses no particular obstacles for the purpose of this work.As pointed out previously, the VG Bio-Q does not allow one to directly extract quantitative threshold information from CID experiments due to several limitations of the commercial instrument [7]. For weakly
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.