Chiroptische Untersuchungen des Photodetachments elektrogesprayter Anionen
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Philipps-Universität Marburg
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Abstract
In the scope of this work a new chirality spectrometer was built for chiroptical studies of electrosprayed anions. To this end, an electrospray ionization (ESI) source was coupled with a linear time-of-flight mass spectrometer (TOF-MS) and an electron projection unit. In the center of the chirality spectrometer ion packages can interact with elliptical polarized nanosecond laser pulses with a wavelength of 355 nm. Created and depleted charge carriers can be detected mass-resolved in case of ions or direction-resolved in case of photoelectrons by two individual detector units. This allows the determination of the circular dichroism in total ion yields (PICD) and the circular dichroism in the photoelectron angular distribution (PECD).
The viability of the presented setup for chirality analysis of electrosprayed anions was demonstrated in proof of principle experiments on amino acids. 3,4-Dihydroxyphenylalanine (DOPA) and glutamic acid (GLU) were chosen as analytes. In case of DOPA the single photon detachment of monoanions was investigated. For GLU the single photon detachment of a distribution of solvated dianions was studied. The latter can be followed by decarboxylation and autodetachment processes. The PICD was determined in different ways using the depletion of the precursor ions, the total photoelectron yields and – if applicable – the yields of created radical anions. Despite standard errors in the permille range, no distinction between enantiomers was possible due to undistinguishable PICD values of the L- and D-amino acids. Yet, in a number of cases non-zero PICD values were caused by an instrument-based anisotropy. Different normalization approaches were compared to take instabilities of the precursor ion density and the mean laser power into account. Yield differences of the charge carriers of interest caused by photodetachment were normalized to the simultaneously measured background ion yield and/or the mean laser power. In some cases, a decrease of standard errors was apparent, but no reliable compensation of systematic errors could be observed. The PECD of anionic precursor distributions created by electrospray ionization of the amino acids DOPA and GLU was determined by evaluation of the photoelectrons emitted in forward and backward direction. PECD values of −4.64 % ± 0.24 % and 4.36 % ± 0.25 % were obtained for L- and D-DOPA, respectively. L- and D-GLU exhibit inverted signs and less pronounced asymmetries in the photoelectron angular distribution, as can be seen in their PECD values of 3.62 % ± 0.15 % and −3.79 % ± 0.14 %, respectively. Using two different amino acids as examples, the PECD was detected in the photodetachment for the first time. Asymmetries in the photoelectron angular distribution of several percent reveal the capability of the new ESI-PECD method for chirality analysis.
Chiroptical studies of the peptide gramicidin (GRA) were conducted to demonstrate the extension of the scope of the PECD technique by the utilization of an ESI source. Single photon detachment of doubly deprotonated gramicidin was enforced by left and right circular polarized laser pulses. PECD and PICD were determined using the photoelectron yields in forward as well as backward direction and their sum, respectively. Objective of the experiments on structurally flexible gramicidin was the affirmation of the conformational sensitivity of the PECD for photodetachment processes. To this end, the influence of the solvent history of the precursor ions on the PECD value was investigated. GRA solutions made from pure methanol and water/methanol mixtures were analyzed. Despite having different conformational distributions in the liquid phase according to conventional CD measurements, comparable PECD and PICD values were obtained in the gas phase. This result indicates similar gas phase conformations and can be interpreted in the light of structural equilibration within the ion trap and a possible loss of non-covalent bonds. Furthermore, the influence of substitution of caesium ions for protons on the PECD of gramicidin dianions has been investigated. For the precursor ion distribution obtained by adding an excess of CsOH to a GRA solution, a PECD of 0.45 % ± 0.14 % was measured. This value deviates from the result of −0.46 % ± 0.21 % for a pure GRA solution. The alteration of the PECD value can be either attributed to changes of the electronic structure or conformation by substitution. Within the standard errors the PICD is not affected.
The ESI-PECD method reported in this work, i.e., measuring the PECD in the photodetachment of electrosprayed anions, offers numerous advantages over the classic PECD approach which is based on the photoionization of neutrals. Apart from the already mentioned extension of the investigable analyte range, the utilization of an ESI source offers access to anionic precursors. In general, these can be photodetached by single UV photons, hence no sharp resonance conditions apply. This suggests a universal applicability of the method even for mixtures. A positive side effect of using anionic precursors is the reduction of technical requirements regarding the light sources used. Compared to multiphoton experiments on neutrals, less intense radiation is needed, and compared to synchrotron studies, lower photon energies are sufficient. Furthermore, the use of charged analytes enables the implementation of precursor mass selection. In this way, mass-resolved measurements of the PECD can be achieved, overcoming the need for mass-tagging electrons by coincidence techniques which limit count rates and thus cause long measurement times.
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Created: 2021Issued: 2022-05-30Updated: 2022-05-30
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Fachbereich Chemie
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Philipps-Universität Marburg
Language
ger
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DoctoralThesis
Keywords
AnionsCircular dichroismPhotodetachmentTime-of-flight mass spectrometryAmino acidsPeptidesElectrospray ionization
DFG-subjects
CirculardichroismusAminosäurenAnionenPeptideElektrosprayionisationPhotodetachmentFlugzeitmassenspektrometrie
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540
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Krüger, Peter (0000-0002-6735-8790): Chiroptische Untersuchungen des Photodetachments elektrogesprayter Anionen. : Philipps-Universität Marburg 2022-05-30. DOI: https://doi.org/10.17192/z2021.0502.
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This item has been published with the following license: In Copyright