VUV Photodynamics and Chiral Asymmetry in the Photoionization of Gas Phase Alanine Enantiomers

Abstract

The valence shell photoionization of the simplest proteinaceous chiral amino acid, alanine, is investigated over the vacuum ultraviolet region from its ionization threshold up to 18 eV. Tunable and variable polarization synchrotron radiation was coupled to a double imaging photoelectron/photoion coincidence (i(2)PEPICO) spectrometer to produce mass-selected threshold photoelectron spectra and derive the state-selected fragmentation channels. The photoelectron circular dichroism (PECD), an orbital-sensitive, conformer-dependent chiroptical effect, was also recorded at various photon energies and compared to continuum multiple scattering calculations. Two complementary vaporization methods-aerosol thermodesorption and a resistively heated sample oven coupled to an adiabatic expansion-were applied to promote pure enantiomers of alanine into the gas phase, yielding neutral alanine with different internal energy distributions. A comparison of the photoelectron spectroscopy, fragmentation, and dichroism measured for each of the vaporization methods was rationalized in terms of internal energy and conformer populations and supported by theoretical calculations. The analytical potential of the so-called PECD-PICO detection technique-where the electron spectroscopy and circular dichroism can be obtained as a function of mass and ion translational energy-is underlined and applied to characterize the origin of the various species found in the experimental mass spectra. Finally, the PECD findings are discussed within an astrochemical context, and possible implications regarding the origin of biomolecular asymmetry are identified.

Determination of accurate electron chiral asymmetries in fenchone and camphor in the VUV range: sensitivity to isomerism and enantiomeric purity

“…In contrast to b1, numerical computations of the β parameter of fenchone 40 and camphor 41 lead to very similar, non--distinguishable behaviour with photon energy. The present results also confirm that the enhanced sensitivity of PECD to the molecular potential is especially marked for slow electrons as already pointed out in the case of conformers 14,15 and dimers 18 . Finally, this sensitivity to isomerism confirms also that PECD is a long--range effect, at least for slow electrons, since in the case of camphor the methyl groups are localized quite far from the site of the ionized orbital.…”

“…The benchmarking versus a conventional chromatographic analytical method, highlights the interest and high accuracy, in the % range, of PECD as a precise enantiomeric analytical tool in the gas phase. Especially when combined in with TOF mass spectrometry of the photoions, recorded in coincidence with the photoelectrons, via the so--called PECD--PICO scheme, we stress the ability to analyze a mixture of compounds simultaneously and to provide highly accurate relative ee measurement of each of these compounds, now with significantly improved accuracy over the first demonstrations of this capability 15,30 . As compared to the well--established GC type of technique, PECD is a direct method with no derivatization, nor use of reagents, directly applicable in the gas phase, and which provides additional electronic information, as a multi--dimensional chiroptical probe (photon energy, electron energy, coincident ion mass).…”

Intense Vibronic Modulation of the Chiral Photoelectron Angular Distribution Generated by Photoionization of Limonene Enantiomers with Circularly Polarized Synchrotron Radiation

“…For the HOMO 2π orbital, the conformer variations are initially very small and even going above 15 eV they remain fairly similar. Such conformer invariance in the predicted PECD is a little unexpected . In contrast, the HOMO‐1 1π orbital predictions show far greater conformer dependent variability, far more in keeping with prior generalised expectations.…”

“…What can be seen unambiguously from this presentationo f calculated PECD in Figure 11 is the relative conformer dependent behaviour.F or the HOMO 2p orbital, the conformer variations are initially very small and even going above 15 eV they remain fairly similar.S uch conformer invariance in the predicted PECD is al ittle unexpected. [24,49,[54][55][56][57] In contrast, the HOMO-1 1p orbitalp redictions show far greater conformer dependentv ariability,f ar more in keeping with prior generalised expectations. The lacko fastrong2 p dependence might be thought not unreasonable given that the initial orbital is localised at the C=Cd ouble bond in the ring, and the conformers differ only by rotationo ft he remote tail grouping.…”

The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid

“…Despite the moderate thermodesorber temperature of 130 °C, the experimental data are indicative of an internal temperature of about 200 °C. Although generally no cooling effects are expected in thermodesorber experiments, increased model temperatures are still counter‐intuitive considering that both collisional and evaporative cooling effects may be at play . We propose three possible reasons for this discrepancy: i) the tip temperature may be somewhat higher than measured, ii) applying the harmonic oscillator approximation in internal energy distribution calculations of the large oleic acid molecule at moderate temperatures may be fraught with uncertainty, similar to its application to small molecules at high temperatures, as also evident from the calculated internal energy distribution in Figure f, which predicts barely any neutrals with less than 1 eV internal energy, and iii) inelastic impact of the oleic acid particles on the heated surface may also enhance heating by ca.…”