Conformation-Specific Circular Dichroism Spectroscopy of Cold, Isolated Chiral Molecules

Quantification of enantiomers and blind identification of erythro-sphingosine non-racemates by cold ion spectroscopy

Enantiomers of a lipid erythro-sphingosine have been quantified with ≈4% accuracy by UV cold ion spectroscopy of their non-covalent complexes with a chiral aromatic molecule. The diastereomeric configuration of such complexes enables the quantification using just a single enantiomeric lipid standard and the identification of non-racemic solutions with no standards at all.

Conformer-Selective Photoelectron Circular Dichroism

Conformational flexibility and chirality both play a key role in molecular recognition. It is therefore very useful to develop spectroscopic methods that simultaneously probe both properties. It has been theoretically predicted that photoelectron circular dichroism (PECD) should be very sensitive to conformational isomerism. However, experimental proof has been less forthcoming and only exists for a very few favorable cases. Here, we present a new PECD scheme based on resonance-enhanced two-photon ionization (RE2PI) using UV/Vis nanosecond laser excitations. The spectral resolution obtained thereby guarantees conformer-selectivity by inducing resonant conformer-specific ππ* S1←S0 transitions. We apply this experimental scheme to the study of chiral 1-indanol, which exists in two conformers linked by a ring inversion and defined by the position of the hydroxyl group, namely axial and equatorial. We show that the PECD of the equatorial and axial forms considerably differ in sign, magnitude and shape. We also discuss the influence of the total ionization energy, vibronic excitation of intermediate and final states, and relative polarization of the excitation and ionization lasers. Conformer-specificity adds a new dimension to the applications of PECD in analytical chemistry addressing now the general case of floppy systems.

Anisotropic circular dichroism of jet-cooled chiral molecules

Anisotropic circular dichroism (CD) refers to the CD of oriented molecules, which varies with the direction of light propagation toward the molecules. Thus, anisotropic CD spectroscopy has been used to investigate the orientations of molecules in anisotropic media such as liquid crystals and thin films. However, it is unclear whether anisotropic CD results from isolated chromophores or their intermolecular interactions with other atoms or molecules that form anisotropically aligned structures. Herein, anisotropic CD of isolated chiral molecules was observed for the first time. The resonant two-photon ionization CD spectra of jet-cooled pseudoephedrine and styrene oxide indicated a difference between the CD values of the P/R and Q branches of the origin bands of the S0-S1 transition. This difference may have resulted from the anisotropic CD phenomena of these molecules, which are oriented via photoselection. Although jet-cooled molecules may have nearly random orientations, those excited to the P/R or Q branch become oriented because the transition probability to these branches depends on the molecular orientation relative to the direction of light propagation. These results demonstrate that the CD spectra of cold, isolated molecules, such as those in an interstellar medium, may exhibit anisotropic CD values.

Dual-Beam Circular Dichroism Spectroscopy of Jet-Cooled Chiral Molecules

Circular dichroism (CD) spectroscopy of jet-cooled molecules provides conformation-specific CD spectra. However, its widespread utilization has been limited by the weak CD effects and the low density of gas-phase molecules. Here, we developed a dual-beam method to improve the sensitivity and accuracy of gas-phase CD measurements. Circularly and linearly polarized pulses were generated from a single laser pulse and used to irradiate a single molecular-beam pulse to produce two ion peaks. The ion peaks induced by linearly polarized pulses were subtracted from those induced by circularly polarized pulses to correct the CD values for the pulse-to-pulse fluctuations in laser power and gas density. The resonant two-photon ionization CD spectrum of (1R,2R)-(-)-pseudoephedrine revealed that the standard deviations of CD values measured using the dual-beam method were three times lower than those measured using a single-beam method. The dual-beam method provides an effective, accurate, and easy-to-use tool to obtain gas-phase CD spectra.

Photoelectron Circular Dichroism as a Signature of Subtle Conformational Changes: The Case of Ring Inversion in 1-Indanol

Chirality plays a fundamental role in the molecular recognition processes. Molecular flexibility is also crucial in molecular recognition, allowing the interacting molecules to adjust their structures and hence optimize the interaction. Methods probing simultaneously chirality and molecular conformation are therefore crucially needed. Taking advantage of a possible control in the gas phase of the conformational distribution between the equatorial and axial conformers resulting from a ring inversion in jet-cooled 1-indanol, we demonstrate here the sensitivity of valence-shell photoelectron circular dichroism (PECD) to both chirality and subtle conformational changes, in a case where the photoelectron spectra of the two conformers are identical. For the highest occupied orbital, we observe a dramatic inversion of the PECD-induced photoelectron asymmetries, while the photoionization cross-section and usual anisotropy (β) parameter are completely insensitive to conformational isomerism. Such a sensitivity is a major asset for the ongoing developments of PECD-based techniques as a sensitive chiral (bio)chemical analytical tool in the gas phase.

Ultraviolet photodissociation circular dichroism spectroscopy of protonated L-phenylalanyl-L-alanine in a cryogenic ion trap

We obtained ultraviolet photodissociation (UVPD) circular dichroism (CD) spectra of protonated L-phenylalanyl-L-alanine (L-H⁺PheAla) near the origin band of the S₀-S₁ transition using cryogenic ion spectroscopy. Infrared (IR) ion-dip, IR-UV hole burning (HB) and UV-UV HB spectra showed that L-H⁺PheAla existed as two different conformers in a cryogenic ion trap, and they had nearly identical peptide backbones but different conformations in the Phe side chain. The UVPD CD spectra revealed that the two conformers had opposite CD signs and significantly different CD magnitudes from each other. These results demonstrate that the CD value of L-H⁺PheAla near the origin band is strongly influenced by the conformation of the Phe side chain.

Conformer-dependent vacuum ultraviolet photodynamics and chiral asymmetries in pure enantiomers of gas phase proline

Proline is a unique amino-acid, with a secondary amine fixed within a pyrrolidine ring providing specific structural properties to proline-rich biopolymers. Gas-phase proline possesses four main H-bond stabilized conformers differing by the ring puckering and carboxylic acid orientation. The latter defines two classes of conformation, whose large ionization energy difference allows a unique conformer-class tagging via electron spectroscopy. Photoelectron circular dichroism (PECD) is an intense chiroptical effect sensitive to molecular structures, hence theorized to be highly conformation-dependent. Here, we present experimental evidence of an intense and striking conformer-specific PECD, measured in the vacuum ultraviolet (VUV) photoionization of proline, as well as a conformer-dependent cation fragmentation behavior. This finding, combined with theoretical modeling, allows a refinement of the conformational landscape and energetic ordering, that proves inaccessible to current molecular electronic structure calculations. Additionally, astrochemical implications regarding a possible link of PECD to the origin of life's homochirality are considered in terms of plausible temperature constraints.

Self-referencing circular dichroism ion yield measurements for improved statistics using femtosecond laser pulses

The combination of circular dichroism with laser mass spectrometry via the measurement of ion yields is a powerful tool in chiral recognition, but the measured anisotropies are generally weak. The method presented in this contribution reduces the measurement error significantly. A common path optical setup generates a pair of counter-rotating laser foci in the interaction region of a time-of-flight spectrometer. As the space focus condition is fulfilled for both foci individually, this becomes a twin-peak ion source with well separated and sufficiently resolved mass peaks. The individual control of polarization allows for in situ correction of experimental fluctuations measuring circular dichroism. Our robust optical setup produces reliable and reproducible results and is applicable for dispersion sensitive femtosecond laser pulses. In this contribution, we use 3-methyl-cyclopentanone as a prototype molecule to illustrate the evaluation procedure and the measurement principle.

Identification and Quantification of Any Isoforms of Carbohydrates by 2D UV-MS Fingerprinting of Cold Ions

Biological functionality of isomeric carbohydrates may differ drastically, making their identifications indispensable in many applications of life science. Because of the large number of isoforms, structural assignment of saccharides is challenging and often requires a use of different orthogonal analytical techniques. We demonstrate that isomeric carbohydrates of any isoforms can be distinguished and quantified using solely the library-based method of 2D ultraviolet fragmentation spectroscopy-mass spectrometry (2D UV-MS) of cold ions. The two-dimensional "fingerprint" identities of UV transparent saccharides were revealed by photofragmentation of their noncovalent complexes with aromatic molecules. We assess the accuracy of the method by comparing the known relative concentrations of isomeric carbohydrates mixed in solution with the concentrations that were mathematically determined from the measured in the gas-phase fingerprints of the complexes. For the tested sets with up to five isomers of di- to heptasaccharides, the root-mean-square deviation of 3-5% was typically achieved. This indicates the expected level of accuracy in analysis of unknown mixtures for isomeric carbohydrates of similar complexity.

Mass-resolved electronic circular dichroism ion spectroscopy

DNA and proteins are chiral: Their three-dimensional structures cannot be superimposed with their mirror images. Circular dichroism spectroscopy is widely used to characterize chiral compounds, but data interpretation is difficult in the case of mixtures. We recorded the electronic circular dichroism spectra of DNA helices separated in a mass spectrometer. We studied guanine-rich strands having various secondary structures, electrosprayed them as negative ions, irradiated them with an ultraviolet nanosecond optical parametric oscillator laser, and measured the difference in electron photodetachment efficiency between left and right circularly polarized light. The reconstructed circular dichroism ion spectra resembled those of their solution-phase counterparts, thereby allowing us to assign the DNA helical topology. The ability to measure circular dichroism directly on biomolecular ions expands the capabilities of mass spectrometry for structural analysis.

Chiral and Isomeric Discrimination of Chiral Molecular Ions by Cold Ion Circular Dichroism Spectroscopy

Circular dichroism (CD) spectra contain information about absolute configurations and conformations of chiral compounds. However, extracting this information from CD spectra in solution is challenging, because the spectra exhibit only the averaged CD values of all different conformers. CD spectroscopy of jet-cooled molecules can provide conformation-specific CD spectra, but its application to biomolecules has been limited due to the difficulty of their production in the gas phase. Here, we obtained the first CD spectra of chiral molecular ions produced by electrospray ionization (ESI) using cold ion CD spectroscopy. Protonated l- or d-phenylalanine ions produced by ESI were stored in a cold quadrupole ion trap and irradiated by multiple laser pulses with left or right circular polarization. The CD spectra exhibited well-resolved CD bands of two conformers, whose signs were opposite to each other. This study will broaden the scope of conformation-resolved CD spectroscopy to large molecular ions without size limitations.

High-resolution resonance-enhanced multiphoton photoelectron circular dichroism

Photoelectron circular dichroism (PECD) is a highly sensitive enantiospecific spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization or multiphoton ionization. In the short pulse limit investigated with femtosecond lasers, resonance-enhanced multiphoton ionization (REMPI) is rather instantaneous and typically occurs simultaneously via more than one vibrational or electronic intermediate state due to limited frequency resolution. In contrast, vibrational resolution in the REMPI spectrum can be achieved using nanosecond lasers. In this work, we follow the high-resolution approach using a tunable narrow-band nanosecond laser to measure REMPI-PECD through distinct vibrational levels in the intermediate 3s and 3p Rydberg states of fenchone. We observe the PECD to be essentially independent of the vibrational level. This behaviour of the chiral sensitivity may pave the way for enantiomer specific molecular identification in multi-component mixtures: one can specifically excite a sharp, vibrationally resolved transition of a distinct molecule to distinguish different chiral species in mixtures.

Induced Circular Dichroism of Jet-Cooled Phenol Complexes with (R)-(-)-2-Butanol

The induced circular dichroism (ICD) of phenol complexed with (R)-(-)-2-butanol [PhOH-(-)BOH] in a supersonic jet is investigated using resonant two-photon ionization circular dichroism (R2PICD) spectroscopy. The R2PICD spectrum of PhOH-(-)BOH exhibits nonzero ICD bands near the absorption region of bare PhOH, where (-)BOH is transparent. Two different conformers containing a single hydrogen bond between PhOH and (-)BOH are identified using ultraviolet-ultraviolet hole-burning and infrared ion-dip spectroscopy combined with quantum theoretical calculations. The ICD values of the two conformers are similar to each other. To understand these similar ICD effects of the conformers, the geometrical asymmetry of the PhOH moiety bound to (-)BOH and the coupling strength of the electric transition dipole moments between PhOH and (-)BOH are estimated. Comparing the ICD values of PhOH-(-)BOH with those of PhOH-(-)-l-methyl lactate in the previous report [ Hong , A. ; J. Phys. Chem. Lett. 2018 , 9 , 476 -480 ], we investigate the physical properties that may govern the differences of the ICD values between the two complexes.

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

Photoionization of the chiral monoterpene limonene has been investigated using polarized synchrotron radiation between the adiabatic ionization threshold, 8.505 and 23.5 eV. A rich vibrational structure is seen in the threshold photoelectron spectrum and is interpreted using a variety of computational methods. The corresponding photoelectron circular dichroism-measured in the photoelectron angular distribution as a forward-backward asymmetry with respect to the photon direction-was found to be strongly dependent on the vibronic structure appearing in the photoelectron spectra, with the observed asymmetry even switching direction in between the major vibrational peaks. This effect can be ultimately attributed to the sensitivity of this dichroism to small phase shifts between adjacent partial waves of the outgoing photoelectron. These observations have implications for potential applications of this chiroptical technique, where the enantioselective analysis of monoterpene components is of particular interest.

Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet-Cooled Phenol Complexes with (-)-Methyl l-Lactate

Induced circular dichroism (ICD) is the CD observed in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions. ICD spectroscopy has been used to probe the binding between molecules, such as protein-ligand interactions. However, most ICD spectra have been measured in solution, which only exhibit the averaged CD values of all conformational isomers in solution. Here, we obtained the first isomer-selective ICD spectra by applying resonant two-photon ionization CD spectroscopy to jet-cooled phenol complexes with (-)-methyl l-lactate (PhOH-(-)ML). The well-resolved CD bands in the spectra were assigned to two conformers, which contained different types of hydrogen-bonding interactions between PhOH and (-)ML. The ICD values of the two conformers have different signs and magnitudes, which were explained by differences both in the geometrical asymmetries of PhOH bound to (-)ML and in the electronic coupling strengths between PhOH and (-)ML.

How and How Much Molecular Conformation Affects Electronic Circular Dichroism: The Case of 1,1-Diarylcarbinols

Chiroptical spectra such as electronic circular dichroism (ECD) are said to be much more sensitive to conformation than their non-chiroptical counterparts, however, it is difficult to demonstrate such a common notion in a clear-cut way. We run DFT and TDDFT calculations on two closely related 1,1-diarylmethanols which show mirror-image ECD spectra for the same absolute configuration. We demonstrate that the main reason for the different chiroptical response of the two compounds lies in different conformational ensembles, caused by a single hydrogen-to-methyl substitution. We conclude that two compounds, having the same configuration but different conformation, may exhibit mirror-image ECD signals, stressing the importance and impact of conformational factors on ECD spectra.

Electronic Circular Dichroism Spectroscopy of Jet-Cooled Phenylalanine and Its Hydrated Clusters

We obtained resonant two-photon ionization circular dichroism (R2PICD) spectra of jet-cooled phenylalanine (Phe) and its hydrated clusters (Phe(H₂O)_n, n = 1-2) near the origin band of the S₀-S₁ transition. The R2PICD spectra of Phe exhibit well-resolved CD bands of six different conformers present in the jet, which vary in sign and magnitude depending on their conformations. We revised the previous structural assignments of the Phe conformers based on the comparison between the experimental and theoretical CD signs, infrared spectra, and rotational band contours. The R2PICD spectra of Phe(H₂O)_n reveal that hydration with one or two water molecule(s) does not affect the CD signs of Phe conformers but significantly increases their CD magnitudes. Furthermore, conformational selection by solvation alters relative populations of Phe conformers, leading to a sign inversion in the CD spectra of Phe(H₂O)_n compared with that of Phe monomer.

Mass-Selective Chiral Analysis

Three ways of realizing mass-selective chiral analysis are reviewed. The first is based on the formation of diastereomers that are of homo- and hetero- type with respect to the enantiomers of involved chiral molecules. This way is quite well-established with numerous applications. The other two ways are more recent developments, both based on circular dichroism (CD). In one, conventional or nonlinear electronic CD is linked to mass spectrometry (MS) by resonance-enhanced multiphoton ionization. The other is based on CD in the angular distribution of photoelectrons, which is measured in combination with MS via photoion photoelectron coincidence. Among the many important applications of mass-selective chiral analysis, this review focuses on its use as an analytical tool for the development of heterogeneous enantioselective chemical catalysis. There exist other approaches to combine chiral analysis and mass-selective detection, such as chiral chromatography MS, which are not discussed here.

Fluorescence-detected circular dichroism spectroscopy of jet-cooled ephedrine

The resonant two-photon ionization circular dichroism (R2PICD) spectrum represents the cumulative circular dichroism (CD) of one-photon excitation and the subsequent one-photon ionization, whereas the fluorescence-detected circular dichroism (FDCD) spectra exhibit only the CD of one-photon excitation, similar to conventional CD spectra. We obtained the FDCD spectra of jet-cooled ephedrine (EPD) near the origin band of the S0-S1 transition to measure the CD of one-photon absorption and thus the CD of the ionization process in R2PI in comparison with the R2PICD spectra. The CD effects of the ionization following excitation of the A (0-0) and C (930 cm(-1)) bands in the spectrum are small, whereas those of the B band (530 cm(-1)) are anomalously large, leading to opposite CD signs for the FDCD and R2PICD spectra. Based on the intermediate state-selective fragmentation patterns in the R2PI spectra, this large CD effect is attributed to the state-selective isomerization that occurs after excitation of the B band. By comparing the experimental and theoretical spectra, we determined that the B band corresponds to an asymmetric ring distortion mode that involves torsional motions of the side chain, which may facilitate the isomerization process. This study demonstrates that FDCD spectroscopy combined with R2PICD spectroscopy provides a powerful tool to measure the CD effects of the excitation and ionization processes separately in R2PI and thus probe the structural changes that occur during the ionization process following excitation to an intermediate state.

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

Photoelectron circular dichroism is a chiroptical effect able to distinguish isomers and to determine accurately the enantiopurity of chiral compounds.

Detection limits of organic compounds achievable with intense, short-pulse lasers

Many organic molecules have strong absorption bands which can be accessed by ultraviolet short pulse lasers to produce efficient ionization. This resonant multiphoton ionization scheme has already been exploited as an ionization source in time-of-flight mass spectrometers used for environmental trace analysis. In the present work we quantify the ultimate potential of this technique by measuring absolute ion yields produced from the interaction of 267 nm femtosecond laser pulses with the organic molecules indole and toluene, and gases Xe, N2 and O2. Using multiphoton ionization cross sections extracted from these results, we show that the laser pulse parameters required for real-time detection of aromatic molecules at concentrations of one part per trillion in air and a limit of detection of a few attomoles are achievable with presently available commercial laser systems. The potential applications for the analysis of human breath, blood and tissue samples are discussed.