Electronic Spectroscopy of Protonated 1-Aminopyrene in a Cold Ion Trap

Vibrational and electronic spectra of protonated vanillin: exploring protonation sites and isomerisation

Photofragmentation spectra of protonated vanillin produced under electrospray ionisation (ESI) conditions have been recorded in the 3000-3700 cm-1 (vibrational) and 225-460 nm (electronic) ranges, using room temperature IRMPD (infrared multiphoton dissociation) and cryogenic UVPD (ultraviolet photodissociation) spectroscopies, respectively. The cold (∼50 K) electronic UVPD spectrum exhibits very well resolved vibrational structure for the S1 ← S0 and S3 ← S0 transitions, suggesting long excited state dynamics, similar to its simplest analogue, protonated benzaldehyde. The experimental data were combined with theoretical calculations to determine the protonation site and configurational isomer observed in the experiments.

Protomers of the green and cyan fluorescent protein chromophores investigated using action spectroscopy

The photophysics of biochromophore ions often depends on the isomeric or protomeric distribution, yet this distribution, and the individual isomer contributions to an action spectrum, can be difficult to quantify. Here, we use two separate photodissociation action spectroscopy instruments to record electronic spectra for protonated forms of the green (pHBDI⁺) and cyan (Cyan⁺) fluorescent protein chromophores. One instrument allows for cryogenic (T = 40 ± 10 K) cooling of the ions, while the other offers the ability to perform protomer-selective photodissociation spectroscopy. We show that both chromophores are generated as two protomers when using electrospray ionisation, and that the protomers have partially overlapping absorption profiles associated with the S₁ ← S₀ transition. The action spectra for both species span the 340-460 nm range, although the spectral onset for the pHBDI⁺ protomer with the proton residing on the carbonyl oxygen is red-shifted by ≈40 nm relative to the lower-energy imine protomer. Similarly, the imine and carbonyl protomers are the lowest energy forms of Cyan⁺, with the main band for the carbonyl protomer red-shifted by ≈60 nm relative to the lower-energy imine protomer. The present strategy for investigating protomers can be applied to a wide range of other biochromophore ions.

Laser Photodissocation, Action Spectroscopy and Mass Spectrometry Unite to Detect and Separate Isomers

The separation and detection of isomers remains a challenge for many areas of mass spectrometry. This article highlights laser photodissociation and ion mobility strategies that have been deployed to tackle...

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.

Influence of the N atom position on the excited state photodynamics of protonated azaindole

We present a study of the photofragmentation of three protonated azaindole molecules - 7-azaindole, 6-azaindole, and 5-azaindole - consisting of fused pyrrole-pyridine bicyclic aromatic systems, in which the pyridinic (protonated) nitrogen heteroatom is located at the 7, 6, and 5 positions, respectively. Photofragmentation electronic spectra of the isolated aforementioned azaindolinium cations reveal that their photodynamics extends over timescales covering nine orders of magnitude and provide evidence about the resultant fragmentation pathways. Moreover, we show how the position of the heteroatom in the aromatic skeleton influences the excited state energetics, fragmentation pathways, and fragmentation timescales. Computed ab initio adiabatic transition energies are used to assist the assignation of the spectra, while geometry optimisation in the excited electronic states as well as ab initio calculations along the potential surfaces demonstrate the role of ππ*/πσ* coupling and/or large geometry changes in the dynamics of these species. Evidence supporting the formation of Dewar valence isomers as intermediates involved in sub-picosecond relaxation processes is discussed.

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.

Selecting and identifying gas-phase protonation isomers of nicotineH+ using combined laser, ion mobility and mass spectrometry techniques

Protonation isomers of gas-phase nicotineH⁺ are separated and assigned using a combination of FAIMS and UV photodissociation action spectroscopy.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

Protonation sites can be controlled by the electrospray source as written in the figure.