Protomer-Specific Photochemistry Investigated Using Ion Mobility Mass Spectrometry

Abstract

The utility of tandem ion mobility mass spectrometry coupled with electronic spectroscopy to investigate protomer-specific photochemistry is demonstrated by measuring the photoisomerization response for protomers of protonated 4-dicyanomethylene-2-methyl-6-para-dimethylaminostyryl-4H-pyran (DCM) molecules. The target DCMH species has three protomers that are distinguished by their different collision cross sections with He, N, and CO buffer gases, trends in abundance with ion source conditions, and from their photoisomerization responses. The trans-DCMH protomers with the proton located either on the tertiary amine N atom or on a cyano group N atom exhibit distinct S← S photoisomerization responses, with the maxima in their photoisomerization action spectra occurring at 420 and 625 nm, respectively, consistent with predictions from accompanying electronic structure calculations. The cis-DCMH protomers are not distinguishable from one another through ion mobility separation and give no discernible photoisomerization or photodissociation response, suggesting the dominance of other deactivation pathways such as fluorescence. The study demonstrates that isobaric protomers and isomers generated by an electrospray ion source can possess quite different photochemical behaviors and emphasizes the utility of isomer and protomer selective techniques for exploring the spectroscopic and photochemical properties of protonated molecules in the gas phase.

FromEtoZand back again: reversible photoisomerisation of an isolated charge-tagged azobenzene

“…16 Parallel investigations of protonated aminoazobenzene suffered the added complexity of multiple possible gas-phase protomers, which may have different absorption profiles and photochemical behaviour. 17 Spectroscopic measurements on gas-phase molecules also allows straightforward connections with theory, providing confidence regarding predictions for photoisomerisation properties of similar molecules.…”

Investigations into the performance of travelling wave enabled conventional and cyclic ion mobility systems to characterise protomers of fluoroquinolone antibiotic residues

“…IM separation of protomers for other classes of compound has been identified, including, porphyrins, pesticides and pharmaceuticals . An alternate strategy could be developed using multi‐protomer CCS measurements as additional identification points.…”

“…IM separation of protomers for other classes of compound has been identified, including, porphyrins, pesticides and pharmaceuticals. 19,45,47,48 An alternate strategy could be developed using multi-protomer CCS measurements as additional identification points. TW CCSN 2 values have been determined here in combination with accurate mass measurement, retention time and characteristic ion mobility product ion spectra, providing a multi-metric fingerprint upon which alternate identification criteria could be based.…”

Protomer-Dependent Electronic Spectroscopy and Photochemistry of the Model Flavin Chromophore Alloxazine

“…It is important to emphasize that these relative energy calculations cannot be used to straightforwardly predict which protomer(s) will be produced in the gas-phase. This has been well-established across a range of molecular systems and is attributed to the kinetic nature of the electrospray process [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. Since the photofragment action spectra ( Section 2.2.3 ) suggest the presence of two distinct electrosprayed protomers of AL∙H + , we performed TDDFT ( Figure 6 ) calculations to identify them.…”

“…In studying the protonated forms of multifunctional organic molecules such as alloxazine, it is clearly important to identify the location of the proton. A number of recent studies have focused on the identity of protonation isomers that are produced via electrospray, and it has been clearly established that a range of experimental factors (e.g., solvent and instrumentation) can affect the gaseous protomer distribution [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. Importantly, there is no guarantee that the most stable solution-phase protomer will be the major gas-phase isomer following electrospray.…”