Measurement of the Population of Electrosprayed Deprotomers of Coumaric Acids Using UV-Vis Laser Photodissociation Spectroscopy.

Natalie G K Wong,Caroline E H Dessent,Conor D Rankine

doi:10.1021/acs.jpca.1c04880

Natalie G K Wong, Caroline E H Dessent + Show 1 more

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https://doi.org/10.1021/acs.jpca.1c04880

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Abstract

The measurement of deprotonation sites in multifunctional molecules following electrospray ionization is important to better inform a wide range of spectroscopic and photophysical studies that use electrospray to prepare molecular species for study in the gas phase. We demonstrate that low-resolution UV–vis laser photodissociation spectroscopy can be applied in situ to identify the deprotomers of three coumaric acids, trans-para-coumaric acid (CMA), trans-caffeic acid (CA), and trans-ferulic acid (FA), formed via electrospray. Electronic absorption spectra of the deprotonated coumaric acids are recorded via photodepletion and photofragmentation following electrospray from solutions of ethanol and acetonitrile. By comparing the experimental spectra to wave function theory calculations, we are able to confirm the presence of phenoxide and carboxylate deprotomers upon electrospray for all three coumaric acids, when sprayed from both protic and aprotic solvents. Ratios of the phenoxide:carboxylate deprotomers are obtained by generating summed theoretical absorption spectra that reproduce the experimental spectra. We find that choice of electrospray solvent has little effect on the ratio of deprotomers obtained for deprotonated CMA and CA but has a greater impact for FA. Our results are in excellent agreement with previous work conducted on deprotonated CMA using IR spectroscopy and demonstrate that UV photodissociation spectroscopy of electrosprayed ions has potential as a diagnostic tool for identifying deprotomeric species.

Highlights

The influence of electrospray ionization (ESI) conditions on the location of protonation and deprotonation sites of electrosprayed ions is a topic of keen debate.[1−13] Acid−base reactions are of key importance throughout chemistry and biology, so correctly identifying the structures of protomers and deprotomers can be crucial to understanding reactive processes
The calculations suggest that the phenoxide deprotomers exist at lower relative energies than the carboxylate deprotomers for the gaseous coumaric acid (CMA), caffeic acid (CA), and ferulic acid (FA) anions, but this trend inverts in solution (EtOH and MeCN), albeit with smaller energy differences between the two deprotomeric forms
A limited number of studies have explored how these photodynamics are affected by pH and, the extent of deprotonation, finding that deprotonation had little effect on excited state decay.[27]

Summary

Introduction

The influence of electrospray ionization (ESI) conditions on the location of protonation and deprotonation sites of electrosprayed ions is a topic of keen debate.[1−13] Acid−base reactions are of key importance throughout chemistry and biology, so correctly identifying the structures of protomers and deprotomers can be crucial to understanding reactive processes. Roithova and co-workers performed what is perhaps the seminal investigation of how the electrospray process affects the gas-phase ratios of deprotonated isomers by studying the parahydroxybenzoic acid molecule.[13] NMR was used to probe the solution-phase structures and ion-mobility mass spectrometry (IM-MS) was used to identify the gas-phase isomers. It is very important to have tools available to determine accurately the identity of protonation/deprotonation isomers and to have a full understanding of how the gaseous population relates to the solution-phase population as a function of the experimental conditions Toward this end, recent studies have investigated the possibility of applying in situ spectroscopy to identify correctly electrosprayed protomers or deprotomers.[2,4,6−8,11]

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