Caroline Elizabeth Helen Dessent

We report the first demonstration of ultraviolet photodissociation (UVPD) mass spectrometry achieved with light-emitting diodes (LEDs) externally interfaced to a linear ion-trap mass spectrometer. LEDs provide a low-cost, compact photon source that is technologically simpler to operate than the high-powered pulsed laser systems that are currently used in UVPD instruments. Mounting the LEDs external to the mass spectrometer provides improved flexibility for interchanging LEDs with different wavelengths. Here, we demonstrate 365 and 280 nm LED-UVPD on flavin mononucleotide, tryptophan, and the pentapeptide Leu-Enk. The UVPD fragments observed are in full agreement with fragments observed using pulsed lasers as the photon source.

We report the first demonstration of ultraviolet photodissociation (UVPD) mass spectrometry achieved with Light-Emitting Diodes externally interfaced to a linear ion-trap mass spectrometer. LEDs provide a low-cost, compact photon source that is technically simpler to operate than the high-powered pulsed laser systems that are currently used in UVPD instruments. Mounting the LEDs external to the mass spectrometer provides advanced flexibility for interchanging LEDs with different wavelengths. Here, we demonstrate 365 and 280 nm LED-UVPD on flavin mononucleotide, tryptophan and the pentapeptide, Leu-Enk. The UVPD fragments observed are in full agreement with fragments observed using pulsedlasers as the photon source.

Mass spectrometric identification of the regioisomers of Sulfobenzoic acid (SBA), an important chemical intermediate, is complicated by the system containing both sulfonic acid and carboxylic acid deprotonation sites. Here, we apply infrared multiple-photon dissociation (IRMPD) spectroscopy for the first time to ortho-, meta-, and para-SBA deprotonated anions, [SBA-H]-, formed via electrospray ionization, to determine the identity of the deprotonation isomers. IRMPD spectra were acquired over the fingerprint region (800 – 2000 cm-1) at the FELIX free-electron laser, with experimental measurements being interpreted with the aid of quantum chemical calculations at the CAM-B3LYP/6-311++G(2d,2p) level. For all three regioisomers, the IRMPD spectra are consistent with deprotonation occurring only at the sulfonic acid group with characteristic sulfonate stretches observed (~1220 and 1060 cm-1, for the symmetric and asymmetric stretches). Importantly, we find no spectroscopic evidence for the presence of carboxylate deprotomers. Our spectroscopic results are fully consistent with the quantum chemical predictions of the relative deprotomer energies of the [SBA-H]- anions, since the meta- and para-[SBA-H]-, gas-phase sulfonate deprotomers are calculated to be substantially lower in energy than the corresponding carboxylates (by 65.6 and 73.3 kJ/mol, respectively). The ortho-[SBA-H]- regioisomers optimizes to a single structure, where the sulfonate group is deprotonated and forms an intramolecular hydrogen bond with the -OH group of the adjacent carboxylic acid. We discuss the collision-induced dissociation patterns of the [SBA-H]- regioisomers, and present a full interpretation of the observed fragmentation pathways consistent with the electrosprayed anions corresponding solely to sulfonate structures.

Laser photodissociation spectroscopy of gaseous ions is a widely applicable technique used to investigate the electronic excited states and photochemistry of charged systems ranging from inorganic nanoclusters through to biomolecules. Spectra obtained by this technique provide important benchmarks for theoretical studies of dissociative excited states and give insight into light‐induced fragmentation pathways. In this concept article, we will highlight the advantages of obtaining and analysing photodissociation action spectra in terms of photon‐energy‐dependent relative ion yield plots . This new approach to analysing ion photodissociation spectra has been demonstrated by our laboratory over recent years and is based on the photochemical concept of quantum yields. Importantly, relative ion yield analysis allows experimental vertical excitation energies of charged systems to be straightforwardly identified for comparison to theory. The application of relative ion yield analysis also provides greater insight into excited‐state decay dynamics than is evident from inspection of individual photofragment action spectra. To provide a suitable introduction to the relative ion yield concept for non‐expert readers, we begin the article by reviewing the broader topic of ionic photofragment action spectroscopy. Results from several UV filter molecules are then presented to illustrate the advantages of relative ion yield analysis.

Cannabinoids are of considerable current interest for use in pharmaceutical and non-medical consumer products. While there have been significant efforts to understand their chemical stability under ambient conditions, only sparse attention has been paid to characterising their photostability. Here, we present UVA (365 nm) and UVB (280 nm) photolysis measurements of eight representative cannabinoids, including natural compounds (THC, CBD, THCA, CBDA), metabolites (THC-COOH, THC-OH), and synthetic analogues (JWH-018, MDMB-FUBINACA). Measurements were performed using a novel online-electrospray mass spectrometry (MS) approach, where online photolysis of cannabinoid solutions was conducted with laser light-emitting diodes. MS detection was used to monitor precursor compound decay and photoproduct formation. Complementary results obtained via UV–Vis spectroscopy of photolysed cannabinoid solutions are also presented. For THC, CBD, THC-COOH, THC-OH, THCA and CBDA, significant photodegradation was observed with 280 nm photolysis, both through the appearance of photoproducts detected by MS and via time-dependent changes in the solution UV–Vis absorption profiles. In contrast, the synthetic cannabinoids (JWH-018 and MDMB-FUBINACA) showed negligible degradation with UVB photolysis, consistent with their relatively low absorbance propensity through the mid-UV region. No significant photodegradation was observed for UVA (365 nm) photolysis of any of the cannabinoids. The results presented here constitute the first directly comparable set of photolysis measurements for key phytocannabinoids.

Cannabinoids are of considerable current interest for use in pharmaceutical and non-medical consumer products. While there have been significant efforts to understand their chemical stability under ambient conditions, only sparse attention has been paid to characterizing their photostability. Here, we present UVA (365 nm) and UVB (280 nm) photolysis measurements of eight representative cannabinoids, including natural compounds (THC, CBD, THCA, CBDA), metabolites (THC-COOH, THC-OH), and synthetic analogues (JWH-018, MDMB-FUBINACA). Measurements were performed using a novel online-electrospray mass spectrometry (MS) approach, where online photolysis of cannabinoid solutions was conducted with laser light emit-ting diodes. MS detection was used to monitor precursor compound decay and photoproduct formation. Complementary results obtained via UV-Vis spectroscopy of photolysed cannabinoid solutions are also presented. For THC, CBD, THC-COOH, THC-OH, THCA and CBDA significant photodegradation was observed with 280 nm photolysis, both through the appearance of photoproducts detected by MS, and via time-dependent changes in the solution UV-Vis absorption profiles. In contrast, the synthetic cannabinoids (JWH-018 and MDMB-FUBINACA) showed negligible degradation with UVB photolysis, consistent with their relatively low absorbance propensity through the mid-UV region. No significant photodegradation was observed for UVA (365 nm) photolysis of any of the cannabinoids. The results presented here constitute the first directly comparable set of photolysis measurements for key phytocannabinoids.

Photochemically triggered control of keto-enol equilibria is of growing importance in rational synthetic strategies. Questions exist, however, about the extent to which the electrostatic effects (i.e., interactions with counterions and solvent) can perturb or direct such photocatalytic approaches. Here, we directly address the question of whether electrostatic tuning via alkali metal binding can affect the photochemistry of the classic β-diketone molecule, avobenzone, which exists in keto and enol forms. A combination of photodissociation mass spectrometry over the ultraviolet (UV) (390-238 nm) and infrared (IR) (800-1800 cm^-1) ranges and quantum chemical calculations (density functional theory (DFT) and SCS-ADC(2)) is applied to isolated (gas-phase) alkali metal-avobenzone complexes for the first time. We find that Na⁺, K⁺, and Rb⁺ binding only modestly perturbs the avobenzone π*-character excited states compared to the bare molecule. However, electrostatic binding has a dramatic overall effect since cation binding reverses the relative stability of the tautomers on the electronic ground state: without cation binding, the enol dominates, whereas upon cation binding, the keto form dominates. This is of critical photochemical importance as the ketone tautomer is the doorway geometry to photoinstability through providing access to optically dark triplet states that are adjacent to bright singlet states. Ion binding therefore opens a route to enhanced photochemical activity and molecular dissociation. Our results demonstrate a paradigm where electrostatic binding, here with a metal cation, perturbs the molecular keto-enol photochemistry through disruption of the ground-state electronic surface.

Unexpected activation of the tetrabutylammonium cation in the presence of hexachloroplatinate(IV) under light to give a dinuclear complex of trans-μ²:η²,η²-1,3-butadiene-bis(trichloroplatinate(II)) along with a proposed mechanism of the activation has been reported. The mechanism has been investigated using a combination of photodissociation photodetachment mass spectrometry, and frozen-matrix EPR spectroscopy, in addition to 1D and 2D NMR spectroscopy. In addition to the Bu₄N⁺ salts of [PtCl₆]²⁻ that were part of the original observations, the reactivity of Bu₄P⁺, Pr₄N⁺, and Pe₄N⁺ (Pe = pentyl) salts has also been investigated, and, in addition, the possible involvement of η²-butene complex intermediates has been investigated. The combined results provide additional evidence and support for the originally proposed mechanism of activation of the Bu₄N⁺ cation.

Riboflavin, Vitamin B2, is a key photoactive biomolecule that has important uses as a food additive and as a photocatalyst. While riboflavin's photodegradation pathways have been studied extensively, open questions exist about the effect of the chemical environment on riboflavin photodegradation and the nature of the subsequent photoproducts. Here, we use time-resolved mass spectrometry (TRMS) and gas-phase infrared multiple-photon dissociation (IRMPD) spectroscopy to characterize 365 nm online photolysis of riboflavin under basic conditions. TRMS allowed for monitoring of the light-induced decay of deprotonated riboflavin along with the formation of photoproducts and photolysis intermediates. IRMPD spectroscopy was performed over the fingerprint region (1100-1800 cm^-1) at the FELIX free-electron laser facility, to obtain the first gas-phase IR spectrum of deprotonated riboflavin, the isolated chromophore, along with the IRMPD spectrum of the deprotonated riboflavin dimer. In addition, spectroscopic characterization was performed for the photoproducts lumichrome and lumiflavin, as well as the photolysis intermediates formylmethylflavin and the riboflavin-lumichrome dimer. Our experiments reveal that 365 nm photolysis of the riboflavin dimer is enhanced compared with the monomer, potentially due to spectral shifting of the chromophore upon complexation. The clear propensity for formation of the dimer that we observe for riboflavin and its photolysis behavior indicates that aggregates play a significant role in accelerating photodegradation of riboflavin. This is the first time, to our knowledge, that such an effect has been identified in flavin photochemistry and provides new insight into why photodegradation of riboflavin is particularly sensitive to solution conditions.