Unravelling the Keto–Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone

Abstract

Avobenzone (AB) is a widely used UVA filter known to undergo irreversible photodegradation. Here, we investigate the detailed pathways by which AB photodegrades by applying UV laser-interfaced mass spectrometry to protonated AB ions. Gas-phase infrared multiple-photon dissociation (IRMPD) spectra obtained with the free electron laser for infrared experiments, FELIX, (600−1800 cm −1 ) are also presented to confirm the geometric structures. The UV gas-phase absorption spectrum (2.5−5 eV) of protonated AB contains bands that correspond to selective excitation of either the enol or diketo forms, allowing us to probe the resulting, tautomer-dependent photochemistry. Numerous photofragments (i.e., photodegradants) are directly identified for the first time, with m/z 135 and 161 dominating, and m/z 146 and 177 also appearing prominently. Analysis of the production spectra of these photofragments reveals that that strong enol to keto photoisomerism is occurring, and that protonation significantly disrupts the stability of the enol (UVA active) tautomer. Close comparison of fragment ion yields with the TD-DFT-calculated absorption spectra give detailed information on the location and identity of the dissociative excited state surfaces, and thus provide new insight into the photodegradation pathways of avobenzone, and photoisomerization of the wider class of β-diketone containing molecules.

Photoproducts of the Photodynamic Therapy Agent Verteporfin Identified via Laser Interfaced Mass Spectrometry

“…The presence of these photofragments indicates a complex photophysical picture, and multiple dissociation pathways [ 30 ]. Comparing the Soret-band (398 nm) and Q-band (730–790 nm) photofragmentation mass spectra, photofragmentation into lighter mass fragments is more prevalent at the higher photon energy, consistent with secondary fragmentation when higher excess energy is present [ 11 , 13 , 15 ].…”

“…In the current work, we use a novel approach to measure the direct photoproducts of Verteporfin. We perform measurements away from the complications of the solution-phase environment, by applying laser interfaced mass spectrometry (LIMS) to verteporfin as an isolated gas phase ion [ 11 , 12 , 13 ]. In this approach, the molecular system of interest is introduced to the gas phase from solution by electrospray ionization, mass-selected, and then isolated in an ion trap where it can interact with a laser beam.…”

Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone

“…Gas-phase UV photodissociation experiments were conducted in an AmaZon SL dual funnel electrospray ionization quadrupole ion trap (ESI-QIT) mass spectrometer (Bruker Daltonics Inc., Billerica, MA, USA), modified for LIMS. 10,24,25 Solutions of OB (1 Â 10 À5 mol dm À3 ) and M + (1 Â 10 À5 mol dm À3 ) in ethanol were electrosprayed using typical instrumental parameters (nebulizing gas pressure of 10.0 psi, injection rate of 0.33 mL h À1 , drying gas flow rate of 8.0 L min À1 ), and run in at capillary temperatures of 100 1C. Oxybenzone (OB) was purchased from Alfa Aesar (Ward Hill, MA, USA) and HPLC-grade ethanol from Fisher Scientific, Inc. (Pittsburgh, PA, USA).…”

“…To check for single-photon conditions, the laser power dependence on signal was determined, as described previously. 8,10 Photofragmentation experiments were conducted with an ion accumulation time of 20 ms and a fragmentation time of 100 ms, thereby ensuring that each mass-selected ion packet interacted with one laser pulse, minimizing the likelihood of multiphoton events. When fluorescence is negligible, 26 the UV excited gaseous ion will fragment upon excited-state relaxation, yielding an action absorption spectrum by photodepletion (PD).…”

“…Recent work from our group has demonstrated how pH can impact at the molecular level on the function of an organic sunscreen. [8][9][10] Upon protonation, both oxybenzone and avobenzone were observed to photodissociate directly from the excited state surface, i.e., undergo non-statistical dissociation, which is an undesirable photodegradation process. A similar effect was also observed for oxybenzone and 2-phenylbenzimidazole-5-sulfonic acid upon deprotonation.…”