Interfacial Solvation and Surface pH of Phenol and Dihydroxybenzene Aqueous Nanoaerosols Unveiled by Aerosol VUV Photoelectron Spectroscopy.

Although the significance of aqueous interfaces has been recognized in numerous important fields, it can be even more prominent for nanoscaled aqueous aerosols because of their large surface-to-volume ratios and prevalent existence in nature. Also, considering that organic species are often mixed with aqueous aerosols in nature, a fundamental understanding of the electronic and structural properties of organic species in aqueous nanoaerosols is essential to learn the interplay between water and organic solutes under the nanoscaled size regime. Here, we report for the first time the vacuum ultraviolet photoelectron spectroscopy of phenol and three dihydroxybenzene (DHB) isomers including catechol, resorcinol, and hydroquinone in the aqueous nanoaerosol form. By evaluating two photoelectron features of the lowest vertical ionization energies originated from the b1(π) and a2(π) orbitals for phenolic aqueous nanoaerosols, their interfacial solvation characteristics are unraveled. Phenolic species appear to reside primarily on/near the aqueous nanoaerosol interface, where they appear only partially hydrated on the aqueous interface with the hydrophilic hydroxyl group more solvated in water. An appreciable proportion of phenol is found to coexist with phenolate at/near the nanoaerosol interface even under a high bulk pH of 12.0, indicating that the nanoaerosol interface exhibits a composition distribution and pH drastically different from those of the bulk. The surface pH of phenol-containing aqueous nanoaerosols is found to be ∼2.2 ± 0.1 units more acidic than that of the bulk interior, as measured at the bulk pH of 12.0. From the photoelectron spectra of DHB aqueous nanoaerosols, the effects of numbers/arrangements of -OH groups are assessed. This study shows that the hydration extents, pH values, deprotonation status, and numbers/relative arrangements of -OH groups are crucial factors affecting the ionization energies of phenolic aqueous nanoaerosols and thus their redox-based activities. The multifaceted implications of the present study in the aerosol science, atmospheric/marine chemistry, and biological science are also addressed.

Molecular Basis of the Antioxidant Capability of Glutathione Unraveled via Aerosol VUV Photoelectron Spectroscopy.

Glutathione (GSH), the most abundant nonenzymatic antioxidant in living systems, actively scavenges various exogenous/endogenous oxidizing species, defending important biomolecules against oxidative damages. Although it is well established that the antioxidant activity of GSH originates from the cysteinyl thiol (-SH) group, the molecular origin that makes the thiol group of GSH a stronger reducing agent than other thiol-containing proteins is unclear. To gain insights into the molecular basis underlying GSH's superior antioxidant capability, here we report, for the first time, the valence electronic structures of solvated GSH in the aqueous aerosol form via the aerosol vacuum ultraviolet photoelectron spectroscopy technique. The pH-dependent electronic evolution of GSH is obtained, and the possible correlations between GSH and its constituting amino acids are interrogated. The valence band maxima (VBMs) for GSH aqueous aerosols are found at 7.81, 7.61, 7.52, and 5.51 ± 0.10 eV at a pH of 1.00, 2.74, 7.00, and 12.00, respectively, which appear to be lower than the values of their corresponding hybrid counterparts collectively contributed from the three isolated constituting amino acids of GSH. An additional photoelectron feature is observed for GSH aqueous aerosols at pH = 12.00, where the thiol group on its Cys residue becomes deprotonated and the relatively well-separated feature allows its vertical ionization energy (VIE) to be determined as 6.70 ± 0.05 eV. Compared to a VIE of 6.97 ± 0.05 eV obtained for a similar thiolate feature observed previously for isolated Cys aqueous aerosols ( Su et al. VUV Photoelectron Spectroscopy of Cysteine Aqueous Aerosols: A Microscopic View of Its Nucleophilicity at Varying pH Conditions . J. Phys. Chem. Lett. 2015 , 6 , 817 - 823 ), a 0.27 eV reduction in the VIE is found for GSH, indicating that the outermost electron corresponding to the nonbonding electron on the thiolate group can be removed more readily from the GSH tripeptide than that from Cys alone. The possible origins underlying the decrease in the VBM of GSH with respect to that of each corresponding hybrid counterpart and the decrease in the VIE of the thiolate feature of GSH with respect to that of the isolated Cys are discussed, providing hints to understand the superior antioxidant capability of GSH from a molecular level.