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Download fileMolecular Basis of the Antioxidant Capability of Glutathione Unraveled via Aerosol VUV Photoelectron Spectroscopy
journal contribution
posted on 2016-09-02, 00:00 authored by Po-Chiao Chang, Youqing Yu, Zhong-Hang Wu, Ping-Cheng Lin, Wei-Ren Chen, Chien-Cheng Su, Meng-Sin Chen, Yu-Lin Li, Tzu-Ping Huang, Yin-Yu Lee, Chia C. WangGlutathione (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.