title: pH-Independent Charge Resonance Mechanism for UV Protective Functions of Shinorine and Related Mycosporine-like Amino Acids.
abstract: The UV-protective ability of mycosporine-like amino acids (MAAs) has been well documented and is believed to serve as a protecting agent for marine organisms from solar radiation. However, the effective UV absorption by MAAs has not been well correlated to MAA (neutral) structures. In this study, the origin of UV-protecting ability of MAAs was elucidated by experimental and theoretical spectroscopic investigations. The absorption maxima of mycosporine-glycine and shinorine in the UVA region were practically unaffected over a wide range of pH 4-10 and only slightly blue-shifted at pH 1-2. It was revealed that the zwitterionic nature of the amino acid residue facilitates the protonation to the chromophoric 3-aminocyclohexenone and 1-amino-3-iminocyclohexene moieties and the operation of the charge resonance in the protonated species well accounts for their allowed low-energy transitions in the UVA region. The RI-CC2/TZVP calculations on model systems in their protonated forms well reproduced the observed transition energies and oscillator strengths of MAAs, only with insignificant systematic overestimations of the both values. The slight hypsochromic shifts at pH 1-2 were explained by (partial) protonation to a carboxylate anion in the amino acid residue, as confirmed by theory. Fluorescence spectral investigations of shinorine were also performed for the first time in water to confirm the effective nonradiative deactivation. Consequently, this study unequivocally demonstrated that the 3-aminocyclohexenone as well as 1-amino-3-iminocyclohexene moieties, which are readily protonated at a wide range of pH, are responsible for the UV-protective ability of aqueous solution of MAAs.
authors.affiliation: Nagase R&D Center, Nagase & Co., Ltd. , 2-2-3 Murotani, Nishi-Ku, Kobe 651-2241, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Matsuyama
authors.affiliation: Nagase R&D Center, Nagase & Co., Ltd. , 2-2-3 Murotani, Nishi-Ku, Kobe 651-2241, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Matsumoto
authors.affiliation: Nagase R&D Center, Nagase & Co., Ltd. , 2-2-3 Murotani, Nishi-Ku, Kobe 651-2241, Japan.
authors.forename: Shogo
authors.initials: S
authors.lastname: Yamamoto
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , 2-1 Yamada-oka, Suita 565-0871, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Nagasaki
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , 2-1 Yamada-oka, Suita 565-0871, Japan.
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Inoue
authors.affiliation: Office for University-Industry Collaboration, Osaka University , 2-8 Yamada-oka, Suita 565-0871, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Nishijima
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , 2-1 Yamada-oka, Suita 565-0871, Japan.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Mori