Photodissociation dynamics of hydroxybenzoic acids.
Aromatic amino acids have large UV absorption cross-sections and low fluorescence quantum yields. Ultrafast internal conversion, which transforms electronic excitation energy to vibrational energy, was assumed to account for the photostability of amino acids. Recent theoretical and experimental investigations suggested that low fluorescence quantum yields of phenol (chromophore of tyrosine) are due to the dissociation from a repulsive excited state. Radicals generated from dissociation may undergo undesired reactions. It contradicts the observed photostability of amino acids. In this work, we explored the photodissociation dynamics of the tyrosine chromophores, 2-, 3- and 4-hydroxybenzoic acid in a molecular beam at 193 nm using multimass ion imaging techniques. We demonstrated that dissociation from the excited state is effectively quenched for the conformers of hydroxybenzoic acids with intramolecular hydrogen bonding. Ab initio calculations show that the excited state and the ground state potential energy surfaces change significantly for the conformers with intramolecular hydrogen bonding. It shows the importance of intramolecular hydrogen bond in the excited state dynamics and provides an alternative molecular mechanism for the photostability of aromatic amino acids upon irradiation of ultraviolet photons.
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
Yi Lin
YL
Yang
Yuri
Y
Dyakov
Y T
YT
Lee
Chi-Kung
CK
Ni
Yi-Lun
YL
Sun
Wei-Ping
WP
Hu
D005453:Fluorescence; D062385:Hydroxybenzoates / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory; D013816:Thermodynamics
D062385:Hydroxybenzoates