[Study on the spectral properties of caffeic acid and its derivatives]

Caffeic acid, chlorogenic acid and forsythiaside are active components of Chinese herbs and have diverse biological activities. As the similar compounds, they all include 3, 4-dihydroxy-phenyl in their structures. In this paper, the fluorescence and UV spectra of the caffeic acid, chlorogenic acid and forsythiaside were studied in different pH. The spectra properties of fluorescence and UV were further discussed in view of the phenol hydroxyl states, free, dissociative and protonated. It is found that the luminescence intensity, the peak shape and peak wavelength change with pH. Experimental results also indicate that caffeic acid, chlorogenic acid and forsythiaside can emit fluorescence in wide range of pH (2-12), and fluorescent intensity is enhanced as the dissociation of 4-phenol hydroxyl. On the other hand, fluorescent intensity may be quenched as the dissociation of 3-phenol hydroxyl. Furthermore, in strong acidic or in strong basic media, the fluorescence is quenched partly, and the shape of UV spectra changed greatly in strong basic media, meaning that their molecular structures are changed considerable.