Abstract:

BACKGROUND: Traditionally, UV/visible spectra, conductivity, electrophoresis and other methods are commonly applied for studying the interaction of metal ions and amino acids, but UV-Vis spectroscopy and fluorescence spectroscopy for L-histidine reacted with silver ion in aqueous solution is rarely reported.
OBJECTIVE: To investigate the interaction between silver ion and histidine using UV/VIS and fluorescence spectra.
METHODS: The influence of pH, multicomponent concentration such as histidine, silver ion, formaldehyde, sodium dodecyl sulfate and trihydroxymethyl aminomethane, as well as illumination strength and time, on the interaction between silver ion and histidine were investigated, and the mechanism of reaction was also explored.
RESULTS AND CONCLUSION: Applied pH potentiometer titration method, the dissociation constants of histidine was defined 9.21. The stepwise stability constants of histidine-silver was ㏒K1=5.56 and ㏒K2=4.05, respectively by using half n(_) method. At    20 ℃, the electric potential of histidine-silver system was 2.10×10^-4 V. According to pH potentiometer titration and Iob method, the compound was consisted of histidine: Ag = 2:1. Compared with histidine, histidine-silver systems reached a shoulder peak at 295.3 nm, which was assigned to conjugate double bond of imidazole ring that easily generated π-π* transition. And an absorption peak close to 242 nm can be assigned to n-π* transition of the C＝O group of the histidine-silver. The fluorescence emission spectra of histidine-silver systems belonged to 5D0→7F2 electric dipole transition. Compared with reference solutions under the same conditions, it not only emitted wavelengths blue shift, but also induced fluorescence quenching. Results showed that, imidazole ring was involved in the bonding action with silver ion. The reaction process is to firstly generate six-coordination complex, secondly reduce the silver ion into ultrafine silver particles which are bound by histidine.