A differentiation of the affinity of uranium(VI) to phosphate and carboxylic groups in native phosvitin studied by ATR FT-IR spectroscopy


A differentiation of the affinity of uranium(VI) to phosphate and carboxylic groups in native phosvitin studied by ATR FT-IR spectroscopy

Li, B.; Raff, J.; Bernhard, G.; Foerstendorf, H.

The toxicity of the uranium to the living organisms is because of its heavy metal characteristic. Proteins, the fundamental component of all living cells and the key to their metabolism, undergo conformational changes upon the heavy metal complexation, thus loss their proper cellular function. In this study, phosvitin, a highly water soluble 34 kDa protein containing roughly 35 phosphate groups and 29 carboxylic residues1, is chosen as an ideal model system for the spectroscopic investigation of the interaction of U(VI) with proteins allowing the differentiation between the U(VI)-phosphate and U(VI)-carboxylic complexation. For this purpose, two different U(VI) concentrations (10−4 M and 10−5 M) are set up at pH 4 with various amounts of phosvitin to acquire complexes with different U(VI)/phosphate group ratios. The aqueous solutions were investigated by ATR FT-IR spectroscopy. For the very first time, soluble protein U(VI) complexes are achieved in aqueous solution providing spectral evidence for U(VI) complexation by the unequivocal identification of the νas(UO22+) mode. The spectra of the soluble complex show that at a low U(VI)/phosphate ratio (1:10.2) U(VI) preferentially binds to the phosphate groups. Interestingly, the νas(UO22+) mode is found at 905 cm−1 which is bathochromic shifted about 60 cm−1 compared to the free uranyl ion2 reflecting a strong coordination to several phosphate groups. With increasing U(VI)/phosphate ratio, U(VI) complexation to carboxylic groups is observed by a hypsochromic shift of the νas(UO22+) mode and characteristic bands of the νs(COO−) and νas(COO−) modes. At a higher U(VI)/phosphate ratio (10:1), complexation between U(VI) and carboxylic groups becomes dominant. From the observed frequency of this mode (925 cm−1) a typical bidental complexation to U(VI) by carboxylic group can be assumed3. In order to reduce the impact of the carboxylic groups on the U(VI) binding, phosvitin is modified using EDC. After subsequent incubation with 10−3 M U(VI) in aqueous solution at pH 4, the obtained IR spectra of the precipitated U(VI)-protein complex confirm this assumption.

Keywords: Phosvitin; U(VI); ATR FT-IR; phosphate groups; carboxylic groups

  • Poster
    European Conference on the Spectroscopy of Biological Molecules 2009, 28.08.-02.09.2009, Palermo, Italy

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Publ.-Id: 12764