Effects of ionic strength on the coordination of Eu(III) and Cm(III) to a Gram-negative bacterium,Paracoccus denitrificans

Abstract

We studied the effect of ionic strength on the interactions of Europium(III) and Curium(III) with a Gram-negative bacteriumParacoccus denitrificans. Bacterial cells grown in 0.5-, 3.5-, and 5.0% NaCl were used in adsorption experiments and laser experiments that were performed at the same ionic strengths as those in the original growth media. The distribution ratio (logKd) for Eu(III) and Cm(III) was determined at pHs 3−5. To elucidate the coordination environment of Eu(III) adsorbed onP. denitrificans, we estimated the number of water molecules in the inner sphere and strength of the ligand field by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pHs 4−6. The logKdof Eu(III) and Cm(III) increased with an increase of pH at all ionic strengths because there was less competition for ligands in cells with H+at higher pHs, wherein less H+was present in solution: cation adsorption generally occurs through an exchange with H+on the functional groups of coordination sites. No significant differences were observed in the logKdof Eu(III) and Cm(III) at each pH in 0.5-, 3.5-, and 5.0% NaCl solutions, though competition for ligands with Na+would be expected to increase at higher NaCl concentrations. The logKdof Eu(III) was almost equivalent to that of Cm(III) under all the experimental conditions. TRLFS showed that the coordination environments of Eu(III) did not differ from each other at 0.5-, 3.5-, and 5.0% NaCl at pHs 4−6. TRLFS also showed that the characteristic of the coordination environment of Eu(III) onP. denitrificanswas similar to that on a halophile,Nesterenkonia halobia, while it significantly differed from that on a non-halophile,Pseudomonas putida. These findings indicate that the number of coordination sites for Eu(III) onP. denitrificans, whose cell surface may have similar structures to that of halophiles, increased with increasing ionic strength, though their structure remained unchanged.