Interaction of curium(III) with surface-layer proteins from Lysinibacillus sphaericus JG-A12

Trivalent actinides such as Cm(III) are able to occupy natural Ca(II) binding sites in biological systems. For this investigation, we studied the formation of aqueous Cm(III) complexes with S-layer proteins by time-resolved laser-induced fluorescence spectroscopy (TRLFS). S-layer proteins serve as protective biointerfaces in bacteria and archaea against the surrounding solution. Experimental assays were performed at a fixed total concentration of Cm(III) (0.88 µM) using an S-layer protein (5 g/L / 39.6 µM) at varying pH levels (2.0 to 9.0), as well as several types of S-layer proteins of L. sphaericus JG-A12. Based on resulting luminescence spectra and lifetime data, specific and unspecific binding sites could be distinguished. Notably, specific Cm(III) binding to S-layer proteins was confirmed by the appearance of a sharp emission band at 602.5 nm, combined with a long lifetime of 310 µs. The high affinity of these specific binding sites was also verified using competing EDTA, wherein only a high EDTA concentration (40 µM) could efficiently remove Cm(III) from S-layer proteins.