Comparison of the Binding of Cadmium(II), Mercury(II), and Arsenic(III) to the de Novo Designed Peptides TRI L12C and TRI L16C

Designed α-helical peptides of the TRI family with a general sequence Ac-G(LKALEEK)4G-CONH2 were used as model systems for the study of metal−protein interactions. Variants containing cysteine residues in positions 12 (TRI L12C) and 16 (TRI L16C) were used for the metal binding studies. Cd(II) binding was investigated, and the results were compared with previous and current work on Hg(II) and As(III) binding. The metal peptide assemblies were studied with the use of UV, CD, EXAFS, 113Cd NMR, and 111mCd perturbed angular correlation spectroscopy. The metalated peptide aggregates exhibited pH-dependent behavior. At high pH values, Cd(II) was bound to the three sulfurs of the three-stranded α-helical coiled coils. A mixture of two species was observed, including Cd(II) in a trigonal planar geometry. The complexes have UV bands at 231 nm (20 600 M-1 cm-1) for TRI L12C and 232 nm (22 600 M-1 cm-1) for TRI L16C, an average Cd−S bond length of 2.49 Å for both cases, and a 113Cd NMR chemical shift at 619 ppm (CdII(TRI L12C)3 -) or 625 ppm (CdII(TRI-L16C)3 -). Nuclear quadrupole interactions show that two different Cd species are present for both peptides. One species with ω0 = 0.45 rad/ns and low η is attributed to a trigonal planar Cd−(Cys)3 site. The other, with a smaller ω0, is attributed to a four-coordinate Cd(Cys)3(H2O) species. At low pH, no metal binding was observed. Hg(II) binding to TRI L12C was also found to be pH dependent, and a 3:1 sulfur-to-mercury(II) species was observed at pH 9.4. These metal peptide complexes provide insight into heavy metal binding and metalloregulatory proteins such as MerR or CadC.