Binding of Iodomercurates to Sulfhydryl-blocked β-Lactoglobulin-A,-B, and -C

Sulfhydryl-blocked β-lactoglobulins (β-LG-S-SCH₂CH₂OH)-A, -B, and -C bind only one iodomercurate species, HgI₃₋, at only one site, with a dissociation constant of 4.0 x 10₋₅M at 25°, pH 5.0, 0.10 ionic strength. (Binding to native β-LG-SH-A, -B, and -C is more complex, involving the sulfhydryl and two other sites and several iodomercurates). Ther red shift of the HgI₃₋ spectrum on binding would ordinarily suggest a hydrophobic site, but the HgI₃₋ site is distinct from, and independent of, the alkane-binding site of native and blocked β-LG:HgI₃₋ may bind a group that shifts its trigonal planar structure toward the tetrahedron of HgI₄²⁻. Binding of HgI₃₋ to blocked β-LG interferes with the well-known association of β-LG-A to octamers at pH 4.6 and low temperature. The relation of the HgI₃₋ site to the crystallographic iodomercurate-binding sites of β-LG-SH is examined.

To facilitate these and future studies if iodomercurate binding, the 200-400 nm spectra of HgI₂. HgI₃₋, and HgI₄²⁻ in aqueous solutions and the thermodynamic formation constants at 25° for the equilibria HgI₂ + I−=HgI₃₋ (4.9 x 10³ M−¹ and HgI₃₋ + I−=HgI₄²⁻ (0.118 x 10³ M−¹) were obtained.