title: Optical and TDPAC spectroscopy of Hg(II)-rubredoxin: model for a mononuclear tetrahedral [Hg(CysS)4]2- center. ISOLDE Collaboration.
abstract: Rubredoxins possess a well-defined mononuclear tetrahedral tetrathiolate metal binding site, a feature exploited by several investigations to study the spectroscopic characteristics and the coordination chemistry of different metal ions at this binding site. In the present work, Hg(II)-substituted rubredoxin (Rd) from Desulfovibrio gigas has been studied by electronic absorption, circular dichroism (CD), magnetic circular dichroism (MCD), and time differential perturbed angular correlation of gamma-rays (TDPAC) spectroscopies. The TDPAC spectrum of 199mHg-Rd at pH 8 exhibits a prevailing nuclear quadrupole interaction (NQI) with a precession frequency of omega1=0.09 Grad/s and an asymmetry parameter eta=0, features characteristic of a slightly distorted tetrahedral tetrathiolate metal coordination, i.e, a HgCysS4 center. In addition, three minor populated NQIs have also been detected. They may represent a trigonal HgS3 (omega1=1.13 Grad/s, eta=0.21), a digonal HgS2 (omega1= 1.34 Grad/s, eta =0.20), and a digonal Hg(II) coordination (omega = 1.58 Grad/s, eta =0.18) with unidentified ligands. Since similar studies at pH 2.5 revealed a time-dependent increase of the HgCysS4 population, the low populated sites may represent intermediate Hg(II) complexes formed prior to the generation of the thermodynamically stable structure. The metal-induced absorption envelope of Hg-Rd reveals three distinct transitions with Gaussian-resolved maxima located at 230, 257, and 284 nm, which are paralleled by dichroic features in the corresponding difference CD spectrum of Hg(II)-Rd versus apo-Rd. Based on the optical electronegativity theory of J*rgensen, the lowest energy transition has been attributed to a CysS-Hg(II) charge-transfer excitation. The Td type of metal coordination in Hg-Rd is supported by the presence of an unresolved A-term with a negative lobe at 295 nm in the difference MCD spectrum. These results point to the usefulness of optical and TDPAC spectroscopies for studying Hg(II) sites in other proteins.
authors.affiliation: Institute of Biochemistry, University of Zürich, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Faller
authors.forename: B
authors.initials: B
authors.lastname: Ctortecka
authors.forename: W
authors.initials: W
authors.lastname: Tröger
authors.forename: T
authors.initials: T
authors.lastname: Butz
authors.forename: M
authors.initials: M
authors.lastname: Vasák
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002942:Circular Dichroism; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism; D003901:Desulfovibrio / Q000737:chemistry* / Q000378:metabolism; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D008280:Magnetics; D008628:Mercury / Q000737:chemistry*; D012416:Rubredoxins / Q000737:chemistry* / Q000378:metabolism; D014965:X-Rays
chemical_list: D001426:Bacterial Proteins; D012416:Rubredoxins; D008628:Mercury; D003545:Cysteine