Spectroscopic and computational studies of metal-thiolate interactions in metalloenzymes and related model complexes.

摘要

The geometric and electronic structures of various transition metal-thiolate complexes have been explored using an assortment of spectroscopic methods, including electronic absorption, circular dichroism (CD), magnetic CD (MCD), resonance Raman, and electron paramagnetic resonance (EPR) spectroscopies, in conjunction with numerous computational methods, such as density functional theory (DFT) and semiempirical INDO/S-CI calculations. This combined spectroscopic/computational approach has been employed to investigate several metalloenzymes that feature metal-thiolate coordination; namely, nickel superoxide dismutase (NiSOD), iron superoxide reductase (FeSOR), and iron-only hydrogenase (FeHases). Studies of these biological systems were conducted on the actual enzyme, as in the case of NiSOD, or on synthetic complexes that mimic the structure and/or function of the enzyme active sites.; Spectroscopic and computational studies of NiSOD have provided a detailed understanding of the electronic structure of the enzyme active site in both its oxidized and reduced states, thereby elucidating the role of the unique ligand set in tuning the Ni center for catalysis. The catalytic mechanism of NiSOD was probed with substrate analogues of superoxide, like azide and hydrogen peroxide, and the resulting spectral data favor an outer sphere mechanism. Additionally, detailed characterization of a series of M2+ complexes with N4S-ligation (M = Mn, Fe, Co, Zn) has yielded fundamental insights into the nature of metal-thiolate bonding interactions, with significant implications for SOR and thiolate-alkylation enzymes.; The active sites of FeHases feature an unusual polynuclear iron-sulfur cluster, known as the H-cluster, that consists of a [Fe4S 4] cubane linked to a diiron subunit. Herein, the DFT-based broken symmetry approach was utilized to explore the geometric, electronic, and magnetic properties of the entire H-cluster. These theoretical studies of the enzyme active site were complimented with extensive spectroscopic and computational investigations of synthetic diiron complexes that model the [2Fe] component of the H-cluster. These models permitted the development of a more complete understanding of the actual H-cluster, and also provided ideal systems for assessing the ability of DFT to accurately compute absorption spectra and vibrational frequencies for dinuclear transition-metal complexes.; Finally, high-field EPR (HF-EPR) and variable-temperature variable-field (VTVH) MCD spectroscopies were employed to quantitatively determine the zero-field splitting parameters for systems difficult to examine with conventional EPR; namely, high-spin Co2+ and V3+ complexes.