| Autor: |
Dent MR; Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States., Milbauer MW; Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States., Hunt AP; Department of Chemistry , University of Michigan , 930 North University Avenue , Ann Arbor , Michigan 48109 , United States., Aristov MM; Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States., Guzei IA; Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States., Lehnert N; Department of Chemistry , University of Michigan , 930 North University Avenue , Ann Arbor , Michigan 48109 , United States., Burstyn JN; Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States. |
| Abstrakt: |
Despite utilizing a common cofactor binding motif, hemoproteins bearing a cysteine-derived thiolate ligand (heme-thiolate proteins) are involved in a diverse array of biological processes ranging from drug metabolism to transcriptional regulation. Though the origin of heme-thiolate functional divergence is not well understood, growing evidence suggests that the hydrogen bonding (H-bonding) environment surrounding the Fe-coordinating thiolate influences protein function. Outside of X-ray crystallography, few methods exist to characterize these critical H-bonding interactions. Electron paramagnetic resonance (EPR) spectra of heme-thiolate proteins bearing a six-coordinate, Fe(III) heme exhibit uniquely narrow low-spin ( S = 1/2), rhombic signals, which are sensitive to changes in the heme-thiolate H-bonding environment. To establish a well-defined relationship between the magnitude of g -value dispersion in this unique EPR signal and the strength of the heme-thiolate H-bonding environment, we synthesized and characterized of a series of six-coordinate, aryl-thiolate-ligated Fe(III) porphyrin complexes bearing a tunable intramolecular H-bond. Spectroscopic investigation of these complexes revealed a direct correlation between H-bond strength and g -value dispersion in the rhombic EPR signal. Using density functional theory (DFT), we elucidated the electronic origins of the narrow, rhombic EPR signal in heme-thiolates, which arises from an Fe-S p π -d π bonding interaction. Computational analysis of the intramolecularly H-bonded heme-thiolate models revealed that H-bond donation to the coordinating thiolate reduces thiolate donor strength and weakens this Fe-S interaction, giving rise to larger g -value dispersion. By defining the relationship between heme-thiolate electronic structure and rhombic EPR signal, it is possible to compare thiolate donor strengths among heme-thiolate proteins through analysis of low-spin, Fe(III) EPR spectra. Thus, this study establishes EPR spectroscopy as a valuable tool for exploring how second coordination sphere effects influence heme-thiolate protein function. |
