Highly Covalent Ferric-Thiolate Bonds in Rubredoxin Exhibit Surprisingly Low Mechanical Stability

Abstract

Depending on their nature, different chemical bonds show vastly different stability with covalent bonds being the most stable ones that rupture at forces above nanoNewton. Studies revealed that Fe-thiolate bonds in metalloprotein are highly covalent and are conceived to be of high mechanical stability. Here we used single molecule force spectroscopy techniques to directly determine the mechanical strength of ferric-thiolate bonds in rubredoxin. We observed that the ferric-thiolate bond ruptures at surprisingly low forces of ∼200 pN, one order of magnitude lower than that of typical covalent bonds, such as C-Si, S-S and Au-thiolate bonds. And the mechanical strength of Fe-thiolate bonds is observed to correlate with the covalency of the bonds in different protein systems. Our results shed new lights on the nature of Fe-thiolate bonds and suggest that highly covalent Fe-thiolate bonds are mechanically labile and clearly distinguish themselves from typical covalent bonds.