Ruthenium(II) complexes with a three-legged piano-stool
structure
based on an arene ring, an N-heterocyclic carbene
(NHC)-carbene ligand with a peracetylated glucose moiety and two chlorides
or one bidentate ligand, were prepared and characterized by spectroscopy
and crystallography. In one case, the sugar substituent is replaced
by an ethyl. The chirality of the sugar results in the formation of
two diastereomers that interconvert through rotations around the Ccarbene–Ru and Carene–Ru bonds. In
water, the complexes undergo a series of equilibrium hydrolysis steps
involving the Ru–Cl bonds. The Ru–arene bond and the
sugar acetyls are also partially hydrolyzed, and the selectivity of
the process is governed by the nature of the arene and the pH of the
solution. The reactivity of the compounds was studied against model
nucleophiles and biological macromolecules. In the former case, mass
experiments demonstrated a variety of binding modes with a trend reflecting
the stability in aqueous environment. In the latter case, protein
crystallography was used to characterize the preferential binding
sites of one of the complexes. The X-ray structure of the adduct formed
upon reaction of one representative complex with hen egg white lysozyme
contains a Ru center, which retains the carbene ligand, close to the
side chain of Asp119. In the adduct with bovine pancreatic ribonuclease,
there are two protein molecules in the asymmetric unit. In one molecule,
two Ru centers are located close to the side chains of His105 and
of His119, which is the protein active site. In the second molecule,
only one Ru center was found in the proximity of the side chain of
His105. The Ru complexes also interact with calf-thymus DNA, although
without displacing the intercalating probe EB. Finally, the complexes
are essentially inactive against the human ovarian carcinoma A2780
cells, the cisplatin-resistant A2780cis cells, and the human embryonic
kidney HEK293T cells.