Solid-State Structural Characterization of Cutinase-ECE-Pincer-Metal Hybrids

The first crystal structures of lipases that have been covalently modified through site-selective inhibition by different organometallic phosphonate-pincer-metal complexes are described. Two ECE-pincer-type d(8)-metal complexes, that is, platinum (1) or palladium (2) with phosphonate esters (ECE = [(EtO)-(O=)P(-O-C6H4-(NO2)-4)(-C3H6-4-(C6H2-(CH2E)(2))](-); E = NMe2 or SMe) were introduced prior to crystallization and have been shown to bind selectively to the Ser(120) residue in the active site of the lipase cutinase to give cut-1 (platinum) or cut-2 (palladium) hybrids. For all five presented crystal structures, the ECE-pincer-platinum or -palladium head group sticks out of the cutinase molecule and is exposed to the solvent. Depending on the nature of the ECE-pincer-metal head group, the ECE-pincer-platinum and palladium guests occupy different pockets in the active site of cutinase, with concomitant different stereochemistries on the phosphorous atom for the cut-1 (S-p) and cut-2 (R-p) structures. When cut-1 was crystallized under halide-poor conditions, a novel metal-induced dimeric structure was formed between two cutinase-bound pincer-platinum head groups, which are interconnected through a single R-Cl bridge. This halide-bridged metal dimer shows that coordination chemistry is possible with protein-modified pincer-metal complexes. Furthermore, we could use NCN-pincer-platinum complex 1 as site-selective tool for the phasing of raw protein diffraction data, which shows the potential use of pincer-platinum complex 1 as a heavy-atom derivative in protein crystallography.