Solid-state structural characterization of cutinase-ECE-pincer-metal hybrids

Abstract

The first crystal structures of lipases that have been covalently modified through site-selective inhibition by different organometallic phosphonatepincer– metal complexes are described. Two ECE-pincer-type d8-metal complexes, that is, platinum (1) or palladium (2) with phosphonate esters (ECE=[(EtO)-(O=)PACHTUNGTRENUNG(-O-C6H4-ACHTUNGTRENUNG(NO2)- 4)ACHTUNGTRENUNG(-C3H6-4-ACHTUNGTRENUNG(C6H2-ACHTUNGTRENUNG(CH2E)2)] ; E= NMe2 or SMe) were introduced prior to crystallization and have been shown to bind selectively to the Ser120 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–platplatinum 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 (SP) and cut-2 (RP) structures. When cut-1 was crystallized under halide-poor conditions, a novel metalinduced dimeric structure was formed between two cutinase-bound pincer– platinum head groups, which are interconnected through a single m-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.