Pd-Oxazolone complexes conjugated to an engineered enzyme: improving fluorescence and catalytic properties.

Different Pd-complexes containing orthometallated push-pull oxazolones were inserted by supramolecular Pd-amino acid coordination on two genetically engineered modified variants of the thermoalkalophilic Geobacillus thermocatenolatus lipase (GTL). The Pd-lipase conjugation was performed on the solid phase in the previously immobilized form of GTL at mild conditions, and soluble conjugated Pd-GTL complexes were obtained by simply desorbing by washing with acetonitrile aqueous solution. Three different Pd complexes were incorporated to two different genetically modified enzyme variants, one containing all the natural cysteine residues changed by serine residues, and another variant including an additional Cys mutation directly in the catalytic Serine (Ser114Cys). The new Pd-enzyme conjugates were fluorescent even at ppm concentrations, while in same conditions free Pd complexes did not show fluorescence at all. The Pd conjugation with the enzyme extremely increases the catalytic profile of the corresponding Pd complex from 200 to almost 1000-fold in the hydrogenation of arenes in aqueous media, achieving in the case of GTL conjugated with orthopalladated 4a an outstanding TOF value of 27,428 min-1. Also the applicability of GTL-C114 conjugated with orthopalladated 4b in a site-selective C–H activation reaction under mild conditions have been demonstrated. Therefore, the Pd incorporation to the enzyme produces a highly stable conjugate, and improves remarkably the catalytic activity and selectivity, as well as the fluorescence intensity of the Pd complexes.