Haloperoxidase Mimics : Heterogeneous and Functional Catalysis of Oxidative Halogenation Reactions

Abstract

Within minutes, all structures in natural waters suffer from biofouling, the colonization of aquatic organisms. This natural phenomenon conserving and expanding habitats as well as species turned out fatal for the marine industry wreaking havoc on shipping and offshore plants each year. Chemical antifouling coatings that are commercially available, currently release toxins like copper and other co-biocides to retard the growth of algae and encrusting organisms. In contrast, Nature’s sophisticated and highly efficient defense strategy against epibiont growth mainly relies on halogenating enzymes, a highly specialized subclass of oxidoreductases. In particular, the enzyme vanadium-haloperoxi-dase (V-HPO) is considered to play an important role in marine organisms by producing brominated organic components within their metabolism as well as in the release of low-molecular hypobromous species. These highly reactive compounds as hypobromous acid and hypobromites (HOBr/BrOˉ) are essential in their defense system against bacterial attack and biofouling by interfering with the colonization process at its lowest level, the bacterial cell-cell communication (“quorum sensing”). This thesis focusses the catalytic activity of these fascinating halogenating enzymes in general as well as possible applications. The first Chapter presents a detailed introduction into the topic including the variety of transition metal V-HPO mimics that had been explored. Additionally, the underlying BASF cooperation project is described with regards to the development of new antifouling paint formulations based on V-HPO enzyme mimetic activity. Of particular significance and inspired by the Belousov-Zhabotinsky reaction, the scope of interest has been extended to cerium dioxide nanopar-ticles (nanoceria) emulating natural iron containing halogenating enzymes. In depend-ency to their morphology and Ce3+/Ce4+ ratio, they radically catalyzed the conversion of bromide ions to reactive intermediates that target bacterial signaling compounds control-ling bacterial quorum sensing. Laboratory and field tests with paint formulations contain-ing 2 wt% of nanoceria showed a reduction in biofouling comparable to copper(I) oxide, the most typical biocidal pigment. Nanoceria has the potential to be the pioneer for highly stable, non-soluble, non-toxic, low-cost and sustainable substitute for conventional inorganic/organic biocides with high damage in bacterial biofilms but minimal collateral ef-fects to the environment.