Second Generation Model Complexes for the Enzyme Purple Acid Phosphatase

Marta Zajaczkowski-Fischer (2010). Second Generation Model Complexes for the Enzyme Purple Acid Phosphatase PhD Thesis, School of Chemistry & Molecular Bioscience, The University of Queensland.

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Author Marta Zajaczkowski-Fischer
Thesis Title Second Generation Model Complexes for the Enzyme Purple Acid Phosphatase
School, Centre or Institute School of Chemistry & Molecular Bioscience
Institution The University of Queensland
Publication date 2010-09
Thesis type PhD Thesis
Supervisor Prof. Dr. Graeme R. Hanson, CAI, UQ
Prof. Dr. Peter Comba, Inst. Inorg. Chem., University of Heidelberg
Total pages 194
Total colour pages 192
Total black and white pages 2
Language eng
Subjects 03 Chemical Sciences
Abstract/Summary The present thesis is concerned with the development and characterization of new diferric purple acid phosphatase (PAP) model systems, which include functional groups that are meant to act as a second coordination sphere in phosphoester hydrolysis. A short review on purple acid phosphatases, including the postulated reaction mechanisms in phosphoester hydrolysis, and published PAP model complexes is given in Chapter 2. Furthermore, important data of published bridged diferric complexes are presented, in order to have a basis for interpretation of the analytical data in the results part. In Chapter 3, the results on the new model complexes are presented and discussed. At first, the cyclam-based ligand L1 and its coordination chemistry are described (Chapter 3.1). The most important findings are the following: Ligand L1 predominantly forms a μ-oxo bridged diferric complex when reacted with [FeCl4]- in situ. This complex, called K1, can readily coordinate phosphate and inactive phosphoesters, also in partly aqueous solution. Interestingly, the inactive phosphodiester diphenylphosphate (DPP) coordinates in a monodentate mode to one FeIII, whereas the monoesters para-nitrophenylphosphate (pNPP) and 1-naphthylphosphate (1-NP) bind in a bridging mode to both FeIII centers. The monodentate coordination of DPP is encouraging in terms of the intended reactivity in phosphoester hydrolysis, as this coordination mode is believed to be the active one, leading to a terminal hydroxide as a possible nucleophile. Chapter 3.2 deals with the phenolatebased ligands HL2 and H3L3 and their coordination chemistry. These ligands are derivatives of the published HBPMP ligand and incorporate amino and amido functional groups as second coordination sphere mimics. Diferric complexes, called K2 and K3, are obtained by in situ reaction of the ligands with a ferric salt. A spectrophotometric pH titration was performed and revealed the pH dependent species distribution and the corresponding pKa values of the complex solutions. K2 has three quilibria between pH 4.6 and 11, where the second equilibrium is the deprotonation of the second coordination sphere amines. In contrast, K3 has only two equilibria due to the low pKa of the amido protons. Regarding the coordination of phosphoesters, K2 shows a similar behavior to K1. DPP is coordinated monodentately to one FeIII, whereas pNPP and 1-NP form bridging complexes. This is not observed with K3, which shows bridging coordination with both, mono- and diesters, possibly due to the lack of hydrogen bond donors in the second coordination sphere. The complexes K1, K2 and K3 were tested for hydrolytic activity towards the activated phosphoester substrates bis-(2,4-dinitrophenyl)phosphate (BDNPP) and 2,4-dinitrophenylphosphate (DNPP). The results of these experiments are presented in Chapter 3.3. K1 and K2 are the first examples of PAP model complexes that catalyze the hydrolysis of the phophomonoester DNPP. So far, only diester hydrolysis with PAP model complexes has been reported in literature, while the inactive bridging coordination mode is observed for phosphomonoesters. We draw the fact, that K1 and K2 can hydrolyze monoesters, back to the hydrogen bonding interaction of the coordinated substrates to the remote ligand parts. A closer analysis of the reactivity of K1 towards DNPP and BDNPP, based on DFT calculations, shows that 1) BDNPP is stabilized by the interaction with the protonated cyclam in the monodentate coordination mode, 2) the hydrolysis of DNPP has a significantly lower activation barrier with the hydrogen bonding interactions than without and 3) this barrier is lower than the energy barrier to a bridging coordination mode. As a conclusion, a mechanism is proposed, where the substrate binds in a monodentate coordination mode and is subsequently attacked by a terminal hydroxide. This active species is in equilibrium with the inactive bridging complex. In the case of the diester BDNPP, the equilibrium is shifted to the active species, while the monoester DNPP is more stable in the bridging coordination mode. The hydrolysis product remains bound to K1 and inhibits catalysis.
Keyword biomimicry
purple acid phosphatase
secondary interactions
diferric complex
EPR spectroscopy
molecular modelling
Additional Notes 3-194

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Created: Wed, 16 Nov 2011, 04:21:43 EST by Marta Zajaczkowski-fischer on behalf of Library - Information Access Service