Monoesterase activity of a purple acid phosphatase mimic with a cyclam platform

The synthesis and characterization of a novel dinucleating ligand L (L=4,11-dimethyl-1,8-bis{2-N-(di-2-pyridylmethyl)amino]ethyl}cyclam) and its μ-oxo-bridged diferric complex (H(2)L){Fe(III)(2)(O)}(Cl)(4)](2+) are reported. This diiron(III) complex is the first example of a truly functional purple acid phosphatase (PAP) mimic as it accelerates the hydrolysis of the activated phosphomonoester 2,4-dinitrophenyl phosphate (DNPP). The spectroscopic and kinetic data indicate that only substrates that are monodentately bound to one of the two ferric ions can be attacked by a suitable nucleophile. This is, most probably, a terminal iron(III)-bound hydroxide. DFT calculations support this assumption and also highlight the importance of secondary interactions, exerted by the protonated cyclam platform, for the positioning and activation of the iron(III)-bound substrate. Similar effects are postulated in the native enzyme but addressed in PAP mimics for the first time.