Dimeric Fe (II, III) complex of quinoneoxime as functional model of PAP enzyme: Moessbauer, magneto-structural and DNA cleavage studies

Purple acid phosphatase, (PAP), is known to contain dinuclear Fe2+2,+3 site with characteristic Fe+3 <- Tyr ligand to metal charge transfer in coordination. Phthiocoloxime (3-methyl-2-hydroxy-1,4-naphthoquinone-1-oxime) ligand L, mimics (His/Tyr) ligation with controlled and unique charge transfers resulting in valence tautomeric coordination with mixed valent diiron site in model compound Fe-1: [μ-OH-Fe2+2,+3 (o-NQCH3ox) (o-NSQCH3ox)2 (CAT) H2O]. Fe-2: [Fe+3(o-NQCH3ox) (p-NQCH3ox)2]2 a molecularly associated dimer of phthiocoloxime synthesized for comparison of charge transfer. 57Fe Moessbauer studies was used to quantitize unusual valences due to ligand in dimeric Fe-1 and Fe-2 complexes which are supported by EPR and SQUID studies. 57Fe Moessbauer spectra for Fe-1 at 300 K indicates the presence of two quadrupole split asymmetric doublets due to the differences in local coordination geometries of [Fe+3]A and [Fe+2]B sites. The hyperfine interaction parameters are δA = 0.152, (ΔEQ)A 0.598 mm/s with overlapping doublet at δB = 0.410 and (ΔEQ)B 0.468 mm/s. Due to molecular association tendency of ligand, dimer Fe-2 possesses 100% Fe+3(h.s.) hexacoordinated configuration with isomer shift δ = 0.408 mm/s. Slightly distorted octahedral symmetry created by NQCH3ox ligand surrounding Fe+3(h.s.) state generates small field gradient indicated by quadrupole split ΔEQ = 0.213 mm/s. Decrease of isomer shifts together with variation of quadrupole splits with temperature in Fe-1 dimer compared to Fe-2 is result of charge transfers in [Fe2+2,+3 SQ] complexes. EPR spectrum of Fe-1 shows two strong signals at g1 = 4.17 and g2 = 2.01 indicative of S = 3/2 spin state with an intermediate spin of Fe+3(h.s.) configuration. SQUID data of χmcorr.T were best fitted by using HDVV spin pair model S = 2, 3/2 resulting in antiferromagnetic exchange (J = -13.5 cm-1 with an agreement factor of R = 1.89 x 10-5). The lower J value of antiferromagnetic exchange leads to Fe+3μ-(OH) Fe+2 bridging in Fe-1 dimer instead of μ-oxo bridge. The intermolecular association through H-bonds may lead to weakly coupled antiferromagnetic interaction between two Fe-2 molecules having Fe+3(h.s.) centers. Using S = 5/2, 5/2 spin pair model we obtained best-fitted parameters such as J = -12.4 cm-1, g = 2.3 with R = 3.58 x 10-5. Synthetic strategy results in non-equivalent iron sites in Fe-1 dimer analogues to PAP enzyme hence its reconstitution results in pUC-19 DNA cleavage activity, as physiological functionality of APase. It is compared with nuclease activity of Fe-2 RAPase.