Copper(II) monohelix complexes with pyrazine-modulated long-chain oligo-α-aminopyridine ligand: synthesis, crystal structures, and bioactivity studies

Using the pyrazine-modulated pentapyridyltetraamine ligand N-2,N-2 '-(pyridine-2,6-diyl)bis(N-6-(pyrazin-2-yl)pyridine-2,6-diamine) H4N9-2pz, three new mononuclear complexes [Cu(H4N9-2pz)](NO3)(2)center dot(CH3CN) 1, [Cu(H4N9-2pz)]Cl-2 center dot 2(H2O) 2 and [Cu(H3N9-2pz)] (CH3COO)center dot 2.5(H2O) 3 have been synthesized, structurally characterized, and their bioactivity properties studied. In vitro analysis of complexes 1-3 revealed that they inhibited AChE and BChE more effectively than the widely available inhibitor tacrine (IC50: 123.58 +/- 6.80 and 146.18 +/- 7.91 mu M). Additionally, their IC50 values for AChE and BChE ranged from 32.87 to 68.15 and 14.60 to 31.68 mu M, respectively. The g(

) and g(perpendicular to) components of the g factor have similar values in the EPR spectrum of complexes 1-3 (g(

) = 2.0179 and g(perpendicular to) = 2.1246 for 1; g(

) = 2.0044 and g(perpendicular to) = 2.1425 for 2; g(

) = 2.0118 and g(perpendicular to) = 2.1356 for 3). The single crystal X-ray approach revealed a distorted trigonal bipyramidal geometry of the complexes, which is consistent with the inverted type of EPR spectra (g(perpendicular to) > g(

)similar to ge), measured magnetic moment susceptibility, and electronic spectrum studies. The comparable structures of complexes 1-3 suggest that the H4N9-2pz ligand, rather than the nitrate, carboxylate, or chloride counterions, is critical for complex formation.