Structural Dependence of Aromatic Ring Stacking and Related Weak Interactions in Ternary Amino Acid−Copper(II) Complexes and Its Biological Implication

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Abstract

Structures and stabilization due to stacking of ternary copper(II) complexes containing an aromatic amino acid (AA) and an aromatic diamine (DA), Cu(AA)(DA), have been investigated by potentiometric, spectroscopic, and X-ray diffraction methods. For the systems with AA = para-X-substituted l-phenylalanine (l-XPhe; X = H, NO2, OH, NH2) and DA = 2,2‘-bipyridine (bpy) or 1,10-phenanthroline (phen), the difference absorption spectra in the region 320−400 nm exhibited a peak assignable to the charge transfer interaction between the aromatic rings of DA and l-XPhe, the intensity being in the order NH2 > OH > H ≫ NO2 with respect to X. The stability constants of Cu(AA)(DA) determined for AA = dl-XPhe (X = F, Cl, Br) and l-XPhe (X = NH2, NO2, I) at 25 °C and I = 0.1 M (KNO3) indicated that stabilization of Cu(l-XPhe)(DA) relative to Cu(l-Ala)(en) (Ala = alanine; en = ethylenediamine) is in the order Br > OH > Cl ≈ NH2 > NO2 ≥ H ≥ F. The structures of [Cu(l-NH2Phe)(bpy)]NO3·H2O (1), [Cu(l-Tyr)(phen)]ClO4·2.5H2O (2), [Cu(l-Phe)(phen)]Cl·3H2O (3), and [Cu(l-Phe)(bpy)]ClO4·H2O (4), isolated as crystals, were determined by the X-ray diffraction method: 1, orthorhombic, P212121, a = 10.292(1) Å, b = 13.576(4) Å, c = 14.407(1) Å, V = 2012.9 Å3, Z = 4, R = 0.037, Rw = 0.038; 2, orthorhombic, P212121, a = 18.20(2) Å, b = 32.63(1) Å, c = 8.14(1) Å, V = 4833 Å3, Z = 4, R = 0.111, Rw = 0.087; 3, monoclinic, P21, a = 11.738(2) Å, b = 16.301(1) Å, c = 11.795(1) Å, β = 102.01(1)°, V = 2207.4 Å3, Z = 2, R = 0.045, Rw = 0.035; 4, monoclinic, P21, a = 9.954(2) Å, b = 24.179(3) Å, c = 9.780(2) Å, β = 107.32(1)°, V = 2257.1 Å3, Z = 4, R = 0.065, Rw = 0.060. All of the complexes have a similar distorted square-pyramidal structure around the central Cu(II) ion. While 1 and 2 have a structure involving aromatic ring stacking in the solid state, 4 has a structure without it and 3 has both types of structures in the unit cell.

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