Coordinate Square Research Articles

Interactions between copper (II) ions and l-threonine, l- allo-threonine and l-serine in aqueous solution

In order to understand the role of the functional side chains in polypeptides and proteins when a metal ion is present, a preliminary investigation of interactions between Cu(II) and l-serine (Ser), l-threonine (Thr), and l- allo-threonine (αThr) has been undertaken. The role of the lateral hydroxyl group was clarified by comparison with O-substituted l-threonine and l-serine. For α-amino-β-hydroxy acids, equilibrium analyses confirmed the formation of the species: Cu-A +, Cu-A 2, Cu-H −1 A 2-, and Cu-H −2 A 2− (A, any amino acid), and enabled the calculation of the corresponding stability constants in 1 M NaClO 4 (25°C). Visible absorption and circular dichroism measurements lead to the conclusion that, at neutral pH, the ligands maintain their “glycine-like” configuration in Cu-A 2, which is modified when one proton from each bound ligand dissociates at pH> 9. The N(amino) and O −(hydroxyl) atoms are donors in the coordination square while the O −(carboxyl) donor atom may compete with O(water) for the axial position of Cu(II), leading to a distorted octahedral structure.

On the structure of manganese (II)- and copper (II)-histidine complexes

l-Histidine offers three groups as potential binding sites for metal ions: the imidazole, amino, and carboxylic acid groups. For stereochemical and geometric reasons, Cu(II) can bind strongly to only two of the three groups at a given moment. There is a contradiction in the literature as to which of the three groups coordinate with the coordination square of Cu(II). On the one hand, it has been suggested that the deprotonated ligand coordinates at the imidazole and the amino group, and on the other hand with the amino and the carboxylic acid function. Most of the evidence presented for one or the other possibility derives from indirect methods; e.g., the determination of stability constants. Since evidence has been given that only those groups are oxidized by H 2O 2 which are in the coordination sphere of Cu(II), this peroxidase-like “probe” was used to investigate 1:1 molar mixtures of Cu(II) and l-histidine or histamine. The results suggested that l-histidine forms a histamine-like complex with Cu(II). NMR spectra of l-histidine and histamine taken in D 2O with increasing amounts of Cu(II), under conditions where Cu(II) 1:2 complexes are formed quantitatively, show also that the imidazole group is involved in the formation of the Cu(II)-histidine 1:2 complex. The NMR spectra of Mn(II) complexes were investigated for the same ligands as with Cu(II). The results show that Mn(II) interacts more strongly with histidine than with histamine, in agreement with estimates of the degree of complex formation. All three binding sites of histidine can bind to Mn(II). The structures of the several species of l-histidine complexes are discussed, as are the known stability and acidity constants. The pH regions in which the protonated species transform to the deprotonated ones are mentioned, as are the structures of some known mixed; i.e., ternary, l-histidine Cu(II)-amino acid complexes.

Molecular Magnetic Susceptibility Tensor in Sulfur Dioxide

The diagonal elements in the magnetic susceptibility tensor in the principal molecule-fixed inertial axis system in SO2 have been determined by observing the molecular rotational Zeeman effect with a high-resolution microwave spectrometer. The signs of gaa, gbb, gcc have been found to be negative from theroetical considerations. The principal values of the magnetic susceptibility tensor, paramagnetic susceptibility tensor, and diamagnetic susceptibility tensor are, respectively: χaa = − 14.4 ± 1.2, χbb = − 19.6 ± 0.5, χcc = − 20.6 ± 0.6; χaap = 38.9 ± 0.3, χbbp = 129.3 ± 1.4, χccp = 139.5 ± 1.2; χaad = − 53.3 ± 1.2, χbbd = − 148.9 ± 1.5, χccd = − 160.1 ± 1.3 (all in units of 10−6 erg/G2·mole). The ground-state averages of the sum of the squares of the electronic coordinates, and approximate values of the diagonal components of the quadrupole moment tensor have also been determined.

Presence of Polarization Bonds in some Copper (II) Complexes

CRYSTAL structure determinations have shown that although the co-ordination of the metal in molecules of bisacetylacetonatocopper(II)1, NN′-ethylenebis(acetylacetoneiminato)copper(II)2 and bissalicylaldehydatocopper-(II)3 is essentially square planar the molecules pack such that there is an axial approach (that is, perpendicular to the co-ordination square) of 3.1–3.4 Å from the copper to a conjugated region of an adjacent molecule. Whereas such an intermolecular approach is not abnormally short it is accompanied by a distinct distortion of the molecules from their expected shape, clearly indicating that there is an interaction between them. It has been suggested that these are examples of polarization bonding, the π-bond system acting as an electron donor and the copper atom as acceptor2.

The Quantum Theory of Quasi-Unimolecular Gas Reactions

In the discussion of [Eq.] (1) they assumed that the energy expression for the molecule consisted of a sum of squares of coordinates and momenta, and treated the problem by the methods of classical statistical mechanics. In this paper the former assumption is retained, but we will attempt to indicate the nature of the modifications which must be made when the motions of the molecule are quantized.