Planar Tetramers Research Articles

A stereospecific complex of poly(I) with ammonium ion

AbstractWe describe conditions which lead to complete helix formation of poly(I) in the presence of NH. Binding of NH is shown to be specific in the presence of Li+, which does not by itself support helix formation under these conditions. The NH–poly(I) complex is characterized by uv, CD, and ir spectroscopy. The CD spectrum is strikingly different from those of the Na+ or K+ complexes, the first extremum being changed from negative for the metal ions to positive for NH. A stereospecific model is proposed for the NH–poly(I) helix in which the N of NH is located on the axis of the four‐stranded helix, midway between planar tetramers formed by the bases. The model is consistent with the tetrahedral symmetry of NH, the requirement for four acceptable hydrogen bonds, the observed stability of the helix, and the accepted geometry of the backbone.

Preparation of mononuclear and polynuclear cyano-complexes of palladium(II)

The abstraction of one cyanide ion from [Pd(dpe)CN) 2] (dpe = Ph 2PCH 2CH 2PPh 2) with silver perchlorate in acetonitrile solution yields a complex that has the empirical formula [Pd(dpe)CN]ClO 2 and is believed to be a mixture of two planar tetramers 4 and 5 both of which involve linear PdCNPd bridges. One of the tetramers has an idealised C 4h structure each palladium having a P 2CN environment, and the other has an idealised D 2h structure two of the palladiums having P 2C 2 environments and two having P 2N 2 environments.

Study of hydracids trapped in monatomic matrices. I. Near infrared spectra and aggregate structures

The infrared spectra of H35Cl, D35Cl, HBr, DBr and their mixtures in solid argon and krypton in the range 10–30 K are presented. Studies have been made over a wide concentration range, for matrix/hydracid ratio from 1000 to 20 which, together with annealing experiments and H/D isotopic dilutions, allow a complete discussion of polymeric species spectra. In the monomer region, Q features are clearly identified at low M/R ratio and assigned to lattice distortion effects on isolated molecules. A new assignment for dimer bands is proposed, in which the two molecules are not equivalent and characterized by different values of force constants and moment derivatives. The trimer is found cyclic, a configuration already mentioned, the three molecules being equivalent and oriented along the sides of an equilateral triangle. After annealing, a new polymeric species associated with a strong band at lower frequency appears and is identified, in the light of H/D mixture spectra, as a cyclic planar tetramer. From accurate frequency measurements one concludes that the H/D isotopic substitution slightly changes the hydracid force constant, a DX molecule acting as a stronger perturber than a HX one on any partner belonging to the same aggregate. At last the analysis of mixed H35Cl–HBr and HCl–HI spectra enables a comparison between the perturbing power of these three molecules, the decreasing electron donor power being in the order HI, HBr, HCl.

Determination of the correlation time for a slowly reorienting [formula omitted] nucleus: Binding of Na + with the 5′-GMP supramolecular assembly

The 23Na NMR lineshape for Na + exchanging between the free state in aqueous solution and the bound state on a slowly reorienting macromolecule is the superposition of two lorentzians, corresponding to a fast relaxation (60% of the total intensity) and a slow relaxation (40% of the total intensity). Separation of these two components is achieved by full deconvolution of the experimental lineshape. An alternate and more easily implemented procedure uses only measurement of the linewidth at one-half and one-eight of the total height. We apply both methods to the determination of the correlation time τ c = 4–9 nsec for Na + bound to the aggregates resulting from self-assembly, in D 2O solution, of the 5′-guanosine monophosphate nucleotide (0.1 M), in the presence of K + ions (0.4–1 M). These correlation times are discussed in terms of the structure of these aggregates, involving plausibly a C 2 h dimer obtained from the interaction of planar tetramers whose core includes a K + ion.

A re-examination of the electron microscopic appearance of pyruvate carboxylase from chicken liver.

Electron microscopic studies of chicken liver pyruvate carboxylase conducted under a variety of conditions show that this enzyme has an overall rhombic appearance and is comprised of four nonspherical subunits. The square planar tetramers originally identified by Valentine et al. (Valentine, R.C., Wrigley, N.G., Scrutton, M.C., Irias, J.J., and Utter, M.F. (1966) Biochemistry 5, 3111-3116) as pyruvate carboxylase have been shown to represent a minor protein contaminant found in many impure preparations of this enzyme. The contaminating protein has been separated from pyruvate carboxylase and further purified. It does not contain biotin and its constituent polypeptides are smaller than those of pyruvate carboxylase. This protein, whose function is not yet identified, shows a strong tendency to aggregate and is highly visible under many conditions of electron microscopy. Several lines of evidence support the thesis that the nonsquare tetramers are pyruvate carboxylase. When essentially homogeneous material is examined with a variety of different negative stains, numbers of intact molecules represent 20 to 70% of the visible protein. These tetramers, like pyruvate carboxylase, are very cold-labile and are protected from dissociation under these conditions by acetyl-CoA, a specific activator of this enzyme. Also, the structures form complexes with avidin and antibiotin antibody and thus, like pyruvate carboxylase, contain biotin.

The purification and properties of the glutamine synthetase from the cytosol of Soya-bean root nodules.

The major portion of glutamine synthetase activity in root nodules of soya-bean plants is associated with the cytosol rather than with Rhizobium japonicum bacteroids. Glutamine synthetase accounts for about 2% of the total soluble protein in nodule cytosol. Glutamine synthetase from nodule cytosol has been purified by a procedure involving fractionation with protamine sulphate, ammonium sulphate and polypropylene glycol, chromatography on DEAE-Bio-Gel A and Bio-Gel A-5m and affinity chromatography on glutamate-agarose columns. The purified preparation appeared to be homogeneous in the analytical ultracentrifuge. From sedimentation-equilibrium experiments a mol. wt. of about 376000 was determined for the native enzyme and 47300 for the enzyme in guanidinium chloride. From these data and measurements of electron micrographs, we have concluded that glutamine synthetase from nodule cytosol consists of eight subunits arranged in two sets of planar tetramers which form a cubical configuration with dimensions of about 10 nm (100 A) across each side. Glutamine synthetase from nodule cytosol has a higher glycine and proline content and a lower content of phenylalanine than the glutamine synthetase that has been prepared from pea seed. The cytosol enzyme contains four half-cystine molecules per subunit, which is in contrast with two reported for the enzyme from pea seed. Enzyme activity is striking influenced by the relative proportion of Mg2+ and Mn2+ in the assay medium. Activity is inhibited by feedback inhibitors and is influenced by energy charge.

Ordered forms of 5′‐8‐aminoguanylic acid

Abstract5′‐8NH2GMP forms an ordered structure in moderately acid (pD 4.7) solution. We propose for this ordered form a novel hemiprotonated G·G structure with a twofold rotation axis and three hydrogen bonds between each pair of guanine residues. Gel formation does not occur with this nucleotide in either neutral or acid solution. In neutral solution 5′‐8NH2GMP also forms a regular, ordered structure, quite different from the acid form and similar to that formed by 5′‐GMP under the same neutral conditions. We suggest that this ordered structure consists of a regularly stacked array of planar tetramers, similar to that proposed for 3′‐GMP at pH 5.2

Magnetic Resonance Studies on a Gel of Guanylic Acid and a Guanosine Monophosphate-Copper(II) Complex

Abstract GMP 3'-(2')-phosphate solutions were studied with proton magnetic resonance over a range of pH 1.6 to 7.7. It was found that the signal of the protons of the NH2 group appears when the solution gelates (pH 5), indicating that the NH2 group is involved in the binding of the gel. The involvement of N7 as a binding site has been confirmed from results on the disruption of the gel by Cu++ ions, and proton magnetic resonance evidence for stacking is given by the pH dependence of the chemical shift of the H8 resonance. These findings are in agreement with Gellert's model (Gellert, M., Lipsett, M. N., and Davies, D. R., Proc. Nat. Acad. Sci. U.S.A. 48, 2013 (1962)) of the molecular structure of the gel, in which a helical structure is assumed to be formed by stacking of planar tetramers with hydrogen bonds between NH2 and N7 (and between O6 and H1). In addition, the proton magnetic resonance measurements on GMP-Cu(II) solutions at acid pH indicated that Cu(II) interacts with the GMP base even when N7 is protonated. This is supported by results from electron spin resonance measurements where the g factor anisotropy of the electron spin resonance spectra at pH 2 indicates that Cu++ is still involved in complexes at pH 2.