Metal ions circular structures

Lunkley JL, Shirotani D, Yamanari K, Kaizaki S, Muller G. Chiroptical Spectra of a Series of Tetrakis((-i-)-3-heptafluatDbutylyrylcamphorato)lanthanide(in) with an Encapsulated Alkali Metal Ion Circularly Polarized Luminescence and Absolute Chiral Structures for the Eu(III) and Sm(ni) Complexes. Inorg Chem 2011 2011 (50) 12724—12732. 

The intron group I ribozymes feature common secondary structure and reaction pathways. Active sites capable of catalyzing consecutive phosphodi-ester reactions produce properly spliced and circular RNAs. Ribozymes fold into a globular conformation and have solvent-inaccessible cores as quantified by Fe(II)-EDTA-induced free-radical cleavage experiments. The Tetrahy-mem group I intron ribozyme catalyzes phosphoryl transfer between guanosine and a substrate RNA strand—the exon. This ribozyme also has been proposed to use metal ions to assist in proper folding, to activate the nucleophile, and to stabilize the transition state. ... 

The heme moiety provides de novo designed heme proteins with an intrinsic and spectroscopically rich probe. The interaction of the amide bonds of the peptide or protein with the heme macrocycle provides for an induced circular dichroism spectrum indicative of protein-cofactor interactions. The strong optical properties of the heme macrocycle also make it suitable for resonance Raman spectroscopy. Aside from the heme macrocycle, the encapsulated metal ion itself provides a spectroscopic probe into its electronic structure via EPR spectroscopy and electrochemistry. These spectroscopic and electrochemical tools provide a strong quantitative base for the detailed evaluation of the relative successes of de novo heme proteins. 

The beautiful circular tetra- 454 [24], penta- 455 [25] and hexa-nuclear 456 [24] helical structures are built of the ligands each of which coordinates with three metal ions. Interestingly, the self-assembly of trisbipyridine ligand 457 to afford the penta- or hexanuclear complex is governed by the counterion. Namely,... 

The circular dichroism (CD) spectrum of the fully zinc-coordinated (native) ZDD as well as of an apo form is shown in Figure 2A. Removal of all zinc ions to produce the apo form of the domain results in extensive loss of ordered secondary structure as judged by the reduction of molar ellipticity in the CD spectrum. As in other structural zinc-binding domains, we conclude that the free energy associated with the coordination of metal ions is necessary for correct folding of ZDD. 

Inorganic double or triple helices are formed by two or three ligand strands wrapped around linearly disposed metal ions [13], Among cyclic transition metal complexes, circular helicates [nJ cH [ ] "cH is a general notation characterizing circular helicates (cH) with n = number of metal ions and m = helicity (m = 2 for a double helix) have specific features and may be considered as toroidal helices [34]. There are two different kinds of circular helical systems. Some structures self-assemble from the metal ions and the ligands only in the presence of an anion, which could act as a template [34,35,64-67], whereas, in other cases, the circular helicates self-assemble from the metal ions and the ligands alone [68-70]. 

Structures of polyether complexes V. Molecular structure of bis(8-quinolyloxyethyl) ether-rubidium iodide, a linear polyether circularly embracing a metal ion, W. Saenger and B. S. Reddy, Acta Cryst., 1979, B35, 56. 

The SH groups of cysteine residues (177) in the rabbit liver protein metallothionein (MT) and two of its fractions (a-MT, /S-MT) can bind transition metal ions. Thus, formation of the silver complexes Agi2-MT, Agis-MT, Ag6-a-MT and Ag6-/S-MT was determined by circular dichroism (CD), when zinc complexes of metallothionein and its fractions were titrated with Ag(I) ions. The intense CD spectrum of one of the silver complexes was attributed to supercoil formation245. The UVV spectra of bilirubin (168) and its complexes with Cu(II), Ag(I) and Au(III) showed that the complexes had different structures, due to differences of ionic charge and ionic radius between the metal ions238. 

The loop sequence of the tpy-modified DNA strand was designed such that the strand could hybridize to a molecular beacon [Fig. 16(a)] only when in its extended, linear form [Fig. 16(h)]. Upon hybridization to the tpy-modified DNA, the molecular beacon switches from its closed, nonfluorescent form to its open, fluorescent form [Fig. 16(h)]. Both Fe and Zn form very stable [M(qjy)2] complexes and consequently, the circular structure adopted by the tpy-modified DNA strands in the presence of 1 equiv of these metal ions represented a steric barrier for its hybridization to the molecular beacon. The metal ion acted as an allosteric off switch for DNA hybridization (148). This study demonstrated that the incorporation of metal-binding ligands in nucleic acid strands can be used to create metal-dependent allosteric switches for the Watson-Crick interactions between the nucleic acid strands, a mechanism that may find applications in creating artificial systems with regulatory function. In contrast, in the presence of Cu + the DNA hybridization to the molecular beacon was energetically more favorable than the formation of circular stmctures from the tpy-containing DNA strands. Therefore, hybridization of a complementary ssDNA to the... 

A review of expanded porphyrin ligands can be found. The texaphyrins (8) can be considered as 22- 7r-electron benzannulene systems with an IS-tt-electron delocalization path, based on crystal structure data as well as NMR. The cadmium complex of the macrocycle is found to be planar with pentadentate coordination of the macrocycle to cadmium, which becomes seven-coordinate as a result of axial coordination to two pyridine molecules. The cavity is neariy circular with a center-to-nitrogen distance of 2.39 A. Because of the larger size of this macrocycle, metal ion coordination is generally seen with the larger transition metals and lanthanides. 

DNA is prone to structural polymorphism its structure can differ markedly from the classical double helix. For example, under superhelical stress, circular closed DNA can adopt an unusual structure, denoted ff-DNA, in which homopurine-homopyrimidine tracts unwind and utilize the pyrimidine-rich strand to form an intramolecular triplex with another homopurine-homopyrimidine tract the remaining purine single strand is left unpaired. The formation of ff-DNA requires the presence of metal ions such as Mg + or... 

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