Double-helical complex

H.-Y. Peng, C.-K. Lam, T. C. W. Mak, Z. Cai, W.-T. Ma, Y.-X. Li, H. N. C. Wong, Chiral Rodlike Platinum Complexes, Double Helical Chains and Potential Asymmetric Hydrogenation Ligand Based on Linear Building Blocks ... 

The amylose single helix is believed to be capable of existing in various states of extension, the diameter probably adjusting to the size of the guest molecules when inclusion complexes are formed. When the guest molecules are removed from such complexes, double helices tend to form [23]. Double helix formation is also associated with retrogradation (below). 

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

The double helix model provides a simple explanation for cell division and reproduction. In the reproduction process, the two DNA chains unwind from each other. As this happens, a new matching chain of DNA is synthesized on each of the original ones, creating two double helices. Since the base pairs in each new double helix must match in the same way as in the original, the two new double helices must be identical to the original. Exact replication of genetic data is thereby accomplished, however complex that data may be. 

The atomic structure of this subunit and its complexes with substrate analogs revealed the enzymatic activity of the rRNA backbone. Thus, the ribosome is in fact a ribozyme P Nissen, J Hansen, N Ban, PB Moore, TA Steitz. Science 289 920-930, 2000. Atomic structure of the ribosome s small 30S subunit, resolved at 5 A WM Clemons Jr, JL May, BT Wimberly, JP McCutcheon, MS Capel, V Ramakrishnan. Nature 400 833-840, 1999. The 8-A crystal structure of the 70S ribosome reveals a double-helical RNA bridge between the 50S and the 30S subunit GM Culver, JH Cate, GZ Yusupova, MM Yusupov, HF Noller. Science 285 2133-2136, 1999. 

The reactivity of a series of closely related substances can either be enhanced or inhibited depending on the type and extent of interaction with double-helical DNA. This was shown in a kinetic study of the substitution of ethylenediamine (en) or A-, A- -dimethylethylcncdiamine (Me2en) by thiourea in the palladium(II) complexes [Pd(4,4,-R2bpy)(en)](PF6)2 (R = H or Me), [Pd(en)2](PF6)2, and [Pd(Me2en)2](PF6)2, in water and in the presence of calf thymus DNA.183... 

Fig, 11. Diagrammatic representation of a planar intercalating guest molecule complexed between adjacent base pairs of the double helical DNA host structure. The base pairs and intercalator are represented by stippled rods. Note the increased base pair separation caused by complexation with the guest. 

A dinuclear silver(i) carbene complex, 22, was also isolated using a pyridyl-bridged biscarbene ligand (Scheme 7).65a,65b Complex 22 possessed a double helical structure with Ag-C bond distances of 2.080(4) and 2.087(4) A and a C-Ag-C angle of 165.5(2)°.65a Weak Ag-Ag interaction could also be observed with an Ag-Ag distance of 3.158 A.65a... 

Fig. 18. X-ray crystal structure of the injectable antiarthritic complex gold(I)-aurothiomalate 88 (side-chain on S omitted) showing the double helical chains. The right-handed helix shown contains thiomalate ligands with the R absolute configuration. Au-S bond lengths 2.283 and 2.286 A S-Au-S angles 178.88° and 169.41°. Adapted from (419).

Figure 4 shows CD spectra of (a) the DNA-lipid complex in organic solution containing a small amount of water (CHCl3/Et0H/Fl20 =4 1 0.07, 790 mM of H2O), and (b) native DNA in an aqueous buffer solution (20 mM NaCl, 10 mM Tris, pH 7.8). The DNA-lipid complex shows a positive Cotton effect at 270 nm and a negative Cotton effect at 245 nm similar to native DNA in aqueous solution, which indicates the B-form structure for the DNA strands [11]. Thus, the DNA-lipid complex forms a double helical B-form... 

Cp6 together with the Intercalator acridine orange solved at atoaiic resolution (36). It can be seen tha the 3 end of the double helical RNA fragment has adopted the C2 endo conformation while the S end maintains the normal C3 endo conformation. A large number of intercalator structures of this type have been solved and they have been summaried elsewhere. Sobell and his colleages pointed out that the Intercalation is associated with a modification of the pucker of the ribonucleotide chain on the 3 end of the intercalator (37-38). Although intercalation is generally associated with the conformations similar to those seen in Fig. 10. a number of alternative conformations have been found with more complex Intercalators. 

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