Circular double-helicate

Much work by Lehn et al. has been dedicated to the self-assembly of helical coordination complexes. Recently, ligand 59 has been shown to self-assemble with five iron(II) ions to form an intricate circular double helicate of 10" charge which surrounds and encapsulates a chloride ion. This chloride ion seemingly cannot be exchanged for larger ions as the inner core of the helicate = 1.75 A) is nearly... 

Hasenknopf B, Lehn JM, Kneisel BO, Baum G, Fenske D, Self assembly of a circular double helicate, Angew. Chem. Int. Ed. Engl., 35 1838-1840, 1996. 

DNA Single, circular, double-helical molecule densely coiled, not bound to histones not membrane bound Very long linear molecules with extensive noncoding regions complexed histones and other proteins organized into chromosomes in membrane-bounded nucleus... 

What feature of circular, double-helical DNA is essential for it to function as a vector for DNA cloning ... 

Figure 21 Cartoon representation of circular double-helicate [5] cH 33 with included Cl...

In the case of the twisted circular double-helical DNA, the introduction of a nick anywhere in one strand (readily done with... 

As the results of molecular hybridization and determination of base sequence have shown, RNA formed during phage reproduction in vivo on the basis of the circular double-helical replicative form of DNA (Hayashi et al., 1964) is complementary to only one of the two DNA chains, i.e., to that which is complementary, in turn, to the single-stranded DNA of the mature phage. In vitro when the circular structure of DNA is disturbed, both strands of replicative DNA can act as substrate for RNA-polymerase. The single-stranded DNA of this phage can also act as template for RNA synthesis in vitro, with the preliminary formation of DNA/ RNA hybrids (Chamberlin and Berg, 1964). 

The unmodified and complementary oligonucleotides were also synthesized, in order to detect thermodynamic and spectroscopic differences between the double helices. Circular dichroism spectra revealed that the covalently bound anthracene does not stack in the centre of the DNA double helix. Mutagenic activity by intercalative binding of the anthracene residue is thus unlikely. Only in vitro and in vivo replication experiments with site-specifically modified... 

Ho et al. were able to verify the a-helical shape of the polymer by circular dichroism (CD) spectra. No structural elements were observed until the formation of the double helical DNA at which point they observed a right-handed a-helix in the polythiophene backbone. Their work demonstrates the power of fluorometric detection as they noted a seven order of magnitude increase in detection sensitivity (20 fM in 200 pi) simply through the use of fluorometric detection as opposed to UV-vis absorption. The polymer in solution has a high fluorescence yield with a maximum at 530 nm (Fig. 11a). Upon formation of the duplex the fluorescence is significantly quenched (Fig. lib), while with the addition of the complementary DNA and triplex formation, the fluorescence intensity is enhanced by a factor of 5 (Fig. 11c). The inherent sensitivity of the spectral shift even allowed distinction between DNA with only one and two mismatched bases (Fig. lOBd, e). 

Aaij and Borst (1972) have also demonstrated the separation of closed circular DNA double helices from their linear counterparts in polyacrylamide gels, and have found that log-linear calibrations of molecular weight against relative mobility can be set up. A conspectus of the differences in electrophoretic behaviour between three conformational types - linear single-stranded, linear double-stranded and circular double-stranded - is available from the curve relating... 

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