Mium and mercury were targeted to specific base

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mium, and mercury were targeted to specific base positions, and lanthanide ions were used to label phosphate positions around the periphery of the molecule. 53 Other techniques can also be exploited to monitor and characterize binding. A recent novel illustration is one from electrochemistry, which has been applied in monitoring the binding of Co(phenh3+ to DNA. 54 Surely other techniques, from EXAFS to scanning tunneling microscopy, will be exploited in the future. Biochemistry also provides very sensitive techniques that have been invalu- able in characterizing interactions of metal complexes with nucleic acids. First are simply gel electrophoresis experiments, which permit an assessment of changes in the nucleic-acid conformation, through its changes in gel mobility, as a func- tion of metal binding. A classic illustration is that of the unwinding of super- helical DNA as a function of intercalation. Closed circular DNA has much the same topological constraints on it as does a rope or a telephone cord; the DNA helices can wind up in coils. We define the duplex turning in a double helix as the secondary helical turns, and turns of the helices about one another as the supercoils or tertiary turns. As long as a DNA double helix is closed in a circle (form I), the total winding, that is, the total number of secondary and tertiary turns, is fixed. Molecules with differing extents of winding have different su- perhelical densities. In a circular molecule with one strand scission, what we call form II (nicked) DNA, the topological constraints are relaxed, and no su- percoils are apparent. The same, by analogy, can be said of a telephone cord Figure 8.9 (facing page) An illustration of two spectroscopic techniques used to probe DNA. (A) The variation in lumi- nescence characteristics of Ru(phenh 2 + with DNA binding. Shown is the emission spectrum of free Ru(phen)3 2 + (----), A-Ru(phenh 2 + in the presence of DNA ( ..... ), and Il-Ru(phenh 2 + in the presence of DNA (--) illustrating the spectroscopic perturbation with DNA binding as well as the associated enantioselectivity in binding of the complexes to the helix. As is evident from the greater luminescence of the Il isomer on binding, it is this Il-isomer that intercalates preferentially into the right-handed helix. (B) An application of para- magnetic broadening by metal complexes in NMR experiments to obtain structural information on their association with nucleic acids. Shown is the IH-NMR spectrum of d(GTGCACh with increasing amounts of A-Ni(phen)3 2 + . Note the preferential broadening of the adenine AH2 res- onance (7.8 ppm), indicating the association of this enantiomer in the minor groove of the helix.
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476 8 / METAUNUCLEIC-ACID INTERACTIONS off the phone receiver, which can turn about itself to relax its many supercoils.
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