To guanine

The secondary structure of DNA is shown in Figure B. This "double helix" model was first proposed in 1953 by James Watson and Francis Crick, who used the x-ray crystallographic data of Rosalind Franklin and Maurice Wilkins. Beyond that, they were intrigued by the results of analyses that showed that in DNA the ratio of adenine to thymine molecules is almost exactly 1 1, as is the ratio of cytosine to guanine ... 

Thus three lines of evidence define the rapidly dissociating receptor as the LR complex. Conditions known to uncouple R from G--first, guanine nucleotide and second, pertussis toxin—produce LR third, reconstitution of G protein restores receptor affinity, sensitivity to guanine nucleotide, and effector activation. In this sense, the ligand and binding behavior of this system is analogous to that of the beta-adrenergic receptor, where the LR and LRG complexes have already been studied with purified proteins and reconstituted membrane preparations (2,i0). 

Lefkowitz, RJ, Cotecchia, S, Samama, P and Costa, T (1993) Constitutive activity of receptors coupled to guanine nucleotide regulatory proteins. TIPS 14 303-307. 

Several studies (for reviews, see Steenken, 1989 Diz-daroglu, 1991) had already shown that OH reacts in a multiplicity of ways with all four DNA bases. Thus, OH can add to guanine residues at C4, C5 and C8 positions to give hydroxyguanine radicals that can have various fates. For example, addition of OH to C8 of guanine produces a radical that can be reduced... 

Scheme 3 Charge transfer from the sugar radical cation to guanine (G)... 

Ni(salen)-DNA adduct formation is closely related to that formed by the Ni(peptide) systems, although there are different mechanisms proposed for both types of complexes. In the case of Ni(salen), the addition of a phenol radical to the guanine heterocycle and formation of a covalent bond to guanine C8 (Equation (9)) is suggested. 

Dohlman H, Caron M, Stader C, Am-laiky N, Lefkowitz R. A family of receptors coupled to guanine nucleotide regulatory proteins. Biochemistry 1988 27 1813-1817. 

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