Rotation around Metal—Purine Bonds

Another question related to the dynamics of platinum—nucleobase chemistry is the rotational freedom or otherwise of the metal—purine bonds. This point is relevant to the formation of, for instance, an intrastrand link upon binding of the platinum to the first base. Studies on the cis-[Pt(NH3)2(Guo)2] cation indicated little restriction around the Pt—purine bond [65, 66] but with bulkier ligands (A, A-Me2-en, N,N,N N -Mq - vl) restricted rotation is observed [65,67]. 

The suggestion has been made that the guanine base is less sterically hindered than the other bases and forms the intrastrand link more easily. Very pertinent to this suggestion is the observation that a 5-[Pt(I 3)2(5 -AMP)2], where the 5 -AMP is bound by Ny with the bases in a head-to-tail (htt) orientation, does exhibit restricted rotation, as evidenced by NMR [67]. The different exocyclic substituents in the 6-position (NH2 for Ade, O for Gua) may be responsible for this difference between 5 -AMP and 5 -GMP [67]. 

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