Guanine metal binding

The absence of protonation shifts proves platination at this site, i.e. when a metal ion is coordinated at e.g. N7 of guanine, protonation cannot occur anymore at this site, resulting in the absence of a protonation pK at about pH 2. An illustration of such a pH dependent chemical shift pattern of nucleobase protons is given in Fig. 11a, b. Metal binding can also alter pKa s of sites where no metal is bound, e.g. the decrease of the pKa at Nl. 

There is a large body of solution data available now on the effects of metal binding on the acidification of bases, notably of the purines guanine, hypox-anthine, and adenine (40-43, 100-104). By in large, these data stem from complexes of Pt(II) as well as Ru(II) and Ru(III). They will be dealt with in more detail in Section V. At this point, the question shall briefly be examined,... 

Established cases of metal binding to the exocyclic amino groups of guanine bases are rare. Amino group mercuration has been reported for 1-methylguanosine (169e), and the X-ray crystal structure of a dianionic 9-methylguanine complex with Pt(II) at N7, and Hg(II) at both N1 and N2 has been determined (237). The compound has been depicted in Fig. 19. The pK values have not been determined. 

A large number of experimental data including stability constant measurements [11] [12] indicate that, at neutral pH, N(7) of guanine is a better metal-binding site than N(7) of adenine. This observation is also supported by modern quantum-chemical calculations with the inclusion of electron correlation effects which reveal that polarity of the bases and the corresponding basicity of the N(7) site decreases in the order guanine > inosine > adenine > 2-aminoadenine [13]. 

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