Metal ion-biomolecule interactions. II. Methylmercuration of the deprotonated amino groups in adenine, guanine, and cytosine derivatives, and its relationship to amino group acidity

Abstract

Proton nmr spectroscopic evidence is presented for methylmercury(II) binding to the deprotonated amino groups in adenosine, 9-methyladenine, guanosine, 1-methylguanosine, and cytidine under basic conditions. Except for the guanosine case, 1H nmr spectra of the products from aqueous or ethanolic 1:1 mixtures of substrate and MeHgOH are consistent with methylmercuration of the deprotonated amino groups. Guanosine undergoes initial binding of MeHg to N 1, and a second equivalent of MeHgOH is necessary to effect amino binding. The nmr spectra of the complexed adenine derivatives suggest that different geometrical isomers exist in (CD 3) 2SO solution, reflecting the partial double bond character of the C 6N bond in these systems. Using a correlation relating the magnitude of the 199Hg- 1H coupling constant ( J) for MeHg-ligand complexes with the ligand p K a ( J = −3.88 p K a + 248.5, extending over 13 p K units, based on a variety of N and O donor ligands), estimates (± 0.3 p K unit) of the p K a s of the amino groups of the above substrates have been made. In this way, p K a values of 15.5 (cytidine), 17.0 (adenosine and 9-methyladenine), 15.1 (guanosine), and 14.9 (1-methylguanosine) are obtained. In the cases where comparisons with literature p K a data can be made, good agreement is found.