The protonation constants of uracil (UR), cytosine (CY), and D-valine (VA) have been calculated at 25 °C and at ionic strength μ = 0.1 (NaNO3). In a first step the stability constants of their Cu(II), Hg(II) and Pt(II) simple complexes were determined. In a second step the mixed complexation of several uracil homologues as thymine, thymidine, orotic acid and cytosine was also investigated under the same conditions by protometric methods. Concerning the mixed complexes, a new simple method of estimating their stability constants has been developed and verified. Keywords: Cu(II), Hg(II), Pt(II) coordination, ternary coordination, uracil homologues, D-valine, protometric methods.

The protonation constants of monomeric (TM) and dimeric (TD) forms of thymine and monomeric (OM) and dimeric (OD) forms of orotic acid and thymidine (TY) have been calculated at 25 and 37 °C and at ionic strength μ = 0.1 (NaNO3). Under the same conditions the stability constants of their Cu(II), Hg(II), and Pt(II) complexes have been determined by protometric methods. The results have been discussed and a comparison has been established in terms of binding sites: nitrogen N3 for monomeric and dimeric forms of thymine and thymidine; carboxylate anion at C6 and adjacent ring nitrogen N1 for monomeric and dimeric forms of orotic acid.