Ternary complexes of cis-(NH 3) 2Pt(II) with model nucleobases (1-methylcytosine, 9-methylguanine) and N- and O-bound amino acids (gly, ala)

Abstract

The formation and properties of isomeric complexes of cis-(NH 3) 2Pt(II) containing the model nucleobase 1-methylcytosine, 1-MeC, and the amino acid glycine, glyH, are reported. Depending on the pH, different protonation states of amino acid and nucleobase are present. In acidic medium, cis-[(NH 3) 2Pt(1-MeC)(H 2O)] 2+ initially forms with glyH cis-[(NH 3) 2Pt(1-MeC)(glyH- O)] 2+ ( 2b) which slowly at pH 2–3 and faster at pH 4–5 converts into cis-[(NH 3) 2Pt(1-MeC)(gly- N)] + ( 1a). 1a is protonated to give the glyH- N species with a p K a ⋍ 2.8 and deprotonated at the NH 2 group of 1-MeC with a p K a ⋍ 12.5–13. Deprotonation of 2b was not detected up to pH 6, at which point conversion 2b → 1a was complete. 1a was prepared and isolated as the nitrate salt via two different routes, starting either from the gly chelate cis-[(NH 3) 2Pt(gly- N,O)] + or from cis-[(NH 3) 2Pt(1-MeC)(H 2O)] 2+. Only the second way led to both O- and N-bound glycine complexes. In the absence of glyH, cis-[(NH 3) 2Pt(1-MeC)(H 2O)] 2+ undergoes a condensation reaction initially to the μ-hydroxo complex cis-[(NH 3) 2Pt(1-MeC)(OH)Pt(1- MeC)(NH 3) 2] 3+ ( 6) and slowly to a bis(1-methylcytosinato) bridged diplatinum(II) complex, cis-[(NH 3) 2Pt(1-MeC −) 2Pt(NH 3) 2] 2+. Analogues of 1a with 9-methylguanine (9-MeGH) instead of 1-MeC as well as mixed 1-MeC, alanine and mixed 9-MeGH, alanine complexes have also been prepared and isolated.