Calorimetric and 195Pt NMR Studies on Aromatic Ring Stacking between Nucleotides and Platinum DNA Intercalators

Abstract

Abstract Calorimetric and spectroscopic investigations have been carried out on the stacking interactions between nucleosides or nucleotides (NMP) and platinum DNA intercalators and other planar platinum(II) complexes with heteroaromatic ligands (L). Pt(L)(diamine) (L = 1,10-phenathroline (phen), its methyl and nitro derivatives, 2,2′-bipyridine; diamine = ethylenediamine (en), L-2,3-diaminopropionate) reacts with NMP (=adenosine 5′-monophosphate (AMP), guanosine 5′-monophosphate (GMP), inosine 5′-monophosphate (IMP), cytidine 5′ -monophosphate (CMP); nicotinamide adenine dinucleotide (NAD), adenosine, cytidine) to give the Pt(L)(diamine)–(NMP)n adducts (n = 1, 2) with log K1 = 1.63—3.00 and log K2 = 1.47—2.51 at 25 °C at ionic strength = 0.1 mol dm−3 (NaCl). The 1 : 1 adduct formation mainly depends on the enthalpy change ΔH1° = −6.5 — −26.2 kJ mol−1, and the entropy change (ΔS1° = −38—20 J mol−1 K−1) makes a negative or small contribution. 1H NMR upfield shifts observed for the Pt(phen)(en)–NMP systems have substantiated the presence of stacked adducts in dilute aqueous solution, whereas diquat, a dicationic organic molecule, did not cause appreciable shifts, and hence the adduct formation was negligible. NAD was found to interact with Pt(phen)(en) through the adenine moiety. 195Pt NMR signals of Pt(phen)(en) in water suffered downfield shifts upon addition of NMP, the Δδ1 values being in the order NAD ≥ AMP ≈ GMP > IMP. This indicates that stacking causes an electron density decrease of the Pt(II) center. Linear relationships have been detected between the ΣΔHn° values and the absorption coefficients εn of the difference spectra in the charge transfer region and between ΣΔHn° and 195Pt NMR shift differences Δδn. These observations serve as evidence for the electronic effects of stacking interactions on the stacked rings and the central Pt(II) ion.