Hetero-association of caffeine and aromatic drugs and their competitive binding with a DNA oligomer

Abstract

NMR spectroscopy has been used to elucidate the molecular basis of the action of caffeine (CAF) on the complexation with DNA of mutagens such as ethidium bromide, propidium iodide, proflavine and acridine orange, and anticancer drugs such as actinomycin D and daunomycin. The hetero-association of CAF and each of the aromatic ligands in 0.1 mol L(-1) phosphate buffer (pD=7.1) has been investigated as a function of concentration and temperature by 500 MHz 1H NMR spectroscopy and analysed in terms of a statistical-thermodynamic model, in which molecules form indefinite aggregates for both self-association and hetero-association. The analysis leads to determination of the equilibrium constants of hetero-association and to the values of the limiting chemical shifts of the heteroassociation of CAF with each of the aromatic molecules. The hetero-association constants between CAF and each of the aromatic drugs/dyes are found to be intermediate in magnitude between those for self-association of CAF and the corresponding drug/dye. The most probable structures of the 1:1 CAF + ligand hetero-association complexes have been determined from the calculated values of the induced limiting chemical shifts of the drug protons. Knowledge of the equilibrium constants for self-association of CAF and the aromatic ligands, for their hetero-association and their complexation with a DNA fragment, the deoxytetranucleotide 5'-d(TpGpCpA), enabled the relative content of each of the CAF-ligand and CAF-ligand-d(TGCA) complexes to be calculated as a function of CAF concentration in mixed solutions. It is concluded that, on addition of CAF to the solution, the decrease in binding of drug or mutagen with DNA is due both to competition for the binding sites by CAF and the aromatic molecules, and to formation of CAF-ligand hetero-association complexes in the mixed solution; the relative importance of each process depends on the drug or mutagen being considered.

Complexation of norfloxacin with DNA in the presence of caffeine

“…As in previous work [12] the reaction (3e) may be safely neglected for CAF-drug systems, which simplifies analytical expressions for the observed dependence of proton chemical shifts of both X and Y components to the form [12] ( ) 1, 2). The monomer concentrations x 1 and y 1 may be derived from the solution of the mass concentration law for scheme (3) [12].…”

“…As in previous work [12] the reaction (3e) may be safely neglected for CAF-drug systems, which simplifies analytical expressions for the observed dependence of proton chemical shifts of both X and Y components to the form [12] ( ) 1, 2). The monomer concentrations x 1 and y 1 may be derived from the solution of the mass concentration law for scheme (3) [12]. It follows that the model (4) is a function of two unknown quantities, δ h and K h , which may be determined from the concentration dependences of δ (Fig.4a) using the numerical procedure described previously [12,22].…”

“…It has been recently shown that CAF, a purine derivative, effectively forms hetero-complexes with a number of aromatic DNA intercalators, which may account for the observed changes of biological activity of these drugs in the presence of Caffeine, i.e. the 'interceptor' action of Caffeine [11,12]. Another mechanism of action has been proposed, in which there is competition between CAF and aromatic drug for the binding sites on DNA, i.e.…”

“…the 'protector' action of Caffeine [12,13]. It is likely that both mechanisms can act simultaneously and therefore must be taken into consideration when analysing a three-component equilibrium of the Drug, CAF and DNA [12,13].…”

“…In the present work we have used 1 H NMR spectroscopy (500MHz) to quantify the complexation of NOR with Caffeine and DNA. As in previous work [12,14] we took the self-complementary DNA oligomer, 5'-d(TpGpCpA), as a model DNA sequence to analyse competitive binding in a three-component system. The choice of the tetramer is dictated by the fact that short self-complementary DNA sequences are thought to be appropriate model systems for a competitive binding studies of molecules in equilibrium [14] and that the 5'-d(TGCA) sequence contains the central the GC step, which was proposed to be the binding site for NOR [9].…”