Spectroscopic sensitivity of linear-benzoadenine nucleotides to divalent metal counterions, side chain conformations, micelles, and enzymes.

From pK(a) data for lin-benzoadenosine 5'-mono-, 5'-di-, and 5'-triphosphates, which are fluorescent "stretched-out" analogues of adenine nucleotides, it was possible to designate the cases of interaction of phosphate with the heteroaromatic moiety. The addition of divalent metal cations or quaternary ammonium micelles diminishes the direct intramolecular interaction between the phosphate(s) and base and consequently brings the pK(a) values close to that of lin-benzoadenosine. Fluorescence spectroscopy was used to investigate the interaction of lin-benzoadenine nucleotides with Mg(2+), Mn(2+), and Co(2+). The association constants for the formation of such complexes were obtained from measurements of steady-state fluorescence quenching. Phase and modulation measurements of the fluorescence lifetimes of lin-benzoadenine nucleotides as a function of Co(2+) concentration permitted determination of the static component of the quenching due to intramolecular complex formation. The association constants of the lin-benzoadenine nucleotides with all of the divalent metal ions studied were greater than those observed for the corresponding adenine nucleotides and were in the order: lin-benzo-ATP > lin-benzo-ADP > lin-benzo-AMP. Fourier transform (1)H NMR of lin-benzo-ATP in the presence of Co(2+) showed broadening of the aromatic proton signals, the 2-H signal (corresponding to the 8-H in ATP) being the most affected. Models are proposed to explain the phosphate-base interaction, the influence of metal ions on base protonation, and the intramo- the intramolecular quenching observed in the complexes due to paramagnetic ion (Co(2+), Mn(2+)) and base interaction.