Fluorescence quenching and spin label electron spin resonance studies of stacking self-association in aqueous solutions of 2-aminopurine riboside and its 5'-mono- and -diphosphate.

The autoassociation of 2-aminopurine riboside (rn2Pur) and its 5'-mono- (P-rn2Pur) and 5'-diphosphate (PP-rn2Pur) in neutral aqueous solutions was investigated using fluorescence quenching and ESR spin-label methods within the range 276-358 K. Respective equilibrium constants and thermodynamic functions were derived therefrom assuming two models of infinite autoassociation: (i) an isodesmic one (K2 = K3 = ... Kp), and (ii) one in which K2 no equal to K2 = K4 ... Kp. Comparative analysis of these data and that of the parent 2-aminopurine, obtained previously, allowed us to formulate the following conclusions: (1) the mechanism of autoassociation of rn2Pur varies with temperature in such a way that a T = 318 K the isodesmic model is fulfilled (K2 = Kp); at high temperatures Kp/K2 greater than 1, i.e. the process is cooperative, while at lower temperatures it becomes anticooperative (Kp/K2 greater less than 1); (2) at 298 K the tendency to autoassociation decreases in the order; rn2Pur greater than P-rn2Pur greater than PP-rn2Pur; (3) rn2Pur forms highly packed complexes with the bases stacked and the ribofuranose residues interacting via hydrogen bonds or water bridges; (4) autoassociation of P-rn2Pur and PP-rn2Pur is mainly governed by stacking of the bases, while the ribose phosphate residues attain a trans configuration corresponding to the lowest electrostatic repulsion between charged phosphate groups; even at high ionic strength (I = 0.8), a positive electrostatic contribution to the free enthalpy of autoassociation is observed; (5) the two methods employed gave similar results for P-rn2Pur, but somewhat different ones for rn2Pur because the presence of the spin label (nitroxide stable radical) at the 2'(3')-OH group of the ribose residues prevents its interaction via hydrogen bonding with an unlabeled one of an adjacent nucleoside.