LONG-RANGE INTERMOLECULAR INTERACTIONS IN DILUTE AQUEOUS-SOLUTIONS OF IONIZED POLY(L-LYSINE) AT LOW IONIC-STRENGTH

Measurements of nonradiative singlet energy transfer (Forster transfer) were performed to investigate the intermolecular interactions of ionized poly(L-lysine) in dilute aqueous solution at low ionic strength. The range of ionic strengths covered in the experiments includes the location of the "ordinary to extraordinary transition". Some of the molecules of the polymer were lightly labeled with donors, and others were lightly labeled with acceptors. Different combinations of labels, giving Forster radii ranging from 1.7 to 5.8 nm, were used. In the presence of low concentrations of added salt, enhanced efficiency for nonradiative singlet energy transfer was detected in the extraordinary regime only for the pair of labels that have the largest Forster radius. Measurements were also performed by dilution of the labeled species with unlabeled poly(L-lysine), at constant total concentration of polymer. A simple simulation was devised for interpretation of the rate of the decrease in the efficiency of Forster transfer upon substitution of unlabeled polymer for some of the labeled polymer. Comparison of the simulation with the experiment shows that each macromolecule "sees" only one or two other macromolecules, on the distance scale determined by the largest Forster radius, in the temporal aggregates that are formed in the "extraordinary" or low salt regime.