Laser flash spectroscopy of methylene blue with nucleic acids. Effects of ionic strength and pH

Laser flash spectroscopic measurements were made on methylene blue complexed to the synthetic polynucleotides poly[d(G-C)] and poly[d(A-T)] in solutions of varying ionic strength and pH. Triplet decay rates and rates of triplet quenching by oxygen have been measured for the polymer/dye solutions. The triplet decay and oxygen quenching rates of methylene blue in complex with poly[d(A-T)] are much less sensitive than those with poly[d(G-C)] with respect to variations in ionic strength. It is also shown that protonation of the triplet state of MB+ with poly[d(A-T)] is slower than that of free dye. These results indicate strong binding of the dye to poly[d(A-T)]. Excitation of the MB+/poly[d(A-T)] complex at 665 nm yielded single exponential decay kinetics, in contrast with excitation at 600 nm where double exponential kinetics were measured. This is tentatively assigned to excitation of dye dimer bound to this polymer.