Triplet energy transfer in polyadenylic acid: Temperature dependence of phosphorescence quenching by Mn2+

The quenching of poly rA phosphorescence by Mn²⁺ is investigated in the temperature range from 4.2 to 77°K. At 4.2°K the phosphorescence intensity is found to decrease linearly whereas at 77°K it decreases nearly logarithmically with increasing quencher concentration. The decay of phosphorescence at 4.2°K is strictly exponential with lifetime, τ_e, of 2.82 sec independent of quencher concentration. At higher temperatures and nonzero quencher concentrations the decays are nonexponential with relatively fast initial decays. The phosphorescence is thought to arise from triplet traps that are formed when an ordered sequence of monomers is interrupted by some other stereochemically allowed conformation of the monomer. The above observations are interpreted in terms of a one dimensional random walk of the triplet excitation over the trap sites. The limiting, thermally activated jump time of triplet excitation from a trap to a nearest neighbor is found to be ${\text{7× 10}}^{\text{−3}}\hspace{0.17em}\sec$ . From the temperature dependence of this jump time the trap depth is calculated to be 115 cm⁻¹. An upper bound to the average length of ordered sections in poly rA in this temperature range is found to be 7 monomer units. The phosphorescence spectra and decays of several adenine containing subunits of poly rA in various environments at 77°K are reported. Evidence is presented for intramolecular hydrogen bonding in single stranded poly rA and is suggested to account for the differences in phosphorescence spectra and decays of the former and of poly dA.