Luminescence and Electron Spin Resonance Studies of Adenine in Various Polynucleotides

The excited states of adenine in different polynucleotides dissolved in ethylene glycol: H2O glasses have been studied by optical emission and ESR at 77°K. In single-stranded poly A the optical properties were very similar to the constituent AMP monomers except for a red shift of the luminescence. The ESR Δm=1 transitions were observed with D=0.116 cm−1 and E=0.027 cm−1 for poly A and D=0.119 cm−1 and E=0.027 cm−1 (all ±0.002 cm−1) for AMP. This means that the triplet transfer time is longer than 2×10−10 sec since more rapid triplet—triplet jumping would give a time averaging of E. In the ordered double-helix form of poly A the excited states are completely quenched when one-half of the bases are protonated. In poly (A+2U) and poly (A+2I) ordered copolymers the excited states are also quenched. However in random distributions of U, C, and I in single strands of poly A no quenching was observed. This leads to the conclusion that the hydrogen bonding rather than energy transfer is responsible for the quenching observed in the ordered polymers.