The phosphorescence of native DNA at 77deg;K is characterized by a broad band peaked at (4480plusmn;30) Aring;, and a decay time of (0.28plusmn;0.03) sec. The phosphorescence quantum yield of 0.002 is considerably lower than the quantum yields of AMP and GMP atpH 7. Upon denaturing or degrading, the native DNA phosphorescence was partly replaced by the characteristic phosphorescence from the purines. The native DNA phosphorescence was efficiently quenched by Mn2+ions bound to the DNA. Poly dAT had a phosphorescence with a decay time of (0.32plusmn;0.03) sec which was similar to thymidine atpH 11 but showed no contribution from the adenine residues. Because of the similarity between the DNA and poly dAT phosphorescence, we inferred that the phosphorescence of native DNA originates in the thymine bases. These inferences were confirmed by observations of the ``Dgr;m=2'' transitions of the triplethyphen;state ESR in DNA, poly dAT, and ionized thymidine. The values ofHminwere 1054, 1046, and 1067 G, respectively, all very different from the values of 1360 and 1425 G for GMP and AMP. The absence of a strong guanine phosphorescence and ESR signal was explained by the quenching observed of the guanine triplet in GMPmdash;CMP gels and GMPmdash;poly C complexes. A mechanism explaining the occurrence of ionized thymidine emission in DNA and poly dAT is discussed in terms of a proton being transferred across the N3sngbnd;Hmiddot;middot;middot;N1hydrogen bond to adenine thereby quenching the adenine triplet and stabilizing the thymine triplet.
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