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Electron distribution in the triplet state and photodimerization of thiothymine
668
SHORT COMMUNICATIONS
SC 93054 Electron distribution in the triplet state and photodimerization of thiothymine In a recent paper 1 we have correlated the yield of photodimerization of thymine and its analogues with the concentration of the uncoupled electrons at the C-5-C-6 bond of these molecules in their first excited triplet state. 2-Thiothymine did not seem to obey this simple correlation straightforwardly. The calculations of ref. I referred to thiothymine in the thione form. Recent nuclear magnetic resonance studies 2 indicate, however, the 2-thiouracil exists essentially in the thiol form. I t seems highly probable that the same is true for 2-thiothymine. The distribution of the uncoupled electrons in the first excited triplet has therefore been recalculated for the thiol form of 2-thiothymine, in fact for two such forms, corresponding to two possible positions of the - - N H group. The results are presented in Fig. I. o.o33 O 0.027
\\
HN/
o. I I 7 ~ 0.245
0,796
o.o47 H3
o.o32 O
C/ / //0.004
o.o
o.2o9I
/ ~ 0.29I
0.08~
N o.125
34/L~\
N/
Io.313
o.8ol
o.o48 H.~ C/ / //0.006
\~o /
268
0.439
NH 0.078
Fig. I. Distribution of the lone electrons in the triplet state of the thiol forms of thiothymine.
They indicate a relatively very small concentration (of the order of o.6-o.7) of free electrons on the C-5-C-6 bond and a very high concentration of such electrons on the extracyclic - S H group. In that respect the behaviour of the compound is very similar to that of 5-nitrouracil. The absence of photodimerization of 2-thiothymine is therefore readily accounted for in the general outline of our theory.
Universitd de Paris, Institut de Biologie Physico-Chimique, Paris 5e (France)
BERNARD PULLMAN MARIE-JOS~ MANTIONE
I M. J. MANTIONE AND B. PULLMAN, Biochim. Biophys. Acta, 91 (1964) 387 . 2 J. KOKKO, L. •ANDELL AND J. GOLDSTEIN, J. Am. Chem. Soc., 84 (1962) lO42.
Received November 5th, I964 Biochim. Biophys. Acta, 95 (1965) 668
SHORT COMMUNICATIONS
SC 93054 Electron distribution in the triplet state and photodimerization of thiothymine In a recent paper 1 we have correlated the yield of photodimerization of thymine and its analogues with the concentration of the uncoupled electrons at the C-5-C-6 bond of these molecules in their first excited triplet state. 2-Thiothymine did not seem to obey this simple correlation straightforwardly. The calculations of ref. I referred to thiothymine in the thione form. Recent nuclear magnetic resonance studies 2 indicate, however, the 2-thiouracil exists essentially in the thiol form. I t seems highly probable that the same is true for 2-thiothymine. The distribution of the uncoupled electrons in the first excited triplet has therefore been recalculated for the thiol form of 2-thiothymine, in fact for two such forms, corresponding to two possible positions of the - - N H group. The results are presented in Fig. I. o.o33 O 0.027
\\
HN/
o. I I 7 ~ 0.245
0,796
o.o47 H3
o.o32 O
C/ / //0.004
o.o
o.2o9I
/ ~ 0.29I
0.08~
N o.125
34/L~\
N/
Io.313
o.8ol
o.o48 H.~ C/ / //0.006
\~o /
268
0.439
NH 0.078
Fig. I. Distribution of the lone electrons in the triplet state of the thiol forms of thiothymine.
They indicate a relatively very small concentration (of the order of o.6-o.7) of free electrons on the C-5-C-6 bond and a very high concentration of such electrons on the extracyclic - S H group. In that respect the behaviour of the compound is very similar to that of 5-nitrouracil. The absence of photodimerization of 2-thiothymine is therefore readily accounted for in the general outline of our theory.
Universitd de Paris, Institut de Biologie Physico-Chimique, Paris 5e (France)
BERNARD PULLMAN MARIE-JOS~ MANTIONE
I M. J. MANTIONE AND B. PULLMAN, Biochim. Biophys. Acta, 91 (1964) 387 . 2 J. KOKKO, L. •ANDELL AND J. GOLDSTEIN, J. Am. Chem. Soc., 84 (1962) lO42.
Received November 5th, I964 Biochim. Biophys. Acta, 95 (1965) 668