Electronic Factors in the Photodimerization of Thymine

Abstract

The role that quantum chemistry has played in the understanding of the photochemistry of pyrimidines is outlined. A variety of increasingly sophisticated calculations have shown that electronic excitation is localized largely at the double bond between carbons 5 and 6. which is the position of photodimerization ih thymine. So far. the theoretical procedures have not been sufficient to permit a distinction between different multiplicities for the excited levels, but refer only to a "first excited state". A discussion is given of why more extended calculations are necessary before any statement can be made as to whether it is the singlet or the triplet that is involved in thymine photodimerization. THE PHOTODIMERIZATION of thymine, first discovered upon U.V. irradiation of frozen thymine[ 1-41, and afterwards upon in uitro [5,6] and in vivo [7,8] irradiation of DNA, is one of the most important reactions in photobiology. The formation of such dimers in irradiated DNA probably constitutes one of the essential factors responsible for the sensitivity of nucleic acids and cells to the effects of u.v. light. It takes place through the C-5-C-6 bond of thymine and involves the formation of a cyclobutane ring. The reaction occurs also with more or less facility with different thymine analogues [9].1 wish to recall the contribution of the molecular orbital study of the problem[ 10,111 to the elucidation of the mechanism of the reaction, and to report some more recent results in this field.The first molecular orbital calculations on the problem[ 10,l I] were carried out within the general frame of the Huckel approximation. The purpose of these calculations was to determine the electronic characteristics of the first excited state of the compounds studied and to correlate these characteristics with the rate of photodimerization of a series of pyrimidine bases.Among the characteristics of the excited states, two quickly appeared to be particularly important to the problem under investigation:I . The first characteristic concerns the distribution on the molecular periphery of the "uncoupled" electrons of the first excited state, which in this approximation may be confounded with the "spin densities". Examination of this distribution has shown that: ( ( 1 ) In the majority of the pyrimidines the highest concentrations of the lone electrons in theirfirst excited state occur ut C-5 and C-6. The only cases in which this dues not happen are those of the bases that do not dimerize: in 5-nitrouraci1. and 2-thiothymine the maximum concentration of the lone electrons is at the extracyclic substituent, NO, or SH. respectively. ( b ) A striking parallel exists between the yield of photodimerizntion crnd the total concentrrrtion of uncoupled electrons at the C-5-C-6 bond of the pyrimidines in their first excited state. This parallel is illustrated in Table 1. The only exception to this rule is 6-azathymine. The inability of this molecule to undergo photodimerization may be due to its general insensitivity to U.V. irradiation or to the dif...

UV-induced crosslinks in the 16S rRNAs of Escherichia coli, Bacillus subtilis and Thermus aquaticus and their implications for ribosome structure and photochemistry

“…In terms of the photochemistry, the crosslinking is apparently less sensitive to base identity than would be predicted from DNA experiments. Theoretical and experimental work has shown that the lone electrons in the excited S1 or T1 states are localized around the C(5)=C(6) double bond in pyrimidines (29)(30)(31), which explains in part the photoreactivity of pyrimidines. Purine crosslinks have also been characterized, although to a much lesser extent.…”

Even-odd alternation in mass spectrum of thymine and uracil clusters: Evidence of intracluster photodimerization

“…We chose 2-thiouracil, 6-azathymine, and cytosine, the photodimerization yields of which are known to be very small (cytosine) or nearly zero (2-thiouracil and 6-azathymine) in the bulk. This characteristic is due to the fact that they cannot undergo the cycloaddition reaction because of insufficient polarization or low electron density in the C5-C6 bond in the excited state (15,16). In contrast to the uracil case (Fig.…”

“…The polarization of the double bond by the conjugated carbonyl group is believed to play a crucial role in this reaction (15). A striking parallel also has been established between the photodimerization yield and the total concentration of uncoupled electrons in this bond (16). The reaction is known to occur in the electronically excited states reached by UV light of 150-to 300-nm wavelength or in the triplet states (8).…”

Selected Topics in Photochemistry of Nucleic Acids, Recent Results and Perspectives

“…In addition, triplet-triplet transfer experiments (Kittler and Lober, 1969) and triplet quenching studies with various paramagnetic metal ions (Kittler and Lober, 1968) indicate strongly that the triplet state of 6-azauracil is involved in the photochemical hydration process. Although theoretical considerations based on quantum chemical data does not exclude dimer formation for 6-azauracil and 6-azathymine (Pullman, 1968), both have not been identified until now. This may be due to the fact that four-membered rings containing nitrogen atoms are energetically not favoured.…”