Cyclobutane dimers formation

The spontaneous polymerization of styrene was studied in the presence of various acid catalysts (123) to see if the postulated reactive intermediate DH could be intentionally aromatized to form inactive DA. The results showed that the rate of polymerization of styrene is significantly retarded by acids, eg, camphorsulfonic acid, accompanied by increases in the formation of DA. This finding gave further confirmation of the intermediacy of DH because acids would have Httie effect on the cyclobutane dimer intermediate in the Flory mechanism. 

Irradiation of crystalline acenaphthylene1121 at temperatures below 25°C results in formation of the tra/w-cyclobutane dimer only. At temperatures above 40°C both cis and trans dimers are formed, with the cis dimer predominating at 60°C (Table 10.12). The formation of the trans dimer, which occurs with low quantum yield at all temperatures studied, is thought to... 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

Yamada H, Hieda K (1992) Wavelength dependence (150-290 nm) of the formation of the cyclobutane dimer and the (6-4) photoproduct of thymine. Photochem Photobiol 55 541-548 Yan M, Becker D, Summerfield S, Renke P, Sevilla MD (1992) Relative abundance and reactivity of primary ion radicals in y-irradiated DNA at low temperatures. 2. Single- vs double-stranded DNA. J Phys Chem 96 1983-1989... 

It should be noted here that thymine photodimerization may occur by a non-concerted mechanism, involving free radical intermediates. Indeed, photoproducts other than cis-syn dimer, such as the next most abundant thymine dimer, so-called 6 4 adduct, were observed in irradiated DNA. However, the quantum yield of cis-syn photodimer formation (r/j 0.02) is more than an order of magnitude higher than that of the 6 4 adduct ( 0.0013) which in turn is an order of magnitude higher than the quantum yields for other thymine isomers [68]. This specificity can lead to the conclusion that the thymine photodimerization occurs predominantly via concerted 2 + 2 cycloaddition mechanism. A time-resolved study of thymine dimer formation demonstrated that thymine cyclobutane dimers are formed on a timescale of less than 200 nsec, while the 6 4 adduct is formed on a timescale of few milliseconds [69]. The delay in the formation of the latter was attributed to the mechanism of its formation through a reactive intermediate. 

There are two different methods of splitting a thymine dimer photochemical and enzymatic. In the photochemical method, the sample containing thymine dimers is irradiated with UV light. Splitting of the thymine cyclobutane dimer follows the same symmetry rules as its formation it is thermally forbidden but photochemically allowed. When a dimer absorbs a photon of suitable wavelength (2 240 nm), it reacts with a quantum yield of nearly 100% forming two thymines [60]. In the enzymatic method an enzyme recognizes thymine dimers and repairs them. It may require the absorption of a photon, or it may happen in the dark. 

Irradiation of lower molecular weight samples in the fluid N phase at 313 or 366 nm led to an unusual result [21]. In the first few seconds of irradiation the perturbed spectrum of the N phase exhibited hyperchromism (an increase in absorbance) and its shape became similar to that of the spectrum of the isotropic melt. This effect is also observed upon triplet sensitization which, like 366-nm irradiation, suppresses photo-Fries rearrangement [28]. It has not yet been proved that this effect is accompanied by a phase change from N to I induced by photoproducts essentially acting as impurities in the mesophase. The effect could be at the microscopic level where formation of a cyclobutane dimer or other photoproduct could interrupt H-type aggregated chromophore stacks, or confor-... 

As in the case of electrochemical reduction, the photochemical transformation of 5-fluorouracil derivatives differs from that of the other 5-halogeno uracils. The primary photoproduct of 5-fluorouracil, its glycosides and poly(5-FU) is the photohydrate. However, at shorter wavelengths of irradiation, e.g. 254 nm where the photohydrate exhibits absorption, there is elimination of HF from the 5,6 bond and formation of barbituric acid 129 13I>. There is also some evidence for acetone photosensitized formation of cyclobutane dimers of 5-fluorouracil132), as well as dimer formation in irradiated poly(5-FU)133>. 

As a second example of intersystem crossing mechanism in biochromophores we include here the case of the DNA pyrimidine nucleobases, starting by the uracil molecule [91]. In previous sections we presented a model for the rapid internal conversion of the singlet excited rationalizes the ultrafast decay component observed in these systems, both in the gas phase and in solution. Despite the short lifetimes associated to this state, which is the main contributor to the photophysics of the system, formation of photodimers PyroPyr has been observed for the monomers in solution, as well as in solid state, for oligonucleotides, and DNA [92], Since the sixties, the determination of the mechanism of the photoinduced formation of cyclobutane dimers has been the subject of numerous studies [92, 93-97], One of the most classic models that has been proposed for the photodimerization of Pyr nucleobases in solution invokes photoexcitation of a molecule to a singlet state followed by population of a triplet state by an intersystem crossing mechanism... 

Morrison and his co-workers have reported the formation of two cyclobutane dimers from the irradiation of a mixture of dimethylthymine and urocanic acid. The photocycloaddition of 3-methoxycarbonylpsoralen (195) to thymidine in a dry film has been studied using irradiation in the range... 

FIGURE 18.3, The formation of cyclobutane dimers, (a) Chemical formula of dimerization. (b) Stereoview of the monomers (black bonds) and the cyclobutane adduct (open bonds). This diagram shows the amount of movement of C=C carbon atoms needed to form a cyclobutane ring. Ph = phenyl (Ref. 28). 

These results can be explained by considering the electrostatic interactions between the cations, the structure of the AOT reversed micelle interface at varying co, and the transition states leading to the formation of the cyclobutane dimers. As mentioned above, p is the only photoproduct formed in studies with /V-octadecyl-trans-4-stilbazolium p-chlorobenzenesulfonate. This can be attributed to the packing constraints of the monomer within the crystal lattice. The fact that the ratio of p/a increases with decreasing co indicates that the way in which the stilbazolium monomer units pack at the interface is affected by to. 

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