Photodimerization pyrimidine bases

The photodimerization of pyrimidine bases continues to attract attention, undoubtedly because of the biological implications of such transformations. 1,3-Dimethyluracil affords four [ 2 + 2] dimers on irradiation in an ice-matrix.192 Analogous dimeric species have been obtained under a variety of conditions from dimethylthymine,193 tetramethyluracil,194 5-methyloro-... 

Cyclic a,P-enones photodimerize via the Tt-state, as in (4.23)426) and (4.24)427). By chosing the appropriate substituents the reactions proceed regio- and stereoselectively. In contrast, pyrimidine bases dimerize from the first excited singlet state to afford head to tail products (4.25) 428). 

It is well known that pyrimidine bases convert to photodimers upon irradiation to UV light near the X max( > 270 nm). This photochemical reaction has a lethal effect in biological systems due to the photochemical transformation of pyrimidine bases of nucleic acids. However the photodimerization is a reversible reaction and the photodimers split to afford the original monomers very efficiently upon irradiation at a shorter wavelengths as shown in Scheme 1(1). 

Little such information is at hand to support proposed mechanisms for the photohydration and photodimerization of the pyrimidine bases and their derivatives discussed in this chapter, and much of what is known is confusing or self-contradictory. 

All these bases absorb around 260 nm. Thymine and cytosine are most sensitive to irradiation. Two most important types of photochemical reactions that have been observed for these pyrimidine bases are photohydration and photodimerization. In vivo systems, interactions between protons and nucleic acids can also be initiated by radiations of wavelength dlorter than 300 nm. 

E Photodimerization occurs between pyrimidine bases when they are Packed in suitable geometry in the helical DNA molecule. Thymine is tost susceptible to photodimerization which occurs at A 280 nm. The... 

Attempts have been made to correlate these characteristics with the rate of photodimerization of a series of pyrimidine bases.467 1 Among these characteristics, two appeared to be particularly important for the photodimerization problem (for review see ref. 466), namely the density of the uncoupled electrons (or spin density) pf at C-5 and C-6 and the change of the C(5)-C(6) bond order P56 upon excitation. The proposal that the high concentration of spin density on the C(5)-C(6) bond as well as its low bond order in the excited states were factors responsible for the case of photodimerization was stated about ten years ago.464,471 The first calculations, within the framework of the 77-HMO approximation, showed the following findings (i) The concentration of uncoupled... 

In a series of our interaction studies, the photodimerization of thymine bases along the polymer chain was carried out to estimate the degree of intramolecular selfassociation of the bases (Fig. 14). The photodimerization reaction was also studied for monomeric and dimeric model compounds (Fig. 15)27). The formation of thymine photodimers by UV irradiation onto nucleic add is well-kown and the photodimerization of nucleic add, its model compounds and pyrimidine bases has been reported by many groups, though the work on the synthetic polymers containing thymine bases has hitherto been unknown. 

Photochemical reactions of the pyrimidine polymers in solution were studied to determine the quantum yields of the intramolecular photodimerization of the pyrimidine units along the polymer chains. Photoreactions of the polymers were carried out in very dilute solutions to avoid an intermolecular(interchain) photodimerization. Quantum yields determined at 280 nm for the polymers (1-6 in Figure 1) are listed in Table I. The quantum yield of the 5-bromouracil polymer [poly(MAOU-5Br)] could not be determined because of side reactions of the base during the irradiation. 

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

Photochemical reactions of purines and pyrimidines in light are well known, and lead to mutations. Although various photochemical reactions can occur with both pyrimidines and purines, photodimerization of thymine is of the utmost importance. Photochemical reaction by UV irradiation of thymine results in formation of dimers with cyclobutane rings, which block DNA replication. Photodimerization was reported with the synthetic polymers having pendant thymine bases. In a study with polyacrylates and polymethacrylates with... 

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