Pyrimidine, singlet excited

Evidence for the delocalization of the lone pairs of pyrazine is deduced from the large differences in the magnetic susceptibilities of the three isomeric diazines. Thus the magnetic susceptibilities of pyrazine and pyridazine have been found to be smaller than that of pyrimidine.6311 Further 77-electron calculations have been carried out on the singlet excited states of pyrazine.63b... 

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... 

Figure 1. The change of the dynamic polarizability of pyrimidine upon excitation to the lowest excited singlet state. Reproduced from [58]...

This photochemieal reaction occurs through the singlet excited state of 8-MOP. As the singlet state lifetime is short (in the nanosecond range), 8-MOP must be in a close proximity to a pyrimidine base. Therefore, the photoaddition is more likely to involve those 8-MOP moleeules that are intercalated into the double-stranded DNA. This type of photo-addition reaction is supposed to be responsible for die phototoxicity of 8-MOP in human skin (Yang et al, 1989). 

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). 

The deviation from first-order kinetics for the uridylic acids is also reflected in quantum yields which vary with concentration and which therefore vary during the course of a particular photolysis.7 These deviations from first-order kinetics have been discussed in terms of a collision-induced transition of an excited-singlet pyrimidine to a long-lived state.7 We shall say more about the probability of long-lived states later. 

These reactions can be interpreted as proceeding via an intermediate of the diazabenzvalene type, formed from the lowest singlet w, it excited state of the pyrazine, and are analogous to the results reported for the photoisomerization of alkylbenzenes.133 Support for this postulate comes from a study of 2,5-dimethylpyrazine which is isomerized to a mixture of 2,5-dimethylpyrimidine and 4,6-dimethyl-pyrimidine, presumably via the two diazabenzvalenes [Eq. (36)]. There is no evidence for products arising via bicyclohexadiene and prismane intermediates which are also involved in the photorearrangement of alkylbenzenes. 

Figure 16-12. Schematic representation of the main paths of photoproduct formation, in particular photodimers of pyrimidines, through die singlet ( DNA ) and triplet (3DNA ) excited states once the energy is absorbed by DNA. The photolesion can ultimately lead to a genetic defect. (Adapted from Ref. [114].)...

MO methods have calculated that the bond order of the C(5)-C(6) bond in the first triplet of a series of pyrimidines is lower than in the first excited singlets of the corresponding molecules, and have concluded that dimerization of the bases occurs via the triplet state. They divided the pyrimidines into three groups the pyrimidines which are known to be easily dimerizable (uracil, 6-methyluracil, thymine, orotic acid—for these molecules the Pse values in their T state are of the order of 0.09-0.12), those which dimerize not so easily (5-aminouracil, cytosine, 5-methylcytosine—Pge 0.13-0.17), and the bases which do not dimerize at all or only with considerable difficulty (2-thiothymine, Pgg = 0.31 isocytosine, Pgg = 0.39 and 5-nitrouracil). The relative distribution of the bases within groups leads sometimes to only limited agreement with available experimental data. For instance, as the Pgg = 0.085 in T of uracil is lower than Pgg = 0.106 in T of thymine, uracil should dimerize more easily than thymine, a conclusion in disagreement with experiment. ... 

The fluorescence polarization excitation spectrum has been measured for thymine in aqueous solution. " The depolarization at the red edge is attributed to the hidden n, ir transition. Ionization of the lowest excited singlet and triplet states have been determined by the effect of pH on the absorption, fluorescence, and phosphorescence spectra of purines and pyrimidines. " Spectral, polarization, and quantum yield studies of cytidylyl-(3, 5 )-adenosine have also been published. Intermediates in the room-temperature flash photolysis of adenine and some of its derivatives have been identified hydrated electron, radical cations and anions, and neutral radicals resulting from their reactions have been assigned. Photoionization occurs via the triplet state. FMN encapsulated in surfactant-entrapped water pools interacts with polar head groups, entrapped water molecules, and outer apolar solvent. ... 

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