Dimer splitting

Thus, the RCOO radicals produced initially are unstable, and before (or while) undergoing dimerization, split up into simpler radicals and CO2 molecules. 

These experiments proved that a light-excited, reduced flavin is indeed able to photoreduce cyclobutane pyrimidine dimers and that these dimers undergo a spontaneous cycloreversion. The quantum yield of about 0=5% clarified that the overall dimer splitting process is highly efficient, even in these simple model systems ((]) photolyase 70%). 

Since anthracene dimers split to form anthracene monomers upon melting, it may be possible that if the excess vibrational energy in the hot dimer is not dissipated by solvent collisions<88> rapidly enough the bonds may rupture and monomers may be produced,... 

Theoretically, trans-p-coumaric acid can produce 12 isomers depending on whether head-to-tail (4,4 -dihydroxytruxillic acid) or head-to-head (4,4 -dihydroxytruxinic acid) dimerizations occur with syn or anti and with cis or trans ring junctions (37). Mass spectrometric analysis of the tetra-TMSi derivatives showed that head-to-tail dimers split symmetrically on electron impact, whereas head-to-head dimers fragment asymmetrically (Figures 2 and 3) (33,35,38,39). Thus the tetra-TMSi derivative of 4,4 -dihydroxytruxillic acid has a mass spectrum similar to that of the bis-TMSi derivative of p-coumaric acid (33). 

Fig. 1. The action spectrum for uracil dimer splitting. The corresponding curve for thymine dimer splitting is included for comparison (Swenson and Setlow48).

Fig. 9. Action spectra for dimer splitting in pTpT and poly T. The dotted line is the action spectrum for splitting of thymine dimer (Deering and Setlow68).

The decrease in absorbancy of poly U upon irradiation is discussed by Swenson and Setlow.48 Their results are shown in Figure 27. In this figure is also shown the photoinduced dimer splitting. The amount of photorecovery of absorbance for any particular total dose was taken to be a measure of the amount of dimer content at that dose. The amounts of dimer so calculated are shown in Figure 28. The amount of hydrates formed were estimated by measuring the thermal recovery of absorbancy. The results of both thermal and radiation reversal are shown in Figure 29. The total recovery of absorbance for poly U irradiated at 265 nm was 87-90% (after certain corrections, it was estimated that 95% of the absorbance decrease could be accounted for by dimers and hydrates). Of this total reversal of 907o about 67% was effected by the thermal treatment, and presumably this number provides an estimate of the total fraction (0.75) of hydrates in the photoproducts. 

In the case of DNX systems, the experimental UV spectra in hexane clearly show the dimeric splitting resulting from the major naphthalene absorption band Ag —B3u, long-axis polarized [36]. In contrast, EETs from naphthalene to anthracene are usually assumed to occur between the 1Lb(lB3u) naphthalene band, long-axis polarized and weakly dipole... 

For an assemblage of two identical molecules spaced d nm apart, the HOMO and LUMO energies split into four levels, each split by 2t eV apart ("dimer splitting") [26] here t is akin to the Hiickel69 resonance integral (i of Section 3.15 Indeed, chemists will remember Eq. (8.6.10) from the simple Hiickel molecular orbital theory for aromatic 7r-electron systems. As the number of molecules N increases, the energy levels become spaced more closely, until they form a quasi-continuous band of bandwidth W, where... 

Fig. 24. Schematic illustration of the exchange mechanism for absorption intensity in a hypothetical Ni +-Mn + dimer subunit of NF+rRbMnClj. Left isolated NP+ ion, with the weak A2 absorption cross section. Right dimer splitting in the Ni +-Mn + exchange-coupled ground state. Exchange coupling introduces a spin-allowed dimer transition which is thermally activated and is more intense than the spin-forbidden dimer transitions...

Photosensitised [2+2] cycloreversions have been reported in the aryl cage compounds (115), yielding the dienes (116). Analogous cycloreversions have also been observed in cis- and trans-bicyclo[5.2.0]non-8-enes.Intramolecular photosensitised pyrimidine dimer splitting in the indole-pyrimidine (117) is remarkably solvent dependent and is thought to proceed via electron transfer from the excited indole moiety. The relative values of the activation parameters in photosensitised pyrimidine dimer splitting have been determined. ... 

With the model systems used in these investigations, dimer splitting can be induced both by reduction and by oxidation, depending on the sensitizer. While it was inferred from the existence of dimer radical anions D that splitting via the former route occurs in two steps, it has been debated for some time whether this also holds for the latter route, that is, whether dimer oxidation and cleavage are concerted or successive. CIDNP spectroscopy is particularly well suited to answer such questions because the intermediates leave their EPR spectrum (their polarization pattern) in the products. Thus, not only can the intermediates be identified by this signature, even if they are rather short lived, but the occurrence of their polarizations in a product... 

The quantum yield of DNA repair by photolyase (the number of cyclobutane pyrimidine dimers split by the enzyme for each photon absorbed by the enzyme in the enzyme-substrate complex) ranges from 0.7 to 1.0. It should be noted, however, that in photolyase FADH is the catalytic cofactor and MTHF (or 8-HDF) is the photoantenna. As a consequence, the quantum yield of photolyase is the product of three reactions (Payne and Sancar, 1990) energy transfer from MTHF (or 8-HDF) to FADH , electron transfer from (FADH ) to the PyrOPyr, and finally splitting of PyrOPyr °. The latter two reactions are very efficient and occur with nearly 100% efficiency, at least in the case of ToT. Therefore, the critical determinant of overall quantum yield of repair is the quantum yield of energy transfer from the photoantenna to the catalytic cofactor (Kim et aL, 1991, 1992). The efficiency of energy transfer by Forster radiationless... 

The transfer integrals are, as usual, calculated using the dimer splitting approximation [6], and the extended Hiickel [7] or CNDO/2 [8] methods the latter includes corrections due to the Fock potential [9]. We also tried to evaluate the two-electron terms U and Vp, using CNDO/2 or MINDO/3 methods. 

The term with does not have an influence on the dimer splitting of the lowest-lying states (due to the particular symmetry of these states) and can therefore be neglected. However, it is not zero as can be seen from the results of the previous section. A convenient basis set for the calculation is given by the functions J1J2JM) with J=J +J2 saA-J J. 

There are many chemical models for photoreactivable dimer splitting the quinone-dimer system is considered as a model of cationic intermediate of dimers and the tryptophan-dimer system is considered as a model of anionic intermediate [15]. Experimental model studies have demonstrated that both the photodimer radical anion and cation undergo facile fragmentation reactions [16-19]. The exact mechanism of the fragmentation, however, is not yet clearly understood for each of the dimer radicals, from the aspect of why the fragmentation reaction, in which orbital symmetries are not retained in going from reactant to product, proceeds thermally. 

With the goal of characterizing the repair process of pyrimidine dimers, it is important to understand the process of the dimer splitting in terms of the movement of electrons and as a time-dependent dynamic process. In this chapter, we describe the electronic structure characteristics of the pyrimidine... 

Macromonomers have also been used in the synthesis of telechelic polymers using benzyl mercaptan dimer in the synthesis of a,Q)-dicarboxyl telechelic PMMA. This reaction proceeds through an addition-fragmentation reaction whereby the macromonomer dimer splits in half One-half forms the co-terminal end of each macromolecule, while the other is released as a fiinclional initiating radical. 

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