Cleaved-dimer model

A variety of pyrimidine dimer-cleaving photochemical model systems have been developed to aid in the study and elucidation of the DNA-PL reaction mechanism (752, 158, 159). Direct excitation of pyrimidine dimers does not occur on ultraviolet or visible irradiation, suggesting that enzyme cofactors might be involved in a photosensitization process 146). Model systems utilizing free flavin derivatives have recently been described 137, 160). Rokita and Walsh have demonstrated that lumiflavin, 5-deazariboflavin, and 8-methoxy-7,8-didemethyl-7V °-ethyl-5-deazaflavin (Scheme 31) are effective photosensitizers for the thymine dimer cleavage reaction 160). These reactions utilized cis-syn-thymine dimer as substrate with irradiation at the Xmax of the flavin derivative, under strict anaerobic conditions and high pH. Flavin derivatives that contain electron-rich substituents at the 8-position, such as 8-hydroxy-5-deazariboflavin,... 

The classical cadherins are translated as precursor because they are N-terminally cleaved to reveal the mature proteins. This processing is required to activate the cell adhesion function of cadherins. Cadherins interact in trans (i.e., from opposite cells) via the most N-terminal cadherin rqDeats. A short amino acid sequence within this repeat, histidine-alanine-valine (HAV), has been implicated in mediating cell-cell contacts as HAV peptides can disrupt cadherin-dependent cell adhesion. Besides the trans-interactions of cadherins, the extracellular domains are also capable of forming cis-dimers through lateral amino acid contacts between cadherin molecules on one cell. This dimerization again mainly involves the first cadherin repeat. A zipper model based on the pattern of alternating cis- and trans-dimers [1] for the adhesive interactions has been proposed. 

The current working model for the beneficial role of protic and halide additives in reactions with aliphatic amines is outlined in Scheme 9.10 [11]. In these reactions, the rhodium dihalide dimer 30 is proposed to enter two different catalytic cycles. In the productive catalytic cycle, the dimer is cleaved by solvation or binding with the substrate to give 31. Oxidative insertion followed by pro to nation of the rhodium alkoxide... 

The mechanisms involved in dimer cleavage have been examined in simple model systems [90,261-263], including bifunctional ones in which a sensitizer (e.g. indole) is linked to the pyrimidine dimer [264, 265]. For example, the cw-head-to-head dimer of dimethylthymine (88) was cleaved efficiently upon irradiation of anthraquinone-2-sulfonate CIDNP effects observed during this reaction failed to reveal any evidence for the existence of the dimer radical cation [90]. Time-resolved... 

Dimers 15-19 represent the ji-5 bonding pattern. Upon thioacidolysis, the ben-zylic ether linkage of phenylcoumaran-type models is cleaved with concomitant thio-ethylation. The C. -methylol group of ji-5 structures is lost to an extent that depends on thioacidolysis duration. In contrast, the total amount oi 16+ 17 or 18 +19 is stable when this duration varies between 3 and 6 hours. Minor dimers of the ji-5 series are observed as dimers with shortened side chains, similar to dimer 15 or with rearranged side chains similar to monomers 4-5 (Figure 2.4). 

A detailed study of a lignin-model dimer with a phenol function, such as l-(3,5-dimethoxy-4-hydroxy-phenol)-2-(4-methoxy)propane-l,3-diol, indicated how this dimer was cleaved by MnP/H202/Mn. ... 

Alternatively, alkoxides or silylamides may first coordinate with the palladium precursor to form an intermediate that might react with the amine to form the required amido-aryl intermediate. Extensive kinetic studies on stoichiometric reaction models support a mechanism in which the amine cleaves the dimeric hydroxy complex 83 to give an amine intermediate 84, which would suffer intramolecular proton transfer to give 85 (Scheme 1.50) [391]. A similar process is proposed for a dppf derivative [392]. 

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