Uracils 2 molecules

This four-atom replacement was observed in some reactions of uracil derivatives, containing at position 5 a substituent with the CCCN moiety. Treatment of the Z-isomer 5-(2-carbamoylvinyl)-l,3-dialkyluracil with ethanolic sodium ethoxide gave in good yield 3-ethoxycarbonylpyridin-6(lf/)-one (84%) together with 3-A-methylcarbamoyl)pyridin-6-(l7 )-one (10%) (85JOC1513) (Scheme 26). The reaction involves an initial attack of the terminal amino group of the side-chain on position 6 of the uracil molecule. C-6-N-1 bond fission and N-C bond formation yield the pyridin-6(l//)-one. A subsequent attack of the ethoxide ion on the carbonyl groups of the side-chain yields both pyridin-2-one derivatives (Scheme 26). Similar results were obtained with the -isomer. 

The uracil molecule is one of foe bases in DNA. Estimate foe approximate values of foe indicated bond angles. Its skeleton (not its Lewis structure) is gfven below. 

It has been argued [235] by analogy with the case of molecules adsorbed on glassy n-hexane [232] that this enhancement is due to the electron transfer to CF2CI2 of an electron previously captured in a precursor state of the solvated electron in the water layer, which lies at and just below the vacuum level [300,301] and the subsequent. Similar results have been reported for HCl adsorbed on water ice [236]. It has been proposed that enhanced DEA to CF2CI2 via electron transfer from precursor-solvated states in ice [235] may explain an apparent correlation between cosmic ray activity (which would generate secondary LEE in ice crystals) and atmospheric ozone loss [11]. The same electron transfer mechanism may contribute to the marked enhancement in electron, and x-ray-induced dissociation for halo-uracil molecules is deposited inside water ice matrices [39]. 

Adsorption and phase formation of uracil on massive Au[ (lll)-(110)] singlecrystal and Au (111 - 20 nm) film electrodes in 0.1 M IT2SO4 has been studied in electrochemical measurements and applying ATR surface-enhanced infrared reflection absorption spectroscopy [299]. At E < 0.15 V (versus trapped hydrogen electrode), uracil molecules are disordered and planar oriented. Close to the pzc, a 2D condensed physisorbed film of planar-oriented molecules interconnected by directional hydrogen bonds, is formed. 

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... 

In addition to these aforementioned fundamental reactions on the intact uracil molecule, suitable substituted uracil derivatives have proven to be versatile starting materials of typical synthon character, leading to novel condensed heterocyclic systems consisting of two, three, and more heterocyclic rings. Many of these possess great biological interest [antibiotics,... 

Figure 3. (a) Internal coordinates of the Cq+ -Uracil system, (b) Geometry of the Uracil molecule. 

The geometries of the ground state as well as of the singly and doubly ionized Uracil molecules have been optimized by means of DFT calculations using the B3-LYP functional. The vertical and adiabatic first and second ionization potentials presented in Table 2 compare favourably to previous calculations [25, 26] and to experimental data [27,28],... 

From these observations, it would be predicted that a partial keto-enol tautomerism of the carbonyl groups (last entry in Table III) such as is found in the singly ionized uracil molecule, would lead to a two and a half-fold decrease in reactivity. Hence, the fivefold decrease in e aq reactivity upon ionization of the pyrimidine ring may be accounted for by a twofold decrease because of the negative charge and a two and a half-fold decrease because of the partial tautomerization. 

A serious loss (—50% ) of tritium from uracil-5-t was observed to be one result of these photolyses. This loss of isotope was probably not caused by cycles of photohydrate formation and decomposition because the photohydrate should have been quite stable under the conditions of the photolyses. We think that this is likely the result of rapid isotope exchange in the protonated excited uracil molecule, by a mechanism... 

Three such equilibria can be considered for the photolysis of uracil (and the substituted uracils), Equations 8a, 8b, and 8c. Equation 8c represents protonation of an excited uracil molecule. The excited species involved... 

The observed sigmoidal dependence of rate constant upon pH is consistent with an ordinary photochemical reaction mechanism which includes an equilibrium between an excited uracil molecule and hydrogen ion. Consider the following set of reactions, a-f... 

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