Purines electron distribution

Free pyrimidines and purines are weakly basic compounds and are thus called bases. They have a variety of chemical properties that affect the structure, and ultimately the function, of nucleic acids. The purines and pyrimidines common in DNA and RNA are highly conjugated molecules (Fig. 8-2), a property with important consequences for the structure, electron distribution, and fight absorption of nucleic acids. Resonance among atoms in the ring gives most of the bonds partial double-bond character. One result is that pyrimidines are planar molecules purines are very nearly... 

Benzannulation (Figure 11) significantly influences both -electron distribution and reactivity of azoles. Compared to imidazole 51, benzimidazole 54 has an increased positive charge at position 2 and this is consistent with its tendency to undergo reaction at this position with hard nucleophiles. This behaviour is analogous to the chemistry of azines. Purine is also notable for its high -deficiency, 7/7-purine 56 being more -deficient than the 9//-tautomer 55. Possibly this is responsible for the natural occurrence of the less -deficient and therefore chemically more stable derivatives of 9/7-purine. 

This contribution reviews applications of NMR spectroscopy in the investigation of the structure and the intra- and intermolecular interactions of purine derivatives. Purines represent a highly important class of heterocyclic compounds that are widely distributed in all living organisms, not only as constituents of nucleic acids, but also as signal molecules. Their structure, electron distribution, and proton-transfer processes determine their chemical reactivity, interactions with solvents, and, subsequently, also... 

To conclude this review, several important aspects should be highhghted. The nitrogen atoms of the purine skeleton are the centers for the intermolecular interactions between the purine molecule and its environment. The changes in the electron distribution induced by these interactions are reflected in changes of several NMR parameters which can be obtained from the samples at natural and " N abundance, namely the C and " N NMR chemical shifts and the and scalar coupling constants. All these... 

Replacement of the proton at N1 of G by a metal ion has qualitatively similar consequences as substitution of the imino proton of T and U. The Pt(II) binding to this site increases the basicity of the purine ring, but it is not clear how the residual electron density is distributed. The N1 platinated guanine bases titrate with a pA a of 5 (176, 177), and from the high pD dependence of the H8 resonances in the H NMR spectra it is concluded that the most likely protonation site is N7 (cf. Fig. 16). In its protonated state, the nucleobase represents again a metal-stabilized rare tautomer. 

Denis, A., and A. Pullman Theoretical study of the electronic properties of biological purines and pyrimidines. III. Theoret. Chim. Acta (Berlin) 7, 110 (1967). Del Re, G. A simple MO-LCAO method for the calculation of charge distributions in saturated organic molecules. J. Chem. Soc. (London) 4031 (1958). 

The biochemical applications involve the electronic nature of the components of DNA and proteins, especially the charge distributions, electron affinities, and gas phase acidities of purines and pyrimidines and amino acids. The role of electron reactions in diverse areas such as cancer, electron conduction, and sequence recognition all depend on fundamental energetic properties such as electron affinities and solution energies. We explain nonadiabatic experimental data from radiation chemistry by excited anionic states of biological molecules [24],... 

As with other aromatic compounds, the distribution of electrons plays a crucial role in the intermolecular interactions and controls the reactivity of the purine base. The electron densities differ substantially in five- and six-membered rings of heterocycles. The electron density in a five-membered ring is usually larger and is responsible for the predominant populations of the N -H and N -H tautomers. The number of tautomers increases when the purine moiety carries substituents such as amino, oxo, or thioxo groups. Amino tautomers and oxo- or thioxo-tautomers are the main species formed in solution. However, minor tautomers are of considerable importance. They can form odd base pairs within nucleic acids, thereby causing mutagenic events. 

The distribution of electrons around the purine skeleton affects not only its chemical properties and reactivity but also the basic parameters of its NMR spectra - chemical... 

Concerning cellular DNA, a two-component hypothesis has been developed. According to this hypothesis, the electron loss centers (radical cations) end up with the purines, particularly with the guanine moiety, whereas the final site of deposition of the ejected electron is with the pyrimidines, particularly with thymine [127]. The two-components hypothesis implies that in DNA there are mechanism of electron and positive hole transfer by which the initially generated and randomly distributed electron gain and loss centers are tunneled into the T and G traps respectively. 

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