Cytosine pairs

Reaction between C in methanol and RTCNQ in acetonitrile yielded three kinds of ionic solids (1) insulators composed of methoxy substituted RTCNQ anions such as (CHC )[F4TCNQ-0Me ](H20) (Fig. 6) [136], (2) semiconducting CT solids with fully ionic RTCNQ radical anions such as (CHC )(TCNQ ) [137, 138], and (3) conducting CT solids of partially ionic or mixed valent RTCNQ radical anions such as (CHC"XMeTCNQ° >2 [138], where CHC" is the hemiprotonated cytosine pair (Fig. 6b). Cation units in aU products were found to be protonated cytosine species, most commonly CHC, where comes from methanol. This result suggests that the intrinsic transport properties of DNA should be studied not in protic solvents but under strictly dried conditions. 

Fig. 7 Crystal structure of (CHC )(TCNQ) salt, (a) Uniform segregated stacks of CHC and TCNQ. (b) CHC ribbons by complementary hydrogen bonds and TCNQ form a layer within a bc-plane (hydrogen bonds red dotted lines), (c) Formation of three-dimensional structure (//a) between hemiprotonated cytosine pair and RTCNQ species...

An interesting feature was discovered by Sharma and co-workers494,495 in the crystal structure of isocytosine. Two tautomers of isocytosine (42 and 43) exist in an exact 1 1 ratio in the crystal. The tautomers are hydrogen-bonded to each other in a manner analogous to that proposed by Watson and Crick496,497 for the guanine-cytosine pair in DNA. It is worth underlining that the base pair of isocytosine was not obtained by expedient cocrystallization of the two components. It seems therefore probable that both forms 42 and 43 of isocytosine are of approximately equal stability and are present in comparable amounts in solution. 

Storoniak P, Kobylecka M, D bkowska I, Rak J, Gutowski M (2007). Comparison of intermolecular proton transfer in the Watson-Crick anionic guanine-cytosine and 8-oxoguanine-cytosine pairs. To be submitted. 

In two papers, the energies of the molecular orbitals of the guanine-cytosine pair have not been given. The evaluation could be made only from the correlation diagram of the molecular orbitals. 

DMA consists of nucleotides. Nucleotides consist of the bases A, T, G, and C, which are attached to residues of deoxyribose-phosphate. This statement is not entirely correct, since about 1,0% of the cytosine bases in human DNA is modified by a methyl group to form 5-methyl-cytosine. 5-Methy 1-cytosine is formed by an enzyme that flips the cytosine residue out of the DNA helix, methylates it, and flips it back into the helix. 5-Methvl-cytosine pairs with guanine, as does cytosine. 

Base pairing between nucleic acid bases can occur in RNA with adenine pairing with uracil, and cytosine pairing with guanine. However, the pairing is between bases within the same chain, and it does not occur for the whole length of the molecule (e.g. Fig. 6.18). Therefore, RNA is not a double helix, but it does have regions of helical secondary structure. 

Kopka et al. (186) solved the X-ray crystal structure of netropsin (1) complexed with CGCGAATTBrCGCG at 2.2 A resolution. Netropsin binds to the minor groove of double helical B-DNA and is selective for four or more A-T (adenine-thymine) base pairs a single G-C (guanosine-cytosine) pair prevents binding (see color insert, Figure 10). Netropsin s amide NH groups... 

In nucleic acids, the pyrimidines match up with specific purine bases. The matching between purines and pyrimidines forms "base-pairs" in which uracil (in RNA) or thymine (in DNA) pairs with adenine. Cytosine pairs with the base guanine in either nucleic acid. 

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