Cytosine structure

Figure 11-6. Structures of representative conical intersections Sj/Sq in the pyrimidine bases, uracil, thymine, and cytosine. Uracil structures (a,d) are taken from Ref. [147, 210]. Thymine structures (b,e) are taken from Ref. [152], Cytosine structures (c,f) are taken from Ref. [157]...

The cytosine structures also reflect thermodynamic tendencies, although, as stated, exocyclic amine binding has been observed. For bis(pyrimidine) complexes, the trends are in fact opposite to those observed for the bis(guanine) systems and result in severe crowding for the [Pt(NH3)2(l-MeCyt)2] cation. 

Write a structural formula for the enol tautomer of cytosine... 

The structure of cytosine is certainly the aminooxo form (60) which has been confirmed by X-ray analyses <73AX(B)1234), UV and NMR data <63JCS3046), Raman spectra <67SA(A)255l) and even by N NMR spectra (72JPC5087). 

Cytosine was isolated from hydrolysis of calf thymus in 1894 and by 1903 its structure was known and it had been synthesized from 2-ethylthiopyrimidin-4(3H)-one. The acid hydrolysis of ribonucleic acid gives nucleotides, among which are two cytidylic acids, 2 -and 3 -phosphates of cytidine further hydrolysis gives cytidine itself, i.e. the 1-/3-D-ribofuranoside of cytosine, and thence cytosine. The deoxyribonucleic acids likewise yield deoxyribonucleotides, including cytosine deoxyribose-5 -phosphate, from which the phosphate may be removed to give cytosine deoxyriboside and thence cytosine. 

Three more antibiotics, all discovered about 1953, are also derivatives of cytosine. Amicetin, bamicetin and plicacetin may all be isolated from Streptomyces plicatus and all have some activity against some acid-fast and Gram-positive bacteria as well as some other microbial systems (69MI21301). Structural work in this area is fascinating (62JOC2991). 

Another property of pyrimidines and purines is their strong absorbance of ultraviolet (UV) light, which is also a consequence of the aromaticity of their heterocyclic ring structures. Figure 11.8 shows characteristic absorption spectra of several of the common bases of nucleic acids—adenine, uracil, cytosine, and guanine—in their nucleotide forms AMP, UMP, CMP, and GMP (see Section 11.4). This property is particularly useful in quantitative and qualitative analysis of nucleotides and nucleic acids. 

DNA is made up ot two intertwined strands. A sugar-phosphate chain makes up the backbone of each, and the two strands are joined by way of hydrogen bonds betwen parrs of nucleotide bases, adenine, thymine, guanine and cytosine. Adenine may only pair with thymine and guanine with cytosine. The molecule adopts a helical structure (actually, a double helical stnrcture or double helix ). 

Although cytosine was assigned structure 109 on the basis of early infrared spectral studies,and other infrared and ultraviolet spectral... 

The proton transfer in these clusters via the water bridge was found to be about three times as fast as a nonassisted transfer, underscoring the importance of the solvent for the reaction mechanism [98IJQ855]. In addition to the relative stabilities of the cytosine tautomers, the structures and properties of some cytosine derivatives have been investigated, mainly those of 5-hydroxycytosine 111 and 5,6-dihydroxycytosine 112 (Scheme 73) [99JST1, 99JST49]. 

Chemical Name 4-amino-1(3-D-arabinofuranosyl-2(1H)-pyrimidinone hydrochloride Common Name (3-cytosine arabinoside Structural Formula ... 

The following molecular models are representations of (a) adenine and (b) cytosine, constituents of DNA. Indicate the positions of multiple bonds and lone pairs for both, and draw skeletal structures (gray = C. red = O, blue = N, ivory = IT). 

Cyclopentenones. from 1.4-diketones. 886-887 Cyclopropane, angle strain in, 115 bent bonds in. 115 from alkenes. 227-229 molecular model of, 111. 115 strain energy of, 114 torsional strain in, 115 Cystathionine, cysteine from. 1177 Cysteine, biosynthesis of, 1177 disulfide bridges from, 1029 structure and properties of, 1018 Cytosine, electrostatic potential map of, 1104... 

The secondary structure of DNA is shown in Figure B. This "double helix" model was first proposed in 1953 by James Watson and Francis Crick, who used the x-ray crystallographic data of Rosalind Franklin and Maurice Wilkins. Beyond that, they were intrigued by the results of analyses that showed that in DNA the ratio of adenine to thymine molecules is almost exactly 1 1, as is the ratio of cytosine to guanine ... 

The two strands of the nucleic acid DNA are held together by four organic bases. The structure of one of these bases, cytosine, is shown below, (a) How many protons can this base accept (b) Draw the structure of each conjugate acid that can be formed, (c) Mark with an asterisk any structure that can show amphiprotic behavior in aqueous solution. 

Attached by a covalent bond to carbon atom 1 of the deoxyribose ring is an amine (and therefore a base), which may be adenine, A (22) guanine, G (23) cytosine, C (24) or thymine, T (25). In RNA, uracil, U (26), replaces thymine. The base bonds to carbon atom 1 of deoxyribose through the nitrogen of the —NH— group (printed in red) and the compound so formed is called a nucleoside. All nucleosides have a similar structure, which we can summarize as the shape shown in (27) the lens-shaped object represents the attached amine. 

The ability of DNA to replicate lies in its double-helical structure. There is a precise correspondence between the bases in the two strands. Adenine in one strand always forms two hydrogen bonds to thymine in the other, and guanine always forms three hydrogen bonds to cytosine so, across the helix, the base pairs are always AT and GC (Fig. 19.29). Any other combination would not be held together as well. During replication of the DNA, the hydrogen bonds, which are... 

---