C-O group

Ramachandran plot (see Figure 1.7a). The a helix has 3.6 residues per turn with hydrogen bonds between C =0 of residue n and NH of residue n + 4 (Figure 2.2). Thus all NH and C O groups are joined with hydrogen bonds except the first NH groups and the last C O groups at the ends of the a helix. As a consequence, the ends of a helices are polar and are almost always at the surface of protein molecules. 

As shown, an asymmetric carboxylic-sulfonic acid anhydride is formed, but the cellulose attack occurs on the C = O group, since a nucleophilic attack on sulfur is slow, and the tosylate moiety is a much better leaving group than the carboxylate group [193]. Similar to other acylation reactions, there is a large preference for tosylation at the 5 position, and cellulose tosylates... 

The 2 1 reaction of 9-BBN with a series of dicarboxylic acids, namely oxalic acid, malonic acid, 2,2-dimethylmalonic acid, and succinic acid, in dimeth-oxyethane gives in some cases dimeric and in other cases macrocyclic (acyloxy)diorganoboranes. This has been proved by IR spectroscopy (all C = O groups are bidentate), B-NMR 5 = 10 ppm) and X-ray crystallography [47]. With oxalic acid two structures are possible (IV and V), of which the first with a five-membered boron heterocycle instead of a four-membered one is the more probable formulation (Fig. 13). 

Compare, also, the spectrum of -decane with those of a -propyl alcohol, (Figure 9.20) and -undecanal (Figure 9.21). The spectrum of the alcohol is characterized by vibrations due to the O—H and C—O groups. The O—H... 

That the two C O-groups of these diketones are adjacent is proved by the fact that they are capable of condensing with o-phenylene-diamine (quinoxalines, Hinsberg). 

Also termed ethyl acetoacetate, or CH3COCH2COOC2-H5. As an ester, it can be hydrolyzed under certain conditions to acetoacetic acid (CH3COCH2COOH) as a ketone, it reacts with reagents for the carbonyl (C = O) group. Peculiarly, it also behaves like a hydroxyl (OH-) compound. It is the prototype of the phenomenon of tautomerism, and its isomeric forms are termed the keto and the enol forms. 

An important element in the three-dimensional structure of a protein is the secondary structure. The secondary structure results from the formation of hydrogen bonds between the—N—H groups and the carbonyl (C O) groups of the peptide bonds —N—H 0=C. There are two basic ways to do this. We can form a helix or we can form a sheet. The great American chemist Linus Pauling won the Nobel Prize in Chemistry in 1954 for the elucidation of these structures. 

Recent Developments in Ring-Chain Tautomerism L Intramolecular Reversible Addition Reactions to the C = O Group ... 

Figure 4.10 shows the UV absorption spectra of a solution of procaine in 0.1 M HCl and O.IM NaOH. In procaine, the benzene chromophore has been extended by addition of a C = O group and under acidic conditions, as in Figure 4.10, the molecule has an absorption at 279 nm with an A (1%, 1 cm) value of 100. In addition to the extended chromophore, the molecule also contains an auxochrome in the form of an amino group, which under basic conditions has a lone pair of electrons that can interact with the chromophore producing a bathochromic shift. Under acidic conditions the amine group is protonated and does not function as an auxochrome but when the proton is removed from this group under basic conditions a bathochromic shift is produced and an absorption with A, max at 270 nm with an A (1%, 1 cm) value of 1000 appears. 

In the spectra of vinyl or phenyl esters, with unsaturation adjacent to the C—O— group, a marked rise in the carbonyl frequency is observed along with a lowering of the C—O frequency. Vinyl acetate has a carbonyl band at 1776 cm-1 phenyl acetate absorbs at 1770 cm-1. 

N other-(c=o)- is the number of C=O groups in ester and carbonate moieties as well as in anhydride groups. 

Vinylsilanes (8, 491 492) attylic alcohols (9, 340). Details are available for conversion of a ketone to a vinylsilane in which the C—Si bond has replaced the C -O group (cncsilylation). The reaction affords the less substituted vinylsilane in the case of unsymmetrical ketones. The paper includes details for use of vinylsilanes for cyclopcntenone annelalion by l riedel-Crafts acylation with acryloyl chlorides and subsequent cyclization of pentadienyl cations (9, 498-499).1... 

Photochemical and Thermal Reactions of Some Heterocycles Containing C=N—O or N=C—O Group T. Mukai, T. Kumagai and O. Seshimoto, Pure Appl. Chem., 1977, 49, 287-304. 

In the center, perhaps between three or more transmembrane helices, there is a narrow cavity, perhaps resembling that of the K+ channel (Fig. 8-21), into which chelating groups (e.g., C = O groups of the peptide chain) protrude. These groups form the three binding sites for the 0.19-nm-diameter Na+ ion. 

R. L. Baxter, and L. Sawyer. Biotin synthesis requires three other enzymes (steps b, c, d). Step b is catalyzed by a PLP-dependent transaminase. At the left is thiamin diphosphate, in the form of its 2-(1 -hydroxyethyl) derivative, an intermediate in the enzyme pyruvate decarboxylase (Dobritzsch et al.,. Biol. Chem. 273,20196-20204,1998). Courtesy of Guoguang Lu. Thiamin diphosphate functions in all living organisms to cleave C-C bonds adjacent to C=O groups. 

The latest proposed mechanisms1462 for several zinc-containing metalloenzymes combine elements from both types of mechanism by suggesting that the substrate binds to the enzyme through the C—O group, but that in the process the metal-bound water molecule is not displaced, so that the reaction proceeds via a five-coordinate intermediate. This hybrid mechanism is discussed below in greater detail for alcohol dehydrogenase. 

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