Acidity resonance

The effect of a carboxy group is illustrated by the reactivity of 2-bromopyridine-3- and 6-carboxylic acids (resonance and inductive activation, respectively) (cf. 166) to aqueous acid under conditions which do not give hydroxy-debromination of 2-bromopyridine and also by the hydroxy-dechlorination of 3-chloropyridine-4-car-boxylic acid. The intervention of intermolecular bifunctional autocatalysis by the carboxy group (cf. 237) is quite possible. In the amino-dechlorination (80°, 4 hr, petroleum ether) of 5-carbethoxy-4-chloropyrimidine there is opportunity for built-in solvation (167) in addition to electronic activation. This effect of the carboxylate ion, ester, and acid and its variation with charge on the nucleophile are discussed in Sections I,D,2,a, I,D,2,b, and II,B, 1. A 5-amidino group activates 2-methylsulfonylpyridine toward methanolic am-... 

In an aromatic nitro compound such as picric acid resonance of the type... 

The experimental data closely resemble the simulation based on the all-trans PSB values. The discrepancy between the two solid-state NMR studies on rhodopsin arises in part from a difference, in signal-to-noise ratio and in part from possible problems associated with a fatty acid resonance which overlaps with the centerband in the previous study. The simulations illustrate the sensitivity of the sideband intensities to changes in the chemical shift tensor, as well as the quality of data necessary to accurately determine the shift tensor values. 

Selected entries from Methods in Enzymology [vol, page(s)] Acquisition of frequency-discriminated spectrum, 239, 162-166, 170 sensitivity, 239, 169-173 constant-time, 239, 23-26 doublequantum filtered, 239, 236 gradient pulse experiments, 239, 185-189 protein structural information, 239, 377-379 pulse sequence and coherence transfer pathway, 239, 148-149 paramagnetic metalloprotein, 239, 494-497 data recording, SWAT method, 239, 166-169, 172 line shapes, effects of gradient pulses, 239, 162-166 identification of protein amino acid resonances, 232, 100 cyclosporin A, 239, 240-241. 

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... 

VII. Multi-parameter Correlation Equations All authors who have seriously considered the scope and limitations of the linear free-energy relationships have recognized the existence of real deviations. Frequently, the limitations of the Hammett eq. (1) for certain substituents in certain situations were considered to be indicative of a duality of u-constants. Hammett noticed that the reactions of anilines and phenols required a special value for aJt NOt, 1.27, in contrast to the value, 0.778, derived from benzoic acids. An example is the increased acidity of p-nitrophenol over that expected on the basis of the a constant based on benzoic acid. Resonance interaction between the substituent and the side-chain is presumed to be responsible ... 

The melting transition of the daunomycin poly(dA-dT) complex can also be monitored at the nucleic acid resonance line widths and the data for the adenosine H-8 resonance are plotted in Figure 28. The resonance is very broad at temperatures below the melting transition of the complexes (dashed curves in Figure 28) indicative of stiffening of the synthetic DNA by the bound anthracycline ring. 

The observation of selective complexation shifts in the nucleic acid resonances of the synthetic DNA demonstrate a change in the glycosidic torsion angles of the adenosine and thymidine residues and a minimal perturbation in the base pair overlaps on addition of netropsin. These structural perturbations at the antibiotic binding site are propagated to adjacent antibiotic-free base pair regions at low netropsin concentrations. 

Keywords Excited-State Structural Dynamics, Nucleic Acids, Resonance Raman Spectroscopy,... 

The three olefinic protons of acrylic acid resonate as a complex higher-order pattern in the chemical shift range from 5.7-6.8 ppm. The most deshielded of these protons is the hydrogen that is cis to the carboxylic acid group, while the geminal proton resonates at higher field. This order of chemical shifts is the reverse of that observed for simple alkenes. 

The aromatic protons of the phenyl esters of Phosphonic Acid resonate as a broad, single peak or a complex series of bands centered at about 7.2 ppm. 

The experimental conditions for the carboxylation of allenyllithium are governed by the consideration that introduction of the COOLi group renders the allenic protons more acidic (resonance stabilization). If the normal order addition is used (introduction of C02 into the solution of allenyllithium), the allenic carboxylate primarily formed may easily be deprotonated by the strongly basic allenyllithium. The new species (either LiCH=C=CHCOOLi or H2C=C=C(Li)COOLi) may react also with C02. To avoid this situation the solution of allenyllithium is gradually added to a strongly cooled solution of carbon dioxide in THF. As usual, liberation of the acid occurs by adding mineral acid. In view of the possibility of an acid-catalyzed cyclization to a lactone too strongly acidic conditions should be avoided ... 

Although this approach works in principle with uniformly 15N-labeled protein, it is more favorable in combination with selective amino acid labeling. First, in the absence of a spin-labeled inhibitor, the specific amino acid resonances are identified when compared to the spectra recorded from uniformly labeled kinases. The number of peaks is reduced and therefore more manageable if the spin-labeled inhibitor is added in a second step. Especially in regions with aggravated signal overlap, the selective labeling technique clearly separates the peaks and, therefore, enables the quantification of the induced peak attenuation. 

Acids in which the aromatic nucleus resonates with neither the acidic resonator nor the acidic group. Examples are C6H5CH2-... 

In acids of the second type the resonance between the aromatic nucleus and the acidic resonator is a cross-conjugation decreasing the acid-strengthening resonance between the acidic group and its resonator. For instance, the polar term in A log for the change from acetic to benzoic acid is ... 

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