Zwitterionic character

As a multidimensional PES for the reaction from quantum chemical calculations is not available at present, one does not know the reason for the surprismg barrier effect in excited tran.s-stilbene. One could suspect diat tran.s-stilbene possesses already a significant amount of zwitterionic character in the confomiation at the barrier top, implying a fairly Tate barrier along the reaction path towards the twisted perpendicular structure. On the other hand, it could also be possible that die effective barrier changes with viscosity as a result of a multidimensional barrier crossing process along a curved reaction path. 

Two concerns about the pharmacokinetic properties of the inhibitor series developed to this point were the zwitterionic character of the pyrrolidine-based compounds and the metabolic liability of the methyl ester (Maring et al. 2005). Attempts to remedy the potential drawback of zwitterionic character through replacement of... 

Zwitterionic character is notable in several therapeutic area series, e.g. in angiotensin-converhng enzyme inhibitors, quinolone anhbacterials and thrombin inhibitors. The aqueous solubiUty measurement of zwitterions is very pH dependent as might be expected. The relationship of aqueous solubiUty to ionization state is extraordinarily complex if the zwitterion is of the type capable of an equi-Ubrium between true zwitterion and formally neutral forms (e.g. as in a quinolone antibacterial). For these types of complex equilibria, salt effects on solubility may be unexpectedly large, e.g. solubility unexpectedly may track with the chaotropic character of the salt... 

In polar solvents, the structure of the acridine 13 involves some zwitterionic character 13 a [Eq. (7)] and the interior of the cleft becomes an intensely polar microenvironment. On the periphery of the molecule a heavy lipophilic coating is provided by the hydrocarbon skeleton and methyl groups. A third domain, the large, flat aromatic surface is exposed by the acridine spacer unit. This unusual combination of ionic, hydrophobic and stacking opportunities endows these molecules with the ability to interact with the zwitterionic forms of amino acids which exist at neutral pH 24). For example, the acridine diacids can extract zwitterionic phenylalanine from water into chloroform, andNMR evidence indicates the formation of 2 1 complexes 39 such as were previously described for other P-phenyl-ethylammonium salts. Similar behavior is seen with tryptophan 40 and tyrosine methyl ether 41. The structures lacking well-placed aromatics such as leucine or methionine are not extracted to measureable degrees under these conditions. 

When derivatives of amino acids are formed, their zwitterionic character is destroyed, a process which requires the supply of energy. We shall first discuss chemical activation, as it is important for the understanding of hypotheses dealing with prebiotic protein formation. In the case of amino acids which are activated at the carbonyl group, the amino group remains unsubstituted. The derivatives are able to react with nucleophilic residues (Y) ... 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

The CASPT2 barrier to enantiomerization of 7 via 1A -4b is 20.5 kcal/mol (Fig. 12), which is almost the same as that calculated for the enantiomerization of 3b via A"-4b. Although the out-of-plane distortion of A -4b to one of the enantiomers of 7 provides dramatic energy lowering, the zwitterionic character of 7 still results in its being calculated to be ca. 25 kcal/mol higher in energy than its isomer 3b. 

It was demonstrated that the transition state of these processes exhibits a weak zwitterionic character and that secondary orbital interactions facilitate the endo approach. 

Structure of the mesoionic derivative of the imidazo[2,l-/][ l, 2,4]triazine ring system 55 has been determined <2005TL5325>. The X-ray analysis unequivocally revealed the zwitterionic character. 

This versatile water-soluble reactant has been evaluated in wool and nylon dyeing. The nucleophilic aminoalkyl derivatives of orthodox aminochlorotriazine dyes behave like traditional acid dyes on wool owing to their zwitterionic character under neutral-dyeing conditions (Scheme 7.65). Improved wet fastness can be achieved using the reactant XLC... 

One characteristic feature of surfactants is their amphiphilic nature. These molecules present two moieties the hydrophobic moiety (usually a hydrocarbon chain) interacts with the nanotube sidewalls, while the hydrophilic part, called polar head group, is generally charged or has zwitterionic character. It has the double function of helping solubility in aqueous solvents and of providing additional stabilization towards tubes aggregation by coulombic charge repulsion. 

Spiropyran merocyanine specffa shift markedly to the blue as the solvent polarity increases [4,25], as shown in Fig. 7a for 6,8-dinitro-BIPS merocyanine. This is generally accepted to imply that they have a zwitterionic character caused by the donation of electron density from the indoline nitrogen to the phenolic C9 oxygen [4,25]. The rational behind this assignment to the zwitterion is based simply on the fact that if the zwitterion is in the highest occupied molecular orbital (HOMO) state, then the corresponding quinoidal resonant form is in the lowest unoccupied molecular orbital (LUMO) state and changing the solvent to a more polar one... 

Several computational studies have addressed whether the dipolar cycloaddition of nitronates is a concerted or stepwise process (93,100). Natural population analysis reveals that their is very little zwitterionic character in the transition state. The formation of the C C bond marginally precedes the C—O bond on the basis of calculated bond lengths and orders in the transition structure. These calculations also show that the reaction is a concerted process that is shghtly asynchronous. In addition, the cycloaddition likely proceeds through an early transition state and is overall an exothermic process. 

Phosphorus ylides are useful synthons77-79 and there is great interest in the bond between phosphorus and carbon80-82. Today, the zwitterionic character of this bond is undisputed82. After an early indication of a thermal fission of alkoxy-substituted al-kylidenephosphoranes83, there have been few subsequent studies that have suggested decomposition of the phosphorus ylides into phosphanes and carbenes84-88. 

Problems with the compatibility of the dyes can also arise in dyeing mixtures of PA and PAC fibers. Such blends are produced, e.g., for skiwear. The cationic dyes used for the PAC component do not dye normal PA fibers. Acid and metal-complex dyes leave PAC fibers practically white. Sulfo group containing 1 1 metal-complex dyes possess a zwitterionic character and therefore can be applied in low concentration in the same bath with cationic dyes, without the occurrence of precipitation. In this manner, one-bath dyeing can be carried out. The stability of... 

P-Substituents have no influence on the diradical or zwitterionic character of tetramethylene. However, their presence favors the gauche conformation over the irons (extended) form [89] ... 

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