Zwitterion amino acids, effect

Aminolysis of active ester 91 can be effected by the amino group of an amino acid or peptide that is not protected at the terminal carboxy group (Scheme 27). Zwitterionic amino acid 92 or peptide is deprotonated by the addition of a base that generates the anionic form of the reactant. A partially aqueous milieu containing a water-miscible solvent such as acetone, dioxane, or acetonitrile is best for solubilizing the reactants. Esters that are hydrolyzed slowly or not at all are preferred because the acid produced is not readily separated from target molecule 66 (obtained from anion 93) because of their similar properties. Piperidino esters do not undergo hydrolysis but their aminolysis is slow.bl 4-Nitrophenyl esters react efficiently but the nitrophenol is difficult to remove. Succinimido esters are employed most frequently and are particularly useful in the preparation of N -protected dipeptides. Use of an excess of amino acid 92 favors complete consumption of active ester 91 by aminolysis before any of it can hydrolyze. 

The second problem that arises from the use of pre-calculated CS hypersurfaces is the neglect of the molecular environment in the calculation of small model systems. For chemical shift tensors of zwitterionic amino acids this effect was studied by de Dios et Quite large effects were observed for the carbonyl carbons when the crystal lattice was included in the ab initio calculations in the form of a point charge distribution. The an tensor component of L-tyrosine changed by 16 ppm and the 072 tensor component by 37 ppm. The changes for L-threonine were only slightly smaller. 

In your communication you use the same term(s) in relation to zwitterionic amino acids. The first group of people refer thus rather to amino acid residues inside the protein chain. It is obvious that different results may be expected for the two types of compound studies. It appears to me on the basis of data presented in my own communication here that free zwitterions should have a marked preference for a highly folded structure, permitting the interaction of the positive and negative ends. The existence of extended structure in solution (or in crystal) must therefore be due to environmental effects. 

Trivalent rare earth cations form complexes with a series of hydroxycarboj lic acids and amino acids in aqueous solutions, some of which are listed in fig. 8. Earlier studies of Katzin (1968, 1969) revealed that these biological substrates effectively act as bidentate ligands toward the rare earth cations. Rare earth complexes with multidentate chelators were recently applied as receptors for zwitterionic amino acids of biological and artificial interests. Aime et al. (2001a) characterized the complexation behaviors of zwitterionic amino acids with oc-... 

Solutions of polyelectrolytes contain polyions and the free (individual) counterions. The dissociation of a polyacid or its salt yields polyanions, and that of a polybase or its salt yields polycations, in addition to the simple counterions. The polyampholytes are amphoteric their dissociation yields polyions that have anionic and cationic functions in the same ion and often are called zwitterions (as in the case of amino acids having HjN and COO groups in the same molecule). Such an amphoter will behave as a base toward a stronger acid and as an acid toward a stronger base its solution properties (particularly its effective charge) will be pH dependent, and an isoelectric point (pH value) exists where anionic and cationic dissociation is balanced so that the polyion s charges add up to zero net charge (and solubility is minimal). 

Some amphoteric softeners such as amino acids (10.237) and sulphobetaines (10.238) are more effective and durable than the nonionic types but less durable than the cationics moreover, they tend to be expensive. Other amphoteric types include the zwitterionic forms of quaternised imidazolines (10.239) long-chain amine oxides (10.240) also exhibit softening properties. 

Typical examples are the conversion of the neutral form of an amino acid into its zwitterionic form, the helix-coil transitions in polypeptides and polynucleotides, and other conformational changes in biopolymers. Reactions of higher molecularity in which reactants and products have different dipole moments are subject to the same effect (association of the carboxylic acids to form hydrogen-bonded dimers). Equilibrium involving ions are often more sensitive to the application of an electric field ... 

Given that hydrolysis is a reversible reaction, the principle of microscopic reversibility implies that biocatalytic aminoacylation should also be applicable as a mild and efficient alternative method of introducing the side chain of both penicillin- and cephalosporin-based antibiotics. This is the case, with PGAs proving to be particularly effective biocatalysts towards the aminoacylation of both penicillin and cephalosporin nuclei with a variety of carboxyhc acids. Amoxicillin and cephalexin, two of the most important (3-lactam antibiotics, contain an (/ )-phenylglycine side chain which cannot be directly introduced as the amino acid due to its zwitterionic nature at the moderate pH values at... 

The imidazole side-chain of histidine has a value of 6.0, making it a weaker base than the unsubstituted imidazole. This reflects the electron-withdrawing inductive effect of the amino group, or, more correctly the ammonium ion, since amino acids at pH values around neutrality exist as doubly charged zwitterionic forms (see Box 4.7). Using the Henderson-Hasselbalch equation, this translates to approximately 9% ionization of the heterocyclic side-chain of histidine at pH 7 (see Box 4.7). In proteins, plCa values for histidine side-chains are estimated to be in range 6-7, so that the level of ionization will, therefore, be somewhere between 9 and 50%, depending upon the protein. 

Some authors based their approach to selective binding of the more lipophilic a-amino acids in water on hydrophobic effects using water-soluble, cavity-containing cyclophanes for the inclusion of only the apolar tail under renouncement of any attractive interaction of the hosts with the zwitterionic head . Kaifer and coworkers made use of the strong affinity of Stoddart s cyclobis(paraquat-p-phenylene) tetracation 33 for electron-rich aromatic substrates to achieve exclusive binding of some aromatic a-amino acids (Trp, Tyr) in acidic aqueous solution [48]. Aoyama et al. reported on selectivities of the calix[4]pyrogallolarene 34 with respect to chain length and t-basicity of aliphatic and aromatic amino acids, respectively [49]. Cyclodextrins are likewise water-soluble and provide a lipophilic interior. Tabushi modified )S-cyclodextrin with a 1-pyrrolidinyl and a carboxyphenyl substituent to counterbalance the... 

The effect of ionic form on the reaction of the hydrated electron with amino acids has been examined. The cationic form could not be examined since appreciable amounts of H + would have to be present, and with currently available techniques the electron would disappear too rapidly. But by making the solutions alkaline it has been possible to study the anionic form. For glycine (Table I), and several other amino acids and peptides (7), it has been shown that the amino acids are less reactive in the anionic form, agreeing with the conclusion drawn by Garrison. The results for glycine however cannot be interpreted on the basis of the known pK together with assumed rate constants for zwitterion and anion. Other factors are evidently present, and further work is required. 

Amino acids such as alanine actually exist as species called zwitterions, with a positive charge on the nitrogen and a negative chaige on the oxygen. Explain what effect you expect this to have on the melting point of alanine. 

Perhaps the most spectacular success of explanations based on solvation of ground states, published to date, is the dissection of activation parameters for solvolysis of t-butyl chloride in mixtures of ethanol and water, first discussed by Winstein and Fainberg (1957). The complex variation of AH and AS (Fig. 21) has been shown to be due almost entirely to ground state solvation effects, at least for the solvents ethanol—40% ethanol/water studied by Arnett et al. (1965). For 90%, 80%, 70%, 60%, 50% and 40% ethanol/water the parameter AH1 for solvation of the transition state (by transfer from the gas phase) was calculated to be linearly proportional to the corresponding value of AS, as expected from the behaviour of simple salts. The point for pure ethanol did not fall on the calculated line, and this was attributed to nucleophilic solvent assistance. The variation in AG, AH and AS (Fig. 21) can be reproduced remarkably well using ethane and the zwitterionic a-amino acid, glycine, as model compounds (Abraham et al., 1975 see also Abraham, 1974 Abraham and Abraham, 1974). 

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