The ionised carboxyl group

The ionised carboxyl group. The position of the absorption bands of the ionised carboxyl group are fairly well established (Chapter 10.5) following the work of Lecomte [25], and Raman studies on both normal acid salts and amino-acids, by Edsall [1—5, 60]. Two absorptions corresponding to the asymmetric and symmetric modes occur near 1550 cm and 1410 cm . The former is usually more easy to identify in infra-red spectra, as many other absorptions can occur in the same region as the second band. 

This correlation is applicable to all types of amino-acids, regardless of whether or not the amine group has the NH2 or NH3 structure. A similar effect is reported in pyridine betaine [26], although in this case the absorption occurs as high as 1652 cm .  

With non-crystalline preparations it is sometimes possible for both the ionised and un-ionised carboxyl absorptions to appear [21 ], and this is a source of possible confusion in work of this kind. The effect is not, however, common. 

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Amino acids. a-Amino acids (and other amino acids also) normally exist as zwitterionic salts (e.g. (1)), and therefore show bands characteristic of the ionised carboxyl group and an amine salt. Hence there is no absorption corresponding to the normal stretching vibrations as exhibited by an amine, but instead a complex series of bands is observed between 3130 and 2500 cm - and this is also the... 

When an amino acid comes into contact with a strong acid, such as hydrochloric acid, it is the ionised carboxyl group (COO ) that accepts the proton and helps to regulate the acid balance. 

Structure/activity correlation, based on the above mentioned results, indicated that the terpenoid nucleus, the substitutions on the tetracyclic ring, and the aliphatic acidic side chain are important for the activity. In addition, the results of kinetic studies suggest that the mechanism of inhibition was competitive, and that binding to the catalytic site was due to the carboxylic group because when it was methylated, the activity disappeared. The amphipathic character of both triterpenes could explain the minor potency of masticadienoic acid in relation to schinol the ionised carboxylate group is probably at the surface whereas the tetracyclic nucleus must be in the hydrophobic zone of the bilayer [50]. 

There are however also points, showing that the polarisability of the ionised carboxyl group of these colloids can only be slightly stronger than of water ... 

It is very interesting, that the sequence of the alkali cations is quite the reverse of that characteristic of the carboxyl colloids of 2d (seep.284, Fig. 14), further that the sequence obtained with the alkaline earth cations is also different, a characteristic transition sequence occurring. All this shows that the ionised carboxyl group in proteins is more polarisable than the same group in the carboxyl colloids of 2d. We have here once more a characteristic example of constitutional influences on the polarisability of the ionised groups, discussed already in 2k (p. 292). 

In the solid state the two frequencies show minor variations with the nature of the metallic ion and also with the nature of the group to which the ionised carboxyl group is attached. Kagarise [83] has shown that for mono- and di-valent elements there is a linear relationship between the electro-negativity of the element and the asymmetric stretching frequency of the salt, and Stimpson [84] has also noted variations of this type. Changes in the nature of the... 

Much information of a qualitative nature is available about participation by various protecting groups in nucleophilic displacements at the anomeric centre during glycoside synthesis, but quantitative information is confined to participation by acetamido, carboxylate and the ionised hydroxyl groups of the sugar itself. 

Hille extended this hypothesis, proposing that the part of the channel where TTX binds is the part of the channel conferring the selectivity properties [51]., The proposed structure included a 3 by 5 A constriction of the Na-channel formed by a ring of six oxygen atoms. One group in the ring was supposed to be an ionised carboxylic group. 

Thus from succinate onwards, lengthening of the carbon chain between the two ionised carboxyl groups lowers the reversal of charge concentration, but starting from the oxalate ion the first two extra CH2 groups have the reverse effect. 

At pHs above the p/STg of the 3-carboxyl group, the rate of degradation of penicillin is acid catalysed up to about pH 6. The pathway for hydrolysis could be an acid catalysed reaction of the penicillin with an ionised carboxylate or the kinetically equivalent mechanism, the spontaneous degradation of penicillin with an unionised carboxyl function as indicated in (3). 

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