Ionization carboxyl

Amino acid esters can be monodentate (1) or bidentate (2), and examples of both types of complex have been isolated and their solid state IR spectra studied.39 41 Many investigations have shown that formation of the monodentate ester species has similar effects to protonation of the a-amino group. Thus the pKa values of MNH2CH(R)C02H and NH3CH2C02H (carboxyl ionization) are usually quite similar.42... 

As noted above, normalization of the carboxylate ionization is likely the source of the reaction heat observed in region IV of the heat capacity isotherm. The carboxylate ionization process must contribute to the enthalpy isotherm in the low-hydration region. 

We reiterate that we do not have knowledge of the experimental configurational states, or the extent of carboxyl ionization, of the cyclic nitrosoamines. The self-consistency of the QSAR is the only indirect evidence for predicting configurational, conformer, and ionization states of the molecules. 

As an example, consider a dehydration of L-malate catalyzed by fumarase, affording fumarate as the product of reaction this reaction is fully reversible. Kinetic studies suggest a model in which a histidine residue and a carboxyl group in the active site of enzyme are necessary for catalysis. According to this model, from the malate side of reaction, the histidine must be protonated and the carboxyl ionized, while in the reverse direction, the states of ionization are reversed (Cleland, 1977) (Fig. 11). Note that, in Fig. 11, proton is not a part of the chemical reaction but, formally, the elements of water (proton and the hydroxyl ion) are removed from the substrate by the enzyme. 

Obviously, the V profiles for the slow nonsticky substrate are the best choice, since these Vprofiles will show the correct piTa vcdues of amino acid side chains on enzyme in the enzyme-substrate complex that are responsible for catalysis. The apparent pK s of the catalytic groups of fumarase have been measured in both directions (Brandt et al, 1963). When malate is used as a substrate, Vprofile is beU-shaped showing two p a s, 6.4 and 9.0, respectively temperature variation of these groups indicates that p Ta 6.4 corresponds to dissociation of a carboxyl and pXa 9.0 to dissociation of a histidine side chain in the active site of enzyme. Thus, from the malate side of reaction, the enzyme is active if histidine is protonated and the carboxyl ionized. With fumarate as a substrate, the V profile is again bell-shaped showing two p a s. 7.0 and 4.9, respectively this time, however, the temperature variation indicates that p a 7.0 corresponds to dissociation of a carboxyl and p/iTa 4.9 to dissociation of a histidine side chain. Thus, from the fumarate side of reaction, the enzyme is active if carboxyl is protonated and histidine is not. 

Positive Cooperativity of Carboxyl Ionization in Elastic Model Proteins Increases with Increasing Hydrophobicity... 

The carboxyl ionization [pK]) is low and easily identified. However, the ammonium and thiol groups have similar pK values (compare methylamine with methyl mercaptan, Table 2.1) and so an uncertainty exists as to which group ionizes first. Ki, K2, and K3 represent macroscopic ionization constants determined experimentally from a titration curve. K2 and are the composite of four microscopic ionization constants. Once the proton is lost from the carboxyl group, one of two ionization pathways may be followed ... 

Figure 2 shows the influence of pH on the dynamic swelling behavior of P(HEMA-co-AA) copolymers containing 60 mol% HEMA. It can be seen that as the pH value increased, the water uptake rate increased as would have been expected from carboxyl ionization. 

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