Amino groups protein, titration

More phenolic and amino groups are titrated slowly above pH 12 as the protein becomes denatured. 

The wool fiber is composed of protein filaments and consists mainly of keratin with a very complex structure. The amino groups of keratin are of decisive importance for the dyeing process. The amount of basic groups titratable with acid is 850 1 mol per gram of wool fiber. In the acidic and neutral range, carboxyl groups are present largely in the undissociated state. 

Simulation Results. A onc-dimensional simulation model based on the Nernst-Planck and Poisson equations [14, in which all the acid-base reactions occurring in the membrane are taken into account, has been used to give a qualitative description of the pH step titration process. In these simulations, a pH step is applied outside a 2 mm thick stagnant layer, which is assumed to be present in front of an 8 mm thick membrane. Diffusion coefficients in the membrane are assumed to be 4/10 of those in water (this value is based on experience with ion step experiments). Lysozyme, used as a model protein, is assumed to contain 11 carboxylic groups (pKa = 4.4), 2 imidazole groups (pKa = 6.0), and 9 amino groups (pKa = 10.4) per molecule. Concern... 

The count of side-chain amino groups corresponds to the analytical figure for lysine side chains in a denaturing solvent (8 Af urea). In the native protein, however, three of the thirteen lysine groups cannot be observed to titrate. The maximum positive proton charge ( S W" ") is however the same in the native and denatured states, within the uncertainty of about... 

The major problems which this titration curve poses are identification of the two unexplained groups in the carboxyl region, and an explanation for the failure to titrate three of the thirteen side-chain amino groups in the native protein. 

Within the limits of error of amino acid analyses available at the time, the count of groups obtained by Cannan et al. agreed with expectation, except in so far as the alkaline part of the curve was concerned. The number of groups titrated here is essentially the same as the number of amino groups, rather than the sum of amino and phenolic groups. This result is in accord with the later spectrophotometric titration of phenohc groups essentially all of these groups are inaccessible to titration in the native protein. 

Titration curves of trypsin were obtained under a variety of conditions by Duke et al. (1952). The most noteworthy feature is a specific effect of calcium, which displaces the acid part of the titration curve to lower pH, and decreases the total number of groups which are titrated between pH 6 to 9. It is likely that the groups titrated between pH 6 and 9 in the absence of Ca " are a-amino groups, produced by self-digestion of the enzyme. The effect of Ca" " thus appears to result from a complex with the carboxyl groups of the protein, which stabilizes the anionic form of these groups so as to produce the displacement of the acid part of the titration curve. This complex is more resistant to self-digestion than the enzyme alone. 

Heat and titration with acid can accomplish the required H bond ruptures. Cavalieri and Rosenberg (358) have described this process, and they illustrate how it relates to the Watson and Crick structure for nucleic acids. They show further that the temperature required in an H bonding solvent is lower than in inert solvent, consequently these two factors can work in the same direction. Denaturation may also occur by ionization of amino groups (23), and the effect of radiation on DNA has been imputed to breaking of H bonds (454, 2147a). Mechanical stress may be able to break H bonds and denature proteins (1105). 

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