Titratable site

Most frequently, protonation or deprotonation of protein occurs at ionizable, also called titratable, side chains, such as aspartate, glutamate or histidine. The ionization equilibrium of a titratable site,... 

Thus, the pKa value equals pH if the concentrations of protonated and deprotonated states are identical. For this reason, pKa is also called p/fi/2- Equation (10-4) can be rearranged and written in a generalized form in the presence of multiple titration sites,... 

Its capability to titrate sites on heterogeneous surfaces makes photoemission of adsorbed xenon in principle a particularly attractive technique for investigating the surfaces of catalysts. Unfortunately, the technique has its limitations, because the Xe 5p /2 peak has a finite linewidth of about 0.4 eV. If a surface possesses more than three to four different adsorption sites, the spectra may become too... 

The effect of temperature on ammonia adsorption by ZSM5 samples has been investigated by microcalorimetry, varying the adsorption temperature from 150 to 400°C [235]. The initial heats of adsorption were independent of temperature up to 300°C. When the adsorption temperature increased, there was a competition between the formation of ammonium ions on Brpnsted sites and their decomposition. The total number of titrated sites decreased with increasing adsorption temperature. It appeared that an adsorption temperature between 150 and 300°C is appropriate for these calorimetric experiments. 

TEOS HSfCH,LSifOMeL Acid loadine Titratable sites Snecific Surface Area... 

If combination requires two titratable sites on the protein molecule, more complex equations are of course obtained, as discussed elsewhere (Tanford, 1961a). 

Tanford and Roxby introduced the approximation that the average influence of site k on site i is proportional to the average charge of site k. The procedure of Tanford and Roxby iterates to a set of self-consistent partial charges on titrating sites at a given pH. The fractional protonation state, 0 is estimated from the Henderson-Hasselbalch equation... 

The reduced-site approach of Bashford and Karplus is used to calculate the protonation state of a molecule with M titrating sites by taking a Boltzmann-weighted sum over all possible protonation states at each pH. These authors noticed that for particular values of pH, some titratable sites can be considered as being in a fixed protonation state (i.e., protonated or deproto-... 

The application of numerical methods to the solution of Poisson-Boltzmann problems with dielectric boundaries determined by the atomic coordinates and radii of a protein made it possible to carry out much more detailed and complete titration calculations using a MEAD-type model. The idealization used is illustrated in Figure 2. These calculations start from a model compound pK for each of the titrating sites and assume that the difference in titration behavior between the model compound and the analogous site in the protein can be accounted for by three kinds of electrostatic terms (i) AAGBom, the interaction of the site with the reaction field as a result of the polarization of the protein/solvent (versus model compound/solvent) system by the site s own charges ... 

In gathering the results of equations (6)-(8) for the purpose of calculating individual site p a values or overall titration curves, it is convenient to define the intrinsic p a value of a titrating site in a protein, as the p a that the site would have if all other sites were held in their neutral states. In the context of the present model, it is given by ... 

The site-site interactions enter in a more complex fashion because the charged states of the other sites are variable and pH-dependent. In principle, the protonation fraction, 0, for a given site, at any pH can be obtained by a Boltzmann average over all possible protonation states of the N titrating sites ... 

Localized conformational change and improvements in pAa calculations were the main focus of a study in which equation (9) was replaced by statistical average over side c in conformers of the titrating site whose pAimr is to be calculated ... 

In the discussion above we considered only a single titratable site. To extend the procedure to multiple sites first requires that the pKa be calculated for each site according to Eqs. [410]-[412]. The pK of a site determined while all other sites are unprotonated is called the intrinsic pKa of the site. We now define a protonation vector x = x, X2, , xn for N sites where each x l is either 1 or 0 depending on whether the site is protonated x thus spans a space of 2N vectors and x denotes one of them. The charge on site p corresponding to state n is then... 

The importance of accurately determining pKa values of titratable sites of proteins is supported by the large number of studies pertaining to their calculation. In addition to method development " " " and review articles,some of the systems most recently studied are noted here a-chymotrypsin, bacteriorhodopsin, myoglobin, ribonuclease ribonuclease protein BPTI, lyso-... 

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