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Proton binding sites

At the same time, the response deviates more from the theoretical value the fewer proton-binding sites are present. On the other hand, both the adsorption effects and the sub-Nernstian behavior vanish if the thickness of the hydrated layer is allowed to increase up to 800 nm (Fig. 6.24). It is seen from this model that as the thickness of the hydrated layer exceeds the thickness of the space the adsorption effects and the sub-Nernstian behavior disappear. [Pg.163]

Fig. 6.23 Dependence of the Nemst potential on the density of the proton-binding sites and on adsorption. Upper curves within the bracketed sets correspond to the absence of adsorption. The lower curves show the effect of 100 mM charged adsorbate (adapted from Sandifer 1998)... Fig. 6.23 Dependence of the Nemst potential on the density of the proton-binding sites and on adsorption. Upper curves within the bracketed sets correspond to the absence of adsorption. The lower curves show the effect of 100 mM charged adsorbate (adapted from Sandifer 1998)...
The dynamics of proton binding to the extra cellular and the cytoplasmic surfaces of the purple membranes were measured by the pH jump methods [125], The purple membranes selectively labeled by fluorescein Lys-129 of bacteri-orhodopsin were pulsed by protons released in the aqueous bulk from excited pyranine and the reaction of the protons with the indicators was measured. Kinetic analysis of the data implied that the two faces of the membrane differ in then-buffer capacities and in their rates of interaction with bulk protons. The extracellular surfaces of the purple membrane contains one anionic proton binding site per protein molecule with pA" 5.1. This site is within a Coulomb cage radius from Lys-129. The cytoplasmic surface of the purple membrane bears four to five pro-tonable moieties that, due to close proximity, function as a common proton binding site. The reaction of the proton with this cluster is at a very fast rate (3 X 1010 M-1 sec ). The proximity between the elements is sufficiently high that even in 100 mM NaCl, they still function as a cluster. Extraction of the chromophore retinal from the protein has a marked effect on the carboxylates of the cytoplasmic surface, and two to three of them assume positions that almost bar their reaction with bulk protons. Quantitative evaluation of the dynamics of proton transfer from photoactivated bacteriorhodopsin to the bulk has been done by using numerical... [Pg.594]

The increase in log with pH is typical of copper binding to weak acid functional groups. For the reaction of copper with a protonated binding site HpL,... [Pg.167]

Direct titrations are well suited to DOM, because the method does not require the separation of DOM from the bulk solution, as is required in indirect methods. Furthermore, because pH is monitored continuously as increments of a titrant are added, direct titrations provide much more detail about the acidic strengths of proton binding sites in DOM than do indirect methods. The lack of distinct inflection points in the titration curves of DOM is a result of the broad spectrum of acidic strengths of its functional groups. [Pg.2551]

The variables f and are computed from four a values, as indicated in Equation (21). This technique appears to locate the K values of some simple ligand mixtures, but it does not properly recognize the discrete nature of those ligand mixtures. From the limited number of attempts this author has made to test this approach, the method always predicts the existence of rather broad distributions of binding sites, even when only a few diserete sites are present. Nevertheless, when a complex mixture of proton binding sites is present, the method may be useful for estimating the actual nature of the binding site distribution. [Pg.523]

Since in the last decade significant results have been achieved in the theory of adsorption from non-electrolytic liquid mixtures on heterogeneous solids (see reviews [187-189] and references therein), these results can be utilized in the study of heterogeneity of proton binding sites on inorganic oxides. [Pg.427]

Contescu, C., Jagiello, J., and Schwarz, J.A., Heterogeneity of proton binding sites at the oxide/solution interface, Langmuir, 9, 1754, 1993. [Pg.944]

A special complication of proton transfer is the interruption of the proton diffusion by the formation of covalent bonds with the various proton-binding sites. Each site that interacts with a free proton will bind it for a time frame that is proper-... [Pg.1500]

The membrane of Stuchebrukchov s model is an infinite surface, where the multitude of proton binding sites (carboxylates with pK = 5) is represented by a density function (cr). The dwell time of a proton on any of the sites is determined by the pK (Tdweii = l disfeon), but dufing this time interval the proton can diffuse on the surface with a diffusion coefficient that is 10% of the bulk value (Dg 0.1 Db), screening an area with a radius Lg. On the surface, there is a proton-channel acting either as an absorbing sink, or a source which affects the immediate proton concentration, both at the surface and in the solution. The bulk phase in this model is an infinite reservoir, which is sufficiently far from the proton-consuming cluster to satisfy the demand AC/Ax = 0, a definition that is based on a chemical function... [Pg.1506]

It must be recalled that the model of Stuchebrukchov is phenomenological in nature and many discrete steps and events have been lumped together . The understanding of all intricate relations between the rate constants of the chemical reaction of the proton binding sites with free proton and the macroscopic descriptors calls for molecular modeling and direct measurements, where the local properties of the reaction space must be included in the analysis. Accordingly, the Stuchebrukchov mechanism can be taken as a guideline and its predictions are approximations. [Pg.1508]

The fiuorescein molecule is a suitable model system for monitoring the mechanism of proton transfer between adjacent sites as it has two distinct proton binding sites 6 A apart, each having a distinct pK and spectral properties. The first is the oxyanion attached to the xanthene ring, whose protonation is associated with a spectral shift of the dye, while the second is the carboxylate on the benzene ring. The protonation of this site has no effect on the absorption spectrum of the dye (Scheme 20.1). [Pg.1509]

The kinetic features recorded for the intra-Coulomb cage proton transfer between the proton binding sites were reconfirmed with other dyes resembling the... [Pg.1510]

The rate constants of the MonNa or the Mo imply that both species are located on the surface of the membrane, with the proton-binding site constantly exposed to... [Pg.1515]

There is a strong resemblance between the mechanism of ion motion next to the protein and the proton-collecting antenna reported for bacteriorhodopsin [78, 79] or cytochrome c oxidase [2]. These domains consist of a cluster of carboxylates that function as proton binding sites. The protonation on any carboxylate of the cluster leads to rapid proton exchange reactions that finally deliver the proton to the immediate vicinity of the proton-conducting channel of the protein. [Pg.1521]


See other pages where Proton binding sites is mentioned: [Pg.128]    [Pg.129]    [Pg.120]    [Pg.49]    [Pg.95]    [Pg.42]    [Pg.809]    [Pg.405]    [Pg.142]    [Pg.360]    [Pg.593]    [Pg.595]    [Pg.155]    [Pg.215]    [Pg.148]    [Pg.241]    [Pg.367]    [Pg.538]    [Pg.2534]    [Pg.233]    [Pg.234]    [Pg.332]    [Pg.799]    [Pg.525]    [Pg.427]    [Pg.128]    [Pg.1503]    [Pg.1504]    [Pg.1504]    [Pg.1507]    [Pg.1509]    [Pg.1510]    [Pg.1511]    [Pg.1511]    [Pg.1512]    [Pg.1517]   
See also in sourсe #XX -- [ Pg.332 ]




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