Basic sites multiple

The most important multiply charged polyatomic positive ions are compounds with two or more basic groups which when protonated lead to doubly or poly-charged ions. Typical examples are diamines such as the double protonated a, to alkyldiamines, H3N(CH2)pNH2+, and the most important class, the polyprotonated peptides and proteins, which have multiple basic residues. Charge reduction for these systems occurs through proton transfer from one of the protonated basic sites to a solvent molecule. Such a reaction is shown below for the monohydrate of a doubly protonated diamine ... 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

For simple processes in which multiple metals bind to a single ligand = 1 and iEL = 1 (AEE1 = 0) and hence the extended site binding model reduces back the basic site binding model. Figure 10.17 shows... 

Rates of deposition were calculated for cases in which sol and collector surfaces are dissimilar and acquire their charge by the dissociation of multiple acidic and/or basic sites. The double-layer interaction between such dissimilar surfaces can induce a sign reversal in the surface charge or the force. These sign reversals are not observed between identical surfaces. 

Surfaces with basic sites form enolates from both the aldehydes and ketones, leading to multiple aldol condensations and Michael additions. " Candidate molecules must be enolizable, i.e., contain an a-hydrogen atom. Aldol condensation / Michael addition products cover the range from a,p-unsaturated aldehydes, saturated aldehydes, hydrogenated products (alcohols), and the heavier aromatics resulting from multiple condensations. The presence of coordina-... 

Intact protein mass spectrometry allows the molecular mass determination of either proteins or complexes of proteins and covalently bound ligands/other proteins. In a first step, the sample is desalted to detach from buffer components and small ions that would interfere through noncovalent complexes in the gas phase. Next, the isolated protein is ionized, for example, by electrospray ionization (ESI). The acid in the eluent causes protonation of the protein at basic sites, particularly lysine and arginine residues, so that m/z values of multiple species with different charges can be measured in a mass spectrometer. These data are then combined during the deconvolution process to yield the mass of the protein or complex. 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

Alternatively, deprotonation enthalpies can be evaluated from probe adsorption calorimetric data or from temperature programmed desorption (TPD) measurements. The strengths of surface Lewis acid sites and of surface basic sites can also be evaluated, in principle, by the heat of probes adsorption or desorption. In all cases, however, probes adsorption on solids can result in multiple interactions for example, van der Waals interactions can be superimposed to true acid-base interactions, which can also be multiple and finally give rise to some kind of solvation effects, in particular, in the zeolite cavities [23-25]. Thus, the pure acidity/basicity... 

Electrospray ionization (ESI) mass spectrometry (MS) and matrix assisted laser desorption ionization (MALDI) have emerged recently as a powerful and sensitive methods for the detection and structural analysis of a wide variety of analytes, including large biomolecules such as nucleic acids and proteins. ESI-MS is a gentle method of ionization based on protonation of the sample at atmospheric pressure. The protonation of multiple basic sites in protein molecules results in multiply... 

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