Protonic activity, control

Diheterolevulosans, 209-211, 240 Dihexulose dianhydrides, 207 -266, see also Caramels Di-D-fructose dianhydrides 13C NMR spectra, 245-246 conformation, electronic control, 224-228 conformational rigidity, energetic outcomes, 228 hexulopyranose rings, 226 historical overview, 210-213 H NMR spectra, 248 -249 intramolecular hydrogen-bonds, 227 isomerization, 231 -232 1,2-linked, ero-anomeric effect, 224-225 listing, 240-241 nomenclature, 208-210 optical rotations and melting points, 242-243 protonic activation... 

Bacterial cell walls contain different types of negatively charged (proton-active) functional groups, such as carboxyl, hydroxyl and phosphoryl that can adsorb metal cations, and retain them by mineral nucleation. Reversed titration studies on live, inactive Shewanella putrefaciens indicate that the pH-buffering properties of these bacteria arise from the equilibrium ionization of three discrete populations of carboxyl (pKa = 5.16 0.04), phosphoryl (oKa = 7.22 0.15), and amine (/ Ka = 10.04 0.67) groups (Haas et al. 2001). These functional groups control the sorption and binding of toxic metals on bacterial cell surfaces. 

The intestinal absorption of dietary cholesterol esters occurs only after hydrolysis by sterol esterase steryl-ester acylhydrolase (cholesterol esterase, EC 3.1.1.13) in the presence of taurocholate [113][114], This enzyme is synthesized and secreted by the pancreas. The free cholesterol so produced then diffuses through the lumen to the plasma membrane of the intestinal epithelial cells, where it is re-esterified. The resulting cholesterol esters are then transported into the intestinal lymph [115]. The mechanism of cholesterol reesterification remained unclear until it was shown that cholesterol esterase EC 3.1.1.13 has both bile-salt-independent and bile-salt-dependent cholesterol ester synthetic activities, and that it may catalyze the net synthesis of cholesterol esters under physiological conditions [116-118], It seems that cholesterol esterase can switch between hydrolytic and synthetic activities, controlled by the bile salt and/or proton concentration in the enzyme s microenvironment. Cholesterol esterase is also found in other tissues, e.g., in the liver and testis [119][120], The enzyme is able to catalyze the hydrolysis of acylglycerols and phospholipids at the micellar interface, but also to act as a cholesterol transfer protein in phospholipid vesicles independently of esterase activity [121],... 

Finally, we note that all transfers to alcohol-water mixtures or additions of alcohol to crystal mother liquor involve changes in the proton activity of the solution. Care must be taken to ensure that the pH does not change too much, or the crystal may be disrupted. Worse still, the enzymatic activity may be abolished. Control of proton activity in mixed solvents is discussed in Section III,D. If dielectric effects are controlled and pH is properly adjusted, the microenvironment of a crystalline protein will correspond closely to that of aqueous solution at room temperature. Such correspondence is essential for temporal resolution of individual steps in a catalytic reaction. 

The proton, the smallest solute in the cellular water, illustrates particularly well the critical role of the aqueous milieu of the cell in governing macromolecular stability and function under both stable and variable thermal conditions. One of the ubiquitous features of cellular regulatory processes involves control of pH. No type of cell allows the proton activity within its cytoplasm to equilibrate with that of the external medium, whether the medium in question is the external environment or the extracellular fluids. Organelles, too, may maintain a steep proton gradient between the intra-organellar fluids, for instance, the mitochondrial matrix, and the cytoplasm. 

Algae can adjust the intracellular concentration of DMSP through the biosynthetic (anabolic) or the degradation (catabolic) pathways. DMSP-lyase enzymes facilitate the degradation pathway, in which DMSP is cleaved to DMS, acrylate and a proton. What controls the activity of DMSP-lyases in phytoplankton is still unknown. Stefels (2000) suggested... 

Activation control of an overall dissolution rate can, of course, reside in the reduction process, in the oxidation process, in a mixture of both, or in a mixture including some transport control. The reduction process is usually more influential in determining the overall rate. Thus, in the absence of transport control, the kinetics of the electrode process for reduction of hydrated protons, or water molecules, or dissolved molecular oxygen plays the major role in metal dissolution kinetics. Indeed the literature confirms the conclusion that many of the systems seen in experiment or in practice are diffusion controlled that most of the rest are under mixed diffusion and activation control and that those with some activation control... 

Control of an electrolytic reaction often requires that the proton activity remains within acceptable limits during the electrolysis. For small-scale electrolytic preparations (less than about 10 g/liter of substrate), a sufficiently high initial concentration of buffer, acid, or base is adequate in aqueous solution for large-scale electrolysis a controlled addition of protons during a reduction must be provided. This addition may be controlled by a pH-stat or coupled to the current integrator. In aprotic media a proton donor, electrophile, or nucleophile may play a similar role as buffers in aqueous media. 

Mahalingam M, Martinez-Mayorga K, Brown MF et al (2008) Two protonation switches control rhodopsin activation in membranes. Proc Natl Acad Sci USA 105 17795-17800... 

The proton-active modifiers which were used for controlled protonolysis of the active Al—C bonds include long chain alcohols, carboxyhc acids, silanols, proton-active surfactants, and sugars (see Table 2.4). 

One of the problems with much of the work on P450 models is that reactions are performed in organic solvents in which it is not possible to obtain detailed information about the reaction mechanisms involved. This is because the proton activity in organic solvents is not easily determined. It is only in aqueous solution that the conditions necessary for oxygen transfer, such as ionic strength, acidity, and ligand species concentration, are best controlled, and data (e.g. electrochemical and kinetic) are best interpreted. To that end, water-soluble iron and manganese tetraarylporphyrins have been prepared by Bruice et al. and their reactions with hydroperoxides studies. ... 

The nonuniform distribution of the proton-active centers in zeolites can be measured by temperature-controlled desorption of adsorbed organic bases. The bases that are adsorbed on the centers of highest activity require the highest temperature for desorption. The IR spectra of adsorbed bases such as ammonia and pyridine give information about the nature of the adsorption centers. For example, the pyridi-nimn ion is indicative of proton-donor sites. NMR and ESR spectroscopy are also useful for elucidating the nature of acid centers. 

Hausdorf S, Wagler J, Mossig R, Mertens FORL. Proton and wata activity-controlled structure formation in zinc carboxylate-based metal aganic framewaks. J Hiys Chem A 2008 112 7567-76. 

Yuflt V, Brandon NP (2011) Development and application of an actively controlled hybrid proton exchange membrane fuel cell-lithium-ion battery laboratory test-bed based on off-the-shelf components. J Power Sources 196 801-807. doi 10.1016/j.jpowsour.2010.06.029... 

It may be usefiil to remind ourselves that water in this context is an oxide present as a second phase in excess and, if we wish, at constant activity (controlled partial pressure). It is thus analogous to the case of an excess of metal oxide. The proton dissolves interstitially forming a positive defect (H or OHq) and thus behaves like a donor When they are the dominating positive defects in an otherwise undoped oxide they may be compensated by electrons or negative... 

A purely ionic hydrogen bond activation mechanism might be involved in the aza-Henry reaction between a-iminoesters, a very reactive subclass of imines, and various nitroalkanes catalyzed by the BINOL phosphoric acid 44 [54]. The corresponding P-nitro-a-amino acid esters were produced in good yields, diastereo- and enantioselectivities (Scheme 29.23). The authors postulated a dual role of catalyst 44 through activation of the a-iminoester by protonation and control over the nitroaUcane/nitronate equilibrium (Scheme 29.24). 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, many organic molecules undergo rapid proton shifts (tautomerism), and the chemical reactivity of the two isomers may be quite different It is not valid, however, to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions will not be controlled by the position of the facile equilibrium. 

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