Zwitterionic buffers

Because of the zwitterion formation, mutual buffering action, and the presence of strongly acid components, soybean phosphoHpids have an overall pH of about 6.6 and react as slightly acidic in dispersions-in-water or in solutions-in-solvents. Further acidification brings soybean phosphoHpids to an overall isoelectric point of about pH 3.5. The alcohol-soluble fraction tends to favor oil-in-water emulsions and the alcohol-insoluble phosphoHpids tend to promote water-in-oil emulsions. 

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

Equilibrium studies in water were performed for [Mc2Sn(IV)] complexes of zwitterion buffers, such as bicine and tricine (L). The results showed the formation of MLH, ML, ML2, MLH i, and MLH 2 with the hydrolysis products of the... 

The distribution profiles are not necessarily symmetrical because the anion and the cation may have different partition coefficients. For both types of profiles, however, zwitterionic compounds have an approximately constant value of lipophilicity in the region around their isoelectric pH, so that they behave as lipophilicity buffers between their two values [143,327]. 

Bushey, M. M. and Jorgenson, J. W., Capillary electrophoresis of proteins in buffers containing high concentrations of zwitterionic salts, /. Chromatogr., 480, 301, 1989. 

Mash, H., Chin, Y., Sigg, L., Hari, R., and Xue, H., Complexation of copper by zwitterionic aminosul-fonic (good) buffers, Anal Chem, 75 (3), 671-677, 2003. 

Holmlin RE, Chen X, Chapman RG, Takayama S, Whitesides GM (2001) Zwitterionic SAMs that resist nonspecific adsorption of protein from aqueous buffer. Langmuir 17 2841-50... 

Naess, O., and Rasmussen, K. E. (1997). Micellar electrokinetic chromatography of charged and neutral drugs in acidic running buffers containing a zwitterionic surfactant, sulfonic acids or sodium dodecyl sulphate — separation of heroin, basic by-products and adulterants. /. Chromatogr. A 760, 245-251. 

Although these approaches seem to be similar, the former method has several limitations it should be recognized that intrinsic mobility (yu,A ) depends on the ionic strength of the buffer, as shown in Ref. 17. Also, the activity coefficients of the zwitterions could not be calculated via the De-bye-Hiickel equation, and other methods, such as melting point depression, should be used to obtain the activity coefficients of the zwitterions (31). On the other hand, the latter method is directly applicable to the zwitterions to obtain their acidity constants. 

As electrostatic interactions exist between the HILIC zwitterionic stationary phase and charged solutes, particular attention has to be paid to the rigorous control of experimental conditions such as pH, buffer type and ionic strength. 

The effectiveness of the coating has been investigated by separating phenolic compounds in the nonaqueous media. The EOF was found to be anodic and dependent on the pH of the separation buffer. In anofher study [66], imidazole containing zwitterionic salt (N-3-(-triethoxysilylpropyl)-4,5-dihy-droimidazole) was attached to the silica capillary wall via the formation of a covalent bond (Figure 6.8). 

Sample extraction and deproteinization is usually accomplished with organic solvents including ethyl acetate (182-187, 189-192), acetone (193-196), methanol (177,197-200), acetonitrile (201,202), and ethanol (188). To optimize the extraction efficiency, acidification of the sample has been suggested by many workers (177, 188, 192, 197-199). In acidic conditions (pH 3), quinolones, being zwitterions, are fully protonated and, therefore, are becoming less bound by the matrix and more soluble in organic extraction solvents. Extraction of quinolones from food samples can also be accomplished using water (203), phosphate buffer, pH 9 (204), or trichloroacetic acid (205). 

Structures and Properties of Several Synthetic Zwitterionic Buffers... 

The addition of zwitterionic or non-ionic detergents to SD buffers has three important functions. Detergents aim to disrupt membranes and consequently solubilize lipids and delipidate proteins, which are bound to membranes or vesicles. The use of SDS as a detergent will affect the overall net charge and so non-ionic or zwitterionic detergents such as Triton X-100 or CHAPS are used (Shaw and Riederer, 2003). Commonly used CHAPS, which will not affect the overall charge, can be added up to 4% in SD buffers to account for... 

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