Divalent cation-free solution

Fohr KJ, Warchol W, Gratzl M (1993) Calculation and control of free divalent cations in solutions used for membrane fusion studies. In Methods Enzymol. 221 49-57. 

Trading A technical question. When you use the Ca2+-free solutions and add Ca2+ back, what do you do about other divalent cations ... 

Use nuclease-free solutions and glassware. Chelaters, such as EDTA, may be used to reduce nuclease activity by binding divalent cations. 

A combination of chelators for divalent cations is suitable to buffer the free Ca " concentration from 0.1 -100 (iM under experimental conditions. Added Mg " and ATP as well as the pH of the medium must be considered, because they alter the equilibrium between Ca and the chelators present. The free Ca and Mg " concentrations are calculated by a computer program and controlled by Ca and Mg " specific electrodes (Fohr et al., 1993). Each Ca " buffer is prepared separately from stock solutions, with a final check of pH, pCa, or pMg. If no Ca electrode is available, the calculated total amount of Ca (as CaCy and Mg (as Mg(CH3COO)2) must be added before the pH adjustment. Buffers can be stored at -20 °C but should be thawed only once, mainly because of decomposition of ATP. 

In recent studies, antibiotic A23187 was found to form complexes of the type AM1 and A2HM1 with monovalent cations, and of the type A 2 M1 with divalent cations in solution63 232 The solution conformation of the (A23187)2-Ca complex and of the free antibiotic were deduced from NMR spectroscopic data63l... 

Hodson and Azam (1977) have reported that up to 20% of the total ATP in seawater occurs in the free form. It is, however, difficult to say whether this finding was a product of sample manipulation. At least in bacterial cultures, it has been shown that nucleotides may be released into the medium (Chapmann et al., 1971). On the other hand, it is questionable whether ATP is stable enough to exist in solution over longer periods owing to its low stability constant or the possibility of complexation with divalent cations and fulvic acids (Tetas and Lowenstein, 1963 Hulett, 1970 Bulleid, 1978). 

Every chemical attack on glass involves water or one of its dissociation products, i. e. H+ or OH- ions. For this reason, we differentiate between hydrolytic (water), acid, and alkali resistance. In water or acid attack, small amounts of (mostly monovalent or divalent) cations are leached out. In resistant glasses, a very thin layer of silica gel then forms on the surface, which normally inhibits further attack (Fig. 3.4-8a,b). Hydrofluoric acid, alkaline solutions, and in some cases phosphoric acid, however, gradually destroy the silica framework and thus ablate the glass surface in total (Fig. 3.4-8c). In contrast, water-free (i. e. organic) solutions do not react with glass. 

To address the theoretical limitation of the Nikolsky-Eisemnan equation, a more general description of the equihbrium responses of hquid membrane ISEs in mixed ion solutions was proposed (41). The model is based on phase boundary potentials under an equilibrium exchange of an analyte and an interfering co-ion at the membrane/sample solution interface. With ionophore-based membranes, the ion-exchange process is followed by complexation of the ions with an ionophore, where free ionophore was assumed to be always present in excess to simplify the model. The charge of the ions was not fixed so that their effect on the potentiometric responses can be addressed by the model. Under equilibrium conditions, the model demonstrated that the Nikolsky-Eisemnan equation is valid only for ions with the same charge (zj = Zj). The selectivity coefficient, however, can still be used in the new model to quantify the potentiometric responses in the mixed ion solution. For example, the potentiometric responses to a monovalent cation in the presence of a divalent cation are given as... 

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